Imagine playing beach volleyball, like the young men in Figurebelow. They may not realize it, but they are being bombarded by electromagnetic radiation as play in the sunlight. The only kinds of radiation they can detect are visible light, which allows them to see, and infrared light, which they feel as warmth on their skin. What other kinds of electromagnetic radiation are they being exposed to in sunlight? In this lesson, you’ll find out.
Electromagnetic radiation from the sun reaches Earth across space. It strikes everything on Earth’s surface, including these volleyball players.
What Is The Electromagnetic Spectrum?
Electromagnetic radiation occurs in waves of different wavelengths and frequencies. Infrared light and visible light make up just a small part of the full range of electromagnetic radiation, which is called the electromagnetic spectrum. The electromagnetic spectrum is summarized in the diagram in Figurebelow.
On the far left of the diagram are radio waves, which include microwaves. They have the longest wavelengths and lowest frequencies of all electromagnetic waves. They also have the least amount of energy.
On the far right are X rays and gamma rays. The have the shortest wavelengths and highest frequencies of all electromagnetic waves. They also have the greatest amount of energy.
Between these two extremes, wavelength, frequency, and energy change continuously from one side of the spectrum to the other. Waves in this middle section of the electromagnetic spectrum are commonly called light.
As you will read below, the properties of electromagnetic waves influence how the different waves behave and how they can be used.
How do the wavelength and frequency of waves change across the electromagnetic spectrum?
Radio waves are the broad range of electromagnetic waves with the longest wavelengths and lowest frequencies. In Figureabove, you can see that the wavelength of radio waves may be longer than a soccer field. With their low frequencies, radio waves have the least energy of electromagnetic waves, but they still are extremely useful. They are used for radio and television broadcasts, microwave ovens, cell phone transmissions, and radar. You can learn more about radio waves, including how they were discovered, at this URL: (3:58).
AM and FM Radio
In radio broadcasts, sounds are encoded in radio waves that are sent out through the atmosphere from a radio tower. A receiver detects the radio waves and changes them back to sounds. You’ve probably listened to both AM and FM radio stations. How sounds are encoded in radio waves differs between AM and FM broadcasts.
AM stands for amplitude modulation. In AM broadcasts, sound signals are encoded by changing the amplitude of radio waves. AM broadcasts use longer–wavelength radio waves than FM broadcasts. Because of their longer wavelengths, AM radio waves reflect off a layer of the upper atmosphere called the ionosphere. You can see how this happens in Figurebelow. This allows AM radio waves to reach radio receivers that are very far away from the radio tower.
FM stands for frequency modulation. In FM broadcasts, sound signals are encoded by changing the frequency of radio waves. Frequency modulation allows FM waves to encode more information than does amplitude modulation, so FM broadcasts usually sound clearer than AM broadcasts. However, because of their shorter wavelength, FM waves do not reflect off the ionosphere. Instead, they pass right through it and out into space (see Figurebelow). As a result, FM waves cannot reach very distant receivers.
AM radio waves reflect off the ionosphere and travel back to Earth. Radio waves used for FM radio and television pass through the ionosphere and do not reflect back.
Television broadcasts also use radio waves. Sounds are encoded with frequency modulation, and pictures are encoded with amplitude modulation. The encoded radio waves are broadcast from a TV tower like the one in Figurebelow. When the waves are received by television sets, they are decoded and changed back to sounds and pictures.
This television tower broadcasts signals using radio waves.
The shortest wavelength, highest frequency radio waves are called microwaves(see Figureabove). Microwaves have more energy than other radio waves. That’s why they are useful for heating food in microwave ovens. Microwaves have other important uses as well, including cell phone transmissions and radar, which is a device for determining the presence and location of an object by measuring the time for the echo of a radio wave to return from it and the direction from which it returns. These uses are described in Figurebelow. You can learn more about microwaves and their uses in the video at this URL (3:23).
Microwaves are used for cell phones and radar.
Mid-wavelength electromagnetic waves are commonly called light. This range of electromagnetic waves has shorter wavelengths and higher frequencies than radio waves, but not as short and high as X rays and gamma rays. Light includes visible light, infrared light, and ultraviolet light. If you look back at Figureabove, you can see where these different types of light waves fall in the electromagnetic spectrum.
The only light that people can see is called visible light. It refers to a very narrow range of wavelengths in the electromagnetic spectrum that falls between infrared light and ultraviolet light. Within the visible range, we see light of different wavelengths as different colors of light, from red light, which has the longest wavelength, to violet light, which has the shortest wavelength. You can see the spectrum of colors of visible light in Figurebelow. When all of the wavelengths are combined, as they are in sunlight, visible light appears white. You can learn more about visible light in the chapter "Visible Light" and at the URL below.
Red light (right) has the longest wavelength, and violet light (left) has the shortest wavelength.
Light with the longest wavelengths is called infrared light. The term infrared means "below red." Infrared light is the range of light waves that have longer wavelengths than red light in the visible spectrum. You can’t see infrared light waves, but you can feel them as heat on your skin. The sun gives off infrared light as do fires and living things. The picture of a cat that opened this chapter was made with a camera that detects infrared light waves and changes their energy to colored light in the visible range. Night vision goggles, which are used by law enforcement and the military, also detect infrared light waves. The goggles convert the invisible waves to visible images. For a deeper understanding of infrared light, watch the video at this URL: (6:46).
Light with wavelengths shorter than visible light is called ultraviolet light. The termultraviolet means "above violet." Ultraviolet light is the range of light waves that have shorter wavelengths than violet light in the visible spectrum. Humans can’t see ultraviolet light, but it is very useful nonetheless. It has higher-frequency waves than visible light, so it has more energy. It can be used to kill bacteria in food and to sterilize laboratory equipment (see Figurebelow). The human skin also makes vitamin D when it is exposed to ultraviolet light. Vitamin D is needed for strong bones and teeth. You can learn more about ultraviolet light and its discovery at this URL: (3:40).
This sterilizer for laboratory equipment uses ultraviolet light to kill bacteria.
Too much exposure to ultraviolet light can cause sunburn and skin cancer. You can protect your skin from ultraviolet light by wearing clothing that covers your skin and by applying sunscreen to any exposed areas. The SPF, or sun-protection factor, of sunscreen gives a rough idea of how long it protects the skin from sunburn (seeFigurebelow). A sunscreen with a higher SPF protects the skin longer. You should use sunscreen with an SPF of at least 15 even on cloudy days, because ultraviolet light can travel through clouds. Sunscreen should be applied liberally and often. You can learn more about the effects of ultraviolet light on the skin at this URL: (5:59).
If your skin normally burns in 10 minutes of sun exposure, using sunscreen with an SPF of 30 means that, ideally, your skin will burn only after 30 times 10 minutes, or 300 minutes, of sun exposure. How long does sunscreen with an SPF of 50 protect skin from sunburn?
X Rays and Gamma Rays
The shortest-wavelength, highest-frequency electromagnetic waves are X rays and gamma rays. These rays have so much energy that they can pass through many materials. This makes them potentially very harmful, but it also makes them useful for certain purposes.
X rays are high-energy electromagnetic waves. They have enough energy to pass through soft tissues such as skin but not enough to pass through bones and teeth, which are very dense. The bright areas on the X ray film in Figurebelow show where X rays were absorbed by the teeth. X rays are used not only for dental and medical purposes but also to screen luggage at airports (see Figurebelow). Too much X ray exposure may cause cancer. If you’ve had dental X rays, you may have noticed that a heavy apron was placed over your body to protect it from stray X rays. The apron is made of lead, which X rays cannot pass through. You can learn about the discovery of X rays as well as other uses of X rays at this URL:http://www.guardian.co.uk/science/blog/2010/oct/26/x-ray-visions-disease-forgeries.
Two common uses of X rays are illustrated here.
Gamma rays are the most energetic of all electromagnetic waves. They can pass through most materials, including bones and teeth. Nonetheless, even these waves are useful. For example, they can be used to treat cancer. A medical device sends gamma rays the site of the cancer, and the rays destroy the cancerous cells. If you want to learn more about gamma rays, watch the video at the URL below.
KQED: Seeing Cells in 3-D
Scientists in Berkeley have developed a powerful new microscope which uses X rays to scan a whole cell and in a manner of minutes, generate a 3D view of the cell and its genetic material. This groundbreaking tool is helping to advance research into the development of biofuels, the treatment of malaria and it may even help to more rapidly diagnose cancer. For more information on X ray microscopes, see http://science.kqed.org/quest/video/x-ray-microscope-seeing-cells-in-3-d/.
The electromagnetic spectrum is the full range of wavelengths and frequencies of electromagnetic radiation. Wavelength, frequency, and energy change continuously across the electromagnetic spectrum.
Radio waves are the broad range of electromagnetic waves with the longest wavelengths and lowest frequencies. They are used for radio and television broadcasts, microwave ovens, cell phone transmissions, and radar.
Mid-wavelength electromagnetic waves are called light. Light consists of visible, infrared, and ultraviolet light. Humans can see only visible light. Infrared light has longer wavelengths than visible light and is perceived as warmth. Ultraviolet light has shorter wavelengths than visible light and has enough energy to kill bacteria. It can also harm the skin.
X rays and gamma rays are the electromagnetic waves with the shortest wavelengths and highest frequencies. X rays are used in medicine and dentistry and to screen luggage at airports. Gamma rays are used to kill cancer cells.
Lesson Review Questions
What is the electromagnetic spectrum?
Describe how wave frequency changes across the electromagnetic spectrum, from radio waves to gamma rays.
List three uses of radio waves.
How are X rays and gamma rays used in medicine?
Create a public service video warming people of the dangers of ultraviolet light. Include tips for protecting the skin from ultraviolet light.
Explain two ways that sounds can be encoded in electromagnetic waves.
Explain how radar works.
Compare and contrast infrared, visible, and ultraviolet light.
Points to Consider
This chapter introduces visible light. The chapter "Visible Light" discusses visible light in greater detail.
In this lesson, you read that visible light consists of light of different colors. Do you know how visible light can be separated into its different colors? (Hint: How does a rainbow form?)
In the next chapter, Visible Light, you’ll read that visible light interacts with matter in certain characteristic ways. Based on your own experiences with visible light, how does it interact with matter? (Hint: What happens to visible light when it strikes a wall, window, or mirror?)
Waves can come in many sizes. Here we see a large wave crashing on the beach. Other waves can be very small and regular. We normally think of waves as being made of water, but there are forms of energy that take on the characteristics of waves. The idea of a wave has played a major role in our understanding of how the atom is put together and why it behaves the way it does.
Properties of Light
The nuclear atomic model proposed by Rutherford was a great improvement over previous models, but was still not complete. It did not fully explain the location and behavior of the electrons in the vast space outside of the nucleus. In fact, it was well known that oppositely charged particles attract one another. Rutherford’s model did not explain why the electrons don’t simply move toward and eventually collide with the nucleus. Experiments in the early twentieth century began to focus on the absorption and emission of light by matter. These studies showed how certain phenomena associated with light reveal insight into the nature of matter, energy, and atomic structure.
Wave Nature of Light
In order to begin to understand the nature of the electron, we first need to look at the properties of light. Prior to 1900, scientists thought light behaved solely as a wave. As we will see later, this began to change as new experiments demonstrated that light also has some of the characteristics of a particle. First, we will examine the wavelike properties of light.
Visible light is one type of electromagnetic radiation, which is a form of energy that exhibits wavelike behavior as it moves through space. Other types of electromagnetic radiation include gamma rays, x-rays, ultraviolet light, infrared light, microwaves, and radio waves. The figure below shows the electromagnetic spectrum, which is all forms of electromagnetic radiation. Notice that visible light makes up only a very, very small portion of the entire electromagnetic spectrum. All electromagnetic radiation moves through a vacuum at a constant speed of 2.998 × 108 m/s. While the presence of air molecules slows the speed of light by a very small amount, we will use the value of 3.00 × 108 m/s as the speed of light in air.
The electromagnetic spectrum encompasses a very wide range of wavelengths and frequencies. Visible light is only a very small portion of the spectrum with wavelengths from 400-700 nm. [Figure2]
The Figureabove shows how the electromagnetic spectrum displays a wide variation inwavelength and frequency. Radio waves have wavelengths of as long as hundreds of meters, while the wavelength of gamma rays are on the order of 10-12 m. The corresponding frequencies range from 106 to 1021 Hz. Visible light can be split into colors with the use of a prism (Figurebelow), yielding the visible spectrum of light. Red light has the longest wavelength and lowest frequency, while violet light has the shortest wavelength and highest frequency. Visible light wavelength ranges from about 400 – 700 nm with frequencies in the range of 1014 Hz.
A small beam of white light is (refracted) bent as it passes through a glass prism. The shorter the wavelength of light, the greater is the refraction, so the light is separated into all its colors. [Figure3]
^ Credit: CK-12 Foundation - Zachary Wilson; License: CC BY-NC 3.0
^ Credit: CK-12 Foundation - Christopher Auyeung; License: CC BY-NC 3.0
What a beautiful sunset! You probably know that sunlight travels in waves through space from the sun to Earth. But do you know what light really is? Is it just energy, or is it something else? In this article you’ll find out that light may be more than it seems.
Electromagnetic radiation, commonly called light, is the transfer of energy by waves calledelectromagnetic waves. These waves consist of vibrating electric and magnetic fields. Where does electromagnetic energy come from? It is released when electrons return to lower energy levels in atoms. Electromagnetic radiation behaves like continuous waves of energy most of the time. Sometimes, however, electromagnetic radiation seems to behave like discrete, or separate, particles rather than waves. So does electromagnetic radiation consist of waves or particles?
This question about the nature of electromagnetic radiation was debated by scientists for more than two centuries, starting in the 1600s. Some scientists argued that electromagnetic radiation consists of particles that shoot around like tiny bullets. Other scientists argued that electromagnetic radiation consists of waves, like sound waves or water waves. Until the early 1900s, most scientists thought that electromagnetic radiation is either one or the other and that scientists on the other side of the argument were simply wrong.
Q: Do you think electromagnetic radiation is a wave or a particle?
A: Here’s a hint: it may not be a question of either-or. Keep reading to learn more.
A New Theory
In 1905, the physicist Albert Einstein developed a new theory about electromagnetic radiation. The theory is often called the wave-particle theory. It explains how electromagnetic radiation can behave as both a wave and a particle. Einstein argued that when an electron returns to a lower energy level and gives off electromagnetic energy, the energy is released as a discrete “packet” of energy. We now call such a packet of energy a photon. According to Einstein, a photon resembles a particle but moves like a wave. You can see this in the Figurebelow. The theory posits that waves of photons traveling through space or matter make up electromagnetic radiation.
Energy of a Photon
A photon isn’t a fixed amount of energy. Instead, the amount of energy in a photon depends on the frequency of the electromagnetic wave. The frequency of a wave is the number of waves that pass a fixed point in a given amount of time, such as the number of waves per second. In waves with higher frequencies, photons have more energy.
Evidence for the Wave-Particle Theory
After Einstein proposed his theory, evidence was discovered to support it. For example, scientists shone laser light through two slits in a barrier made of a material that blocked light. You can see the setup of this type of experiment in the Figurebelow. Using a special camera that was very sensitive to light, they took photos of the light that passed through the slits. The photos revealed tiny pinpoints of light passing through the double slits. This seemed to show that light consists of particles. However, if the camera was exposed to the light for a long time, the pinpoints accumulated in bands that resembled interfering waves. Therefore, the experiment showed that light seems to consist of particles that act like waves.
Electromagnetic radiation behaves like waves of energy most of the time, but sometimes it behaves like particles. From the 1600s until the early 1900s, most scientists thought that electromagnetic radiation consists either of particles or of waves but not both.
In 1905, Albert Einstein proposed the wave-particle theory of electromagnetic radiation. This theory states that electromagnetic energy is released in discrete packets of energy—now called photons—that act like waves.
After Einstein presented his theory, scientists found evidence to support it. For example, double-slit experiments showed that light consists of tiny particles that create patterns of interference just as waves do.
Effect of translucent, transparent and opaque objects
Visible Light and Matter
A mime is an actor who uses movement and facial expressions rather than words to communicate with an audience. The mime in this picture is using a mirror to apply stage makeup that will accentuate her features so she can communicate more expressively. When light strikes a mirror, it is reflected back from the shiny surface. The reflected light forms an image of whatever is in front of the mirror. Reflection is just one way that visible light may interact with matter.
Reflection of light occurs when light bounces back from a surface that it cannot pass through. Reflection may be regular or diffuse.
If the surface is very smooth, like a mirror, the reflected light forms a very clear image. This is called regular, or specular, reflection. In the Figurebelow, the smooth surface of the still water in the pond on the left reflects light in this way.
When light is reflected from a rough surface, the waves of light are reflected in many different directions, so a clear image does not form. This is called diffuse reflection. In theFigurebelow, the ripples in the water in the picture on the right cause diffuse reflection of the blooming trees.
Transmission of light occurs when light passes through matter. As light is transmitted, it may pass straight through matter or it may be refracted or scattered as it passes through.
A branch of science called seismology deals with the study of waves that travel through the Earth. These waves are produced naturally by earthquakes, or by large amounts of explosives planted in the ground. Scientists look at how and where these waves refract as they travel from one type of rock layer into another. This helps them to figure out what layers are in the Earth, and how thick and deep those layers are.
When light is refracted, it changes direction as it passes into a new medium and changesspeed. The straw in the Figurebelow looks bent where light travels from water to air. Light travels more quickly in air than in water and changes direction. For a detailed explanation of how this happens, watch the animation at this URL:
Let's examine the how the above parameters change when the wave slows down or speeds up:
Scattering occurs when light bumps into tiny particles of matter and spreads out in all directions. In the Figurebelow, beams of light from car headlights are shining through fog. The light is scattered by water droplets in the air, giving the headlights a “halo” appearance.
Q: What might be another example of light scattering?
A: When light passes through smoky air, it is scattered by tiny particles of soot.
Light may transfer its energy to matter rather than being reflected or transmitted by matter. This is called absorption. When light is absorbed, the added energy increases the temperature of matter. If you get into a car that has been sitting in the sun all day, the seats and other parts of the car’s interior may be almost too hot to touch, especially if they are black or very dark in color. That’s because dark colors absorb most of the sunlight that strikes them.
Q: In hot sunny climates, people often dress in light-colored clothes. Why is this a good idea?
A: Light-colored clothes absorb less light and reflect more light than dark-colored clothes, so they keep people cooler.
Classifying Matter in Terms of Light
Matter can be classified on the basis of its interactions with light. Matter may be transparent, translucent, or opaque. An example of each type of matter is pictured in the Figurebelow.
Transparent matter is matter that transmits light without scattering it. Examples of transparent matter include air, pure water, and clear glass. You can see clearly through transparent objects, such as the top panes of the window below, because just about all of the light that strikes them passes through to the other side.
Translucent matter is matter that transmits light but scatters the light as it passes through. Light passes through translucent objects but you cannot see clearly through them because the light is scattered in all directions. The frosted glass panes at the bottom of the windowabove are translucent.
Opaque matter is matter that does not let any light pass through it. Matter may be opaque because it absorbs light, reflects light, or does some combination of both.
Examples of opaque objects are objects made of wood, like the shutters in the Figurebelow. The shutters absorb most of the light that strikes them and reflect just a few wavelengths of visible light. The glass mirror below is also opaque. That’s because it reflects all of the light that strikes it.
Reflection of light occurs when light bounces back from a surface that it cannot pass through. If the surface is very smooth, the reflected light forms an image.
Transmission of light occurs when light passes through matter. As light is transmitted, it may pass straight through matter or it may be refracted or scattered by matter.
Absorption of light occurs when light transfers its energy to matter rather than being reflected or transmitted by matter. The temperature of matter increases with the added energy.
Matter can be classified as transparent, translucent, or opaque depending on how it interacts with light.
Describe three ways that light can interact with matter.
Transmitted light may be refracted or scattered. When does each process occur?
Why does matter increase in temperature when it absorbs light?
Compare and contrast transparent, translucent, and opaque matter.
The tiny object on this man’s finger is life-changing for him. It lets him see clearly without wearing glasses. You probably recognize the object as a contact lens. You may even wear contact lenses yourself.
What Is a Lens?
A lens is a transparent object with one or two curved surfaces. It is typically made of glass (or clear plastic in the case of a contact lens). A lens refracts, or bends, light and forms an image. An image is a copy of an objected formed by the refraction (or reflection) of visible light. The more curved the surface of a lens is, the more it refracts the light that passes through it. There are two basic types of lenses: concave and convex. The two types of lenses have different shapes, so they bend light and form images in different ways.
A concave lens is thicker at the edges than it is in the middle. You can see the shape of a concave lens in the Figurebelow. From the diagram, it’s clear that the lens causes rays of light to diverge, or spread apart, as they pass through it. Note that the image formed by a concave lens is on the same side of the lens as the object. It is also smaller than the object and right-side up. However, it isn’t a real image. It is a virtual image. Your brain “tricks” you into seeing an image there. The light rays actually pass through the glass to the other side and spread out in all directions. You can explore the formation of images by a concave lens at this URL:http://phet.colorado.edu/sims/geometric-optics/geometric-optics_en.html
A convex lens is thicker in the middle than at the edges. You can see the shape of a convex lens in the Figurebelow. A convex lens causes rays of light to converge, or meet, at a point called the focus (F). A convex lens forms either a real or virtual image. It depends on how close the object is to the lens relative to the focus. You can interact with an animated convex lens at this URL: http://www.phy.ntnu.edu.tw/ntnujava/index.php?topic=1395.msg5241#msg5241
Q: An example of a convex lens is a hand lens. Which of the three convex lens diagrams in theFigureabove shows how a hand lens makes an image?
A: You’ve probably looked through a hand lens before. If you have, then you know that the image it produces is right-side up. Therefore, the first diagram must show how a hand lens makes an image. It’s the only one that produces a right-side up image.
Try It Out! Convex Lenses
Be very careful not to aim the beam of light into your eyes, or anyone else’s!
Place a magnifying glass in front of a beam of light from a flashlight. Aim the beam towards a blank wall, then move the magnifying glass back and forth until you produce a small bright point of light on the wall. Why did this happen?
Now use the magnifying glass to look at the fine details of a postage stamp or some other very small picture. Place the magnifying glass between your eye and the stamp, but as close to your eye as you can and still see a clear image. How does the image of the stamp appear?
Next, slowly move the magnifying glass away from your face until you can no longer see an image through the glass. Where do you think your eyes are located relative to the glass? Hint: remember what you learned with mirrors.
Predict what you will see if you move the magnifying glass further away than in question 3. Test your prediction. What do you notice about the position of the image?
How are concave mirrors and convex lenses:
Complete the chart.
Position of object from lens
Size of image
Orientation of image
more than two focal lengths away
from one to two focal lengths away
less than one focal length away
The convex lens:
makes light rays converge to a focal point
produces no image when an object is at the focal point
orientation (erect or inverted) and size of image varies with the distance between the object and the lens
A lens is a transparent object, typically made of glass, with one or two curved surfaces. A lens refracts light and forms an image.
A concave lens is thicker at the edges than it is in the middle. This causes rays of light to diverge. The light forms a virtual image that is right-side up and smaller than the object.
A convex lens is thicker in the middle than at the edges. This causes rays of light to converge. The light forms a real or virtual image depending on the distance of the object from the lens.
What does this picture show? Is it a photo of identical twin sisters, or is it just one girl looking in a mirror? The picture shows a single girl and her mirror image.
How Mirrors Form Images
A mirror is typically made of glass with a shiny metal backing that reflects all the light that strikes it. When a mirror reflects light, it forms an image. An image is a copy of an object that is formed by reflection or refraction. Mirrors may have flat or curved surfaces. The shape of a mirror’s surface determines the type of image it forms. For example, some mirrors form real images, and other mirrors form virtual images. What’s the difference between real and virtual images?
A real image forms in front of a mirror where reflected light rays actually meet. It is a true image that could be projected on a screen.
A virtual image appears to be on the other side of the mirror. Of course, reflected rays don’t actually go through the mirror to the other side, so a virtual image doesn’t really exist. It just appears to exist to the human brain.
Q: Look back at the image of the girl pointing at her image in the mirror. Which type of image is it, real or virtual?
A: The image of the girl is a virtual image. It appears to be on the other side of the mirror from the girl.
The mirror in the opening photo is a plane mirror. This is the most common type of mirror. It has a flat reflective surface and forms only virtual images. The image formed by a plane mirror is also right-side up and life sized. But something is different about the image compared with the real object in front of the mirror. Left and right are reversed. Look at the girl brushing her teeth in theFigurebelow. She is using her left hand to brush her teeth, but her image (on the left) appears to be brushing her teeth with the right hand. All plane mirrors reverse left and right in this way. The term mirror image refers to how left and right are reversed in an image compared with the object.
Some mirrors have a curved rather than flat surface. Curved mirrors can be concave or convex. A concave mirror is shaped like the inside of a bowl. This type of mirror forms either real or virtual images, depending on where the object is placed relative to the focal point. The focal point is the point in front of the mirror where the reflected rays meet. You can see how concave mirrors form images in the Figurebelow and at the following URL. Concave mirrors are used behind car headlights. They focus the light and make it brighter. Concave mirrors are also used in some telescopes. http://www.splung.com/content/sid/4/page/concavemirrors
The other type of curved mirror, a convex mirror, is shaped like the outside of a bowl. Because of its shape, it can gather and reflect light from a wide area. As you can see in the Figurebelow, a convex mirror forms only virtual images that are right-side up and smaller than the actual object. You can see how a convex mirror forms an image in the animation at this URL:
Q:Convex mirrors are used as side mirrors on cars. You can see one in the Figurebelow. Why is a convex mirror good for this purpose?
A: Because it gathers light over a wide area, a convex mirror gives the driver a wider view of the area around the vehicle than a plane mirror would.
When a mirror reflects light, it forms an image. An image is a copy of an object formed by reflection (or refraction). A real image is a true image that forms in front of a mirror where reflected light rays actually meet. A virtual image appears to be on the other side of the mirror and doesn’t really exist.
Most mirrors are plane mirrors that have a flat reflective surface. A plane mirror forms only virtual, right-side up, and life-sized images.
A concave mirror is shaped like the inside of a bowl. The type of image it forms depends on where the object is relative to the focal point. The image may be real, upside down, and reduced in size; or it may be virtual, right-side up, and enlarged.
A convex mirror is shaped like the outside of a bowl. It forms only virtual images that are right-side up and reduced in size relative to the object.
A plane mirror forms an image where the reflected rays
The image formed by a plane mirror appears to be
on the same side of the mirror as the object.
on the opposite side of the mirror from the object.
either on the same side or the opposite side, depending on the distance of the object from the mirror.
on both sides of the mirror, regardless of the distance of the object from the mirror.
The distance from the object to a plane mirror equals the distance from the mirror to the
Anyone who sees the image formed by a plane mirror is sighting at the same image
What is an image? How do real and virtual images differ?
Define the focal point of a mirror.
Describe the image formed by a plane mirror.
What type of image is formed by a concave mirror if the object is between the mirror and the focal point?
Mirrors like the one in the Figurebelow are sometimes placed at street intersections so drivers can see around blind corners. What type of mirror is used for this purpose? What type of image does it form?
4. Ways of Sensing
4.1. Near and Far Sightedness
The lens of the eye is flexible and soft. The muscles attached to it can change the shape of the lens to make it thinner for seeing things far away, or thicker for close-up viewing. Unfortunately some people have an eyeball that is too long or a cornea or lens that is too convex. Either way, they are able to see things up close without problem, but they can’t see things at a distance because the image forms in front of the retina and is blurry.
Because the person can see near objects, this condition is called nearsightedness, or myopia. Since nearsighted eyeballs focus too much, a lens that diverges light can improve vision. A concave lens causes the light rays to spread apart before they reach the lens in the eye, which forces the image back onto the retina so the person can see clearly.
In this condition, the eyeball is too short or the cornea and lens are not convex enough. Farsighted people can see distant objects but are unable to focus on close-up objects. Because they can see things at a distance, this condition is called farsightedness, or hyperopia. Images are formed behind the retina, not on it, and this causes close-up images to be blurry.
Since the light rays do not converge enough, a lens is needed to correct that. Convex lenses help hyperopic eyes to see clearly by moving the image forward from behind the retina to focus on the retina
When we think of blindness we often think of not being able to see anything at all. Total vision loss is only one kind of blindness, but most blind people can detect some light. For this reason the term blindness is not quite right. Low vision may be a better description for people who are able to detect light but whose vision is impaired.
Many things can cause blindness and low vision. Most common are birth defects, injuries, and disease.
When you focus on something, the area to either side of what you’re focusing on is called the periphery and being able to see that is called peripheral vision. In one type of low vision, a person can only see what is directly ahead of them but nothing to the side (no peripheral vision). If you get an empty paper towel roll, hold it up to your eye, and look through it, you can simulate a type of low vision called tunnel vision.
Another type of low vision is the exact opposite of this. A person may have peripheral vision, but be unable to see what’s directly ahead.
Diabetes can sometimes cause vision loss, and some diabetics may become legally blind. With this type of blindness, the retina changes its sensitivity to light, creating many blind spots or areas where the cones and rods do not work well.
Colour blindness is not a form of blindness. It is a deficiency in the way you see colour. A person with this vision problem may have difficulty distinguishing certain colours. This is a hereditary condition, which means it is genetic and a person is born with it.
Colour blindness is caused by a defect in the cone cells in the retina that causes the loss of colour detection. There is no treatment it. For some people, tinted lenses and contact lenses can be used to help with colour perception but normal colour vision is never achieved.
This is a non-permanent, painful condition caused by overexposure to the glare of sunlight reflecting off snow, causing the cornea to become inflamed.
The Inuit of northern Canada carved goggles from caribou antlers to prevent snow blindness. The antler would be curved to fit the shape of the face, with a groove cut out for the nose and eye-slits carved to reduce the amount of light that entered the eye. The goggles were held in place with caribou sinew.
Welder’s flash is a condition similar to snow blindness, caused by not wearing a proper helmet when welding.
4.3. Optical Instruments
This colorful burst of “spaghetti” is really a bundle of optical fibers. These are hair-thin threads of glass that transmit laser light that has been encoded with messages. Optical fibers are a crucial component of modern communications. The use of light in devices such as these is possible because of optics.
Optics and Optical Instruments
Optics is the study of visible light and the ways it can be used to extend human vision and do other tasks. Knowledge of light was needed for the invention of optical instruments such asmicroscopes, telescopes, and cameras, in addition to optical fibers. These instruments usemirrors and lenses to reflect and refract light and form images.
Q: What is an image?
A: An image is a copy of an object created by the reflection or refraction of visible light.
A light microscope is an instrument that uses lenses to make enlarged images of objects that are too small for the unaided eye to see. A common type of light microscope is a compound microscope, like the one shown in the Figurebelow. A compound microscope has at least two convex lenses: one or more objective lenses and one or more eyepiece lenses. The objective lenses are close to the object being viewed. They form an enlarged image of the object inside the microscope. The eyepiece lenses are close to the viewer’s eyes. They form an enlarged image of the first image. The magnifications of all the lenses are multiplied together to yield the overall magnification of the microscope. Some light microscopes can magnify objects more than 1000 times! For more on light microscopes and the images they create, watch the video at this URL:
Q: How has the microscope advanced scientific knowledge?
A: The microscope has revealed secrets of the natural world like no other single invention. The microscope let scientists see entire new worlds, leading to many discoveries—especially in biology and medicine—that could not have been made without it. Some examples include the discovery of cells and the identification of bacteria and other single-celled organisms. With the development of more powerful microscopes, viruses were discovered and even atoms finally became visible. These discoveries changed our ideas about the human body and the nature of life itself.
Like microscopes, telescopes use convex lenses to make enlarged images. However, telescopes make enlarged images of objects—such as distant stars—that only appear tiny because they are very far away. There are two basic types of telescopes: reflecting telescopes and refracting telescopes. The two types are compared in the Figurebelow. They differ in how they collect light, but both use convex lenses to form enlarged images.
A camera is an optical instrument that forms and records an image of an object. The image may be recorded on film or it may be detected by an electronic sensor that stores the image digitally. Regardless of how the image is recorded, all cameras form images in the same basic way, as shown in the Figurebelow.
Light passes through the lens at the front of the camera and enters the camera through an opening called the aperture.
As light passes through the lens, it forms a reduced real image. The image focuses on film (or a sensor) at the back of the camera. The lens may be moved back and forth to bring the image into focus.
The shutter controls the amount of light that actually strikes the film (or sensor). It stays open longer in dim light to let more light in.
Did you ever see a cat chase after a laser light, like the one in Figurebelow? A laser is a device that produces a very focused beam of visible light of just one wavelength and color. Waves of laser light are synchronized so the crests and troughs of the waves line up. The diagram inFigurebelow shows why a beam of laser light is so focused compared with ordinary light from a flashlight.
The following Figurebelow provides a closer look at the tube where laser light is created.Electrons in a material such as a ruby crystal are stimulated to radiate photons of light of onewavelength. At each end of the tube is a concave mirror. The photons of light reflect back and forth in the tube off these mirrors. This focuses the light. The mirror at one end of the tube is partly transparent. A constant stream of photons passes through the transparent part, forming the laser beam. You can see an animation showing how a laser works at this URL
Besides entertaining a cat, laser light has many other uses. One use is carrying communication signals in optical fibers. Sounds or pictures are encoded in pulses of laser light, which are then sent through an optical fiber. All of the light reflects off the inside of the fiber, so none of it escapes. As a result, the signal remains strong even over long distances. More than one signal can travel through an optical fiber at the same time, as you can see in the Figurebelow. Optical fibers are used to carry telephone, cable TV, and Internet signals.
The optical fiber in the diagram is much larger than a real optical fiber, which is only about as wide as a human hair.
Q: When lasers were invented in 1960, they were called "a solution looking for a problem.” Since then, they have been put to thousands of different uses. Can you name other ways that lasers are used?
A: The first widespread use of lasers was the supermarket barcode scanner, introduced in 1974. The compact disc (CD) player was the first laser-equipped device commonly used by consumers, starting in 1982. The CD player was quickly followed by the laser printer. Some other uses of lasers include bloodless surgery, cutting and welding of metals, guiding missiles, thermometers, laser light shows, and acne treatments.
Optics is the study of visible light and the ways it can be used to extend human vision and do other tasks. Optical instruments are based on optics. They use mirrors and lenses to reflect and refract light and form images.
The light microscope and telescope use convex lenses and mirrors to make enlarged images of very tiny or distant objects. A camera uses a convex lens to make a reduced image of an object.
A laser is a device that produces a very focused beam of visible light of just one wavelength and color. Pulses of laser light carry communication signals through optical fibers.
At the following URL, practice using a telescope with the telescope simulator. Select an object to view, and then try different combinations of aperture and eyepiece size. For each combination, adjust the focus until the image is sharp and clear. Make a data table to record 10 different combinations of aperture and eyepiece size and the resulting magnification. Draw one conclusion based on the data in your completed table.