Energy can be transferred as both a particle and a wave.

2. Light Properties

Properties
  • Acts like both a wave and a particle
  • Wavelength, amplitude, frequency
Properties of Light

An ocean wave

What are waves?

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.

Picture of the electromagnetic spectrum

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 Figure above 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.

Beam of white light broken up into a rainbow by a prism

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]

Summary

  • Electromagnetic radiation is a form of energy.
  • Visible light has wavelengths from 400-700 nm.
  • The speed of light in air is 3.00 × 108 m/s.

Practice


Review

Questions

  1. What did Rutherford’s nuclear atomic model not explain?
  2. Prior to 1900, what did scientists believe about the nature of light?
  3. What is visible light?
  4. What is the range of wavelengths for visible light?

Image Attributions

  1. [1]^ Credit: Jon Sullivan / pdphoto.org; Source:http://commons.wikimedia.org/wiki/File:Beach_3_bg_121402-1-.jpg; License: CC BY-NC 3.0
  2. [2]^ Credit: CK-12 Foundation - Zachary Wilson; License: CC BY-NC 3.0
  3. [3]^ Credit: CK-12 Foundation - Christopher Auyeung; License: CC BY-NC 3.0
Wave-Particle Theory

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.  

The Question

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?

The Debate

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 Figure below. The theory posits that waves of photons traveling through space or matter make up electromagnetic radiation.

Diagram illustrating how photons are created

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 Figure below. 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.

Double slit experiment

Summary

  • 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.

Explore More

Watch the animation “Let There Be Light” at the following URL. Then create a timeline of ideas and discoveries about the nature of light.http://www.abc.net.au/science/explore/einstein/lightstory.htm

Review

  1. Why did scientists debate the nature of electromagnetic radiation for more than 200 years?
  2. State Einstein’s wave-particle theory of electromagnetic radiation.
  3. What is a photon?
  4. After Einstein proposed his wave-particle theory, how did double-slit experiments provide evidence to support the theory?