Electromagnetic radiation (e.g., radio, microwaves, light) can be modeled as a wave of changing electric and magnetic fields or as particles called photons. For some situations, one model is more useful than the other.
Study the tabs to learn about some of the features of light and to see some examples of situations that can BEST be described by the wave model of light.
Refraction is the bending of a wave as it enters a new medium at an angle. The medium is the material that the wave travels through.
A prism is a triangular-shaped piece of glass or plastic. In this image, you can see that light is going from air into glass. The white light that reaches the prism contains all the colors of the visible spectrum even though you can't see them.
As the white light passes through the edge or boundary of the prism, the light waves refract. Each of the different wavelengths contained in the white light refracts differently and results in a different color. The white light is broken up by the prism into all the colors of the rainbow. The longest wavelength bends (refracts) the least and is red. The shortest wavelength bends the most and is violet.
The effect of the prism on the white light is to spread out the different wavelengths so that we can see them, each a different color. The spreading out of colors from the refraction of light is called dispersion. When white light passes through a prism, each color of the visible spectrum can be seen on the other side.
Interference is the interaction of two or more waves that combine in a region of overlap. Interference happens because two waves can occupy the same space at the same time.
Imagine you are walking down the street and bump into someone. What happens? You bounce off that person--perhaps even falling down. That's because two pieces of matter cannot occupy the same space at the same time. But what happens when two waves meet at the same place at the same time? Since waves carry energy and not matter, they can occupy the same space. Each wave either increases, decreases, or is neutralized (canceled out). These effects are called wave interference.
If the crest of one wave overlaps the crest of another (or two troughs overlap), they add together and result in a wave of increased amplitude. This means it is a bigger wave. This is called constructive interference.
If the crest of one wave overlaps the trough of another wave that has exactly the same amplitude, they will cancel each other out. If the crest of one wave overlaps the trough of another wave that has a different amplitude, the resulting wave will have a reduced amplitude (the wave will shrink). These are examples of destructive interference.
Diffraction is the bending of a wave as it moves around an obstacle or passes through a narrow opening.
The diffraction of light was shown in 1801 by an English physicist named Thomas Young. When light goes through the narrow openings in the obstacle, it spreads out on the other side of the obstacle. This spreading is called diffraction. The waves that went between the two slits start to interfere with each other. Interference happens because two waves can occupy the same space at the same time. They either add to each other or one subtracts from the other, which leads to the light and dark lines on the screen.
Particles cannot produce this pattern because two particles cannot be in the same place at the same time as waves can.
Polarized light is light with waves that vibrate in only one plane.
Recall that electromagnetic waves are produced by oscillating electric and magnetic fields that are at right angles to each other. These vibrations can be in more than one plane, in which case the light is called unpolarized light. This light can be turned into polarized light by using a polarizing filter. This filter blocks all the planes of the light except for one and it lets that part of the light through. This picture shows a polarizer that has slits in the vertical direction, so it lets the waves that are in the vertical direction through.
The lenses on the 3D glasses allow only one direction of oscillation to pass through. The lenses each let one of these polarizations through so the eyes each see a different picture, which the brain puts together into the 3D image.
Let's Practice
Match each situation on the left to the feature of light that causes it.
Reflection
How does the diffraction of light provide evidence for the wave nature of light?
