• Background
• Instructions
• Illustration
• Quiz

## Background

Vision is the system that allows us to perceive light, so at the most basic level, the visual system is a light-detection system. Thus, before we begin describing the visual system, it is important to know just a little bit about the physics of light. Visible light is an example of electromagnetic energy (Feynman, 1985, 1995). Visible light is the same basic phenomenon as infrared radiation, ultraviolet radiation, gamma rays, and even the electricity that flows in our walls or the magnetic fields that guide our compasses. According to modern physics, light is made up of particles called photons that also behave in a wavelike manner (see Feynman, 1985, for a more thorough explanation of our current understanding of light in layman’s terms). The wavelike behavior allows us to describe the behavior of light using many concepts that are familiar, so it will be emphasized.

The beauty of general theories is that if the theory applies to two different phenomena, you can use one to understand the other. They will behave the same way. So, waves before they reach the beach are waves and light can be described as waves. To the extent both are waves, they behave the same way. So, consider waves at the beach before they break, that is, collapse on the sand. The waves are up-and-down undulations of the water surface. As a result, waves are usually drawn as going up and down. If you watch the waves coming in, you can tell that not all waves are the same. Some are taller than others, and some peaks are closer together, and other peaks are farther apart. Surfers look for “sets” of waves that come in with higher peaks, though also often closer together. We can quantify differences among waves, both in terms of how high they go (amplitude or intensity) and how close they are to adjacent waves (wavelength). Thus, the distance between the peaks of waves is called the wavelength and the height of the wave is called its intensity. In perceptual terms, wavelength correlates with color, and intensity correlates with brightness.

## Instructions

### Full Screen Mode

To see the illustration in full screen, which is recommended, press the Full Screen button, which appears at the top of the page.

### Settings

Below is a list of the ways that you can alter the illustration. The settings include the following:

You can display up to two different waves on this illustration. Each wave can be handled in the same way.
Wave #: check to display. Uncheck to not display. The # refers to the particular wave to examine.
Amplitude or Intensity: change the height of the wave; as the wave gets taller, the wave gets more intense—think of ripples (small amplitude) hitting you versus a tsunami (large amplitude).
Wavelength: the distance between successive peaks.
Show Intensity: If checked, draw a vertical line from the 0 value to the first peak. The height of this line is the intensity of that wave.
Show Wavelength: If checked, draw a line connecting the first two peaks of the wave. The longer the line, the greater the wavelength.

### Reset

Pressing this button restores the settings to their default values.