Why does prism make a rainbow

White light, in particular, is a fairly uniform mixture of all visible wavelengths. Sunlight, considered to be white, actually appears to be a bit yellow because of its mixture of wavelengths, but it does contain all visible wavelengths. The sequence of colors in rainbows is the same sequence as the colors plotted versus wavelength in Figure 2. What this implies is that white light is spread out according to wavelength in a rainbow.

Dispersion is defined as the spreading of white light into its full spectrum of wavelengths. More technically, dispersion occurs whenever there is a process that changes the direction of light in a manner that depends on wavelength. Dispersion, as a general phenomenon, can occur for any type of wave and always involves wavelength-dependent processes.

Figure 2. Even though rainbows are associated with seven colors, the rainbow is a continuous distribution of colors according to wavelengths. Refraction is responsible for dispersion in rainbows and many other situations. The angle of refraction depends on the index of refraction, as we saw in The Law of Refraction.

We know that the index of refraction n depends on the medium. But for a given medium, n also depends on wavelength. See Table 1. Note that, for a given medium, n increases as wavelength decreases and is greatest for violet light. Thus violet light is bent more than red light, as shown for a prism in Figure 3b, and the light is dispersed into the same sequence of wavelengths as seen in Figure 1 and Figure 2. Figure 3. Since the index of refraction varies with wavelength, the angles of refraction vary with wavelength.

A sequence of red to violet is produced, because the index of refraction increases steadily with decreasing wavelength. Any type of wave can exhibit dispersion. Sound waves, all types of electromagnetic waves, and water waves can be dispersed according to wavelength. Dispersion occurs whenever the speed of propagation depends on wavelength, thus separating and spreading out various wavelengths. Dispersion may require special circumstances and can result in spectacular displays such as in the production of a rainbow.

This is also true for sound, since all frequencies ordinarily travel at the same speed. If you listen to sound through a long tube, such as a vacuum cleaner hose, you can easily hear it is dispersed by interaction with the tube. Dispersion, in fact, can reveal a great deal about what the wave has encountered that disperses its wavelengths.

The dispersion of electromagnetic radiation from outer space, for example, has revealed much about what exists between the stars—the so-called empty space. This is called refraction. However, because of differences of refractive index, this refraction angle varies for each color or according to the wavelength of the light. This change of the angle of refraction, or refractive index, in accordance with the wavelength of light is called dispersion.

In conventional media, the shorter the wavelength or the bluer the light , the larger the refractive index. The angle of refraction depends on the speed at which light travels through a medium. People have noticed the phenomenon of refraction throughout history. But the first to discover the law of refraction was Willebrord Snell , a Dutch mathematician. The refractive index of water to the orange sodium-vapor light emitted by streetlamps on highways is 1.

The refractive index of water to violet, which has a short wavelength, is nearly 1. To red light, which has a long wavelength, the refractive index of water is almost 1. Sunlight hitting a water droplet sphere in the atmosphere will be refracted on the surface of the droplet, and enters the droplet. When the refraction process occurs, the light breaks up into seven colors inside the water droplet, and is next reflected at the other surface of the droplet after traveling inside it.

Note that in reflection the angle of reflection is the same as the angle of incidence, which means that reflected light travels in a predetermined path while maintaining the difference of angle of refraction. The light is refracted again when it exits the droplet, further emphasizing the dispersion. Students often benefit from scientific demonstrations because the visual evidence gives them another mode for remembering key concepts.

This works especially well for intangible concepts like light and light travel. You can explain to students that light is actually made up of a spectrum of colors and talk about how rainbows form and then cement the information with a demonstration.

The simplest light demonstrations involve prisms. Prisms are long, clear, triangular crystals usually made of quartz that split the light spectrum into different colors when used properly.

Tack your white paper or canvas to a wall with thumb tacks. Make sure the paper or canvas is flat and smooth so it can capture the rainbow perfectly. You can set it up across the room from a sunny window or use a flashlight if windows are unavailable. Hold your prism up in front of the paper or canvas, making sure to catch the light from the window.