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In this lesson, you’ll learn about the prism and what prisms do to white light. You’ll explore the properties of light waves, as well as the concept of refraction, which causes white light to bend in a prism and spread out into the different spectrum of colors.

What is a Prism?

Have you ever wondered why there are rainbows? Did you ever see a prism work and wonder why a prism seems to create rainbows? Well, in this lesson you’ll learn about prisms, how they work, why they work, and what applications they may have to everyday phenomena.A prism is an object made up of a transparent material, like glass or plastic, that has at least two flat surfaces that form an acute angle (less than 90 degrees). White light is comprised of all the colors of the rainbow.

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When white light is passed through a prism, the colors of the rainbow emerge from the prism much like in the figure here. We’ll learn more about why a prism spreads white light out into the colors of the rainbow.

Speed of Light

The table here is a sampling of different materials and their corresponding index of refraction:

Medium Index of Refraction
Vacuum 1.00
Air 1.0003
Water 1.33
Flint Glass 1.66
Diamond 2.


Based on the index of refraction, the speed of light is fastest in a vacuum and slowest in diamond. The reason why light slows down is because it bends as it goes from one medium to another. The figure here illustrates an incident light beam entering water (red light ray) and refracting as it goes from air to water (blue light ray).

Refraction while changing mediums

Snell’s Law

The law that governs the bending of light as it goes from one medium to another is called Snell’s Law and is given by the equation here:


All of the angles in Snell’s Law are measured from the normal (the imaginary dashed vertical line shown here).


Every wave has two important properties, a wavelength and a frequency. The wavelength is the distance over which the shape of a wave repeats. The figure here illustrates the concept of a wavelength.


The frequency of a wave is defined as the number of oscillations, or complete wavelengths, per second. The speed of any wave, including light waves, can be written as a product of wavelength and frequency.

When a light wave goes from one medium to another, the frequency of the wave remains unchanged. However, the wavelength changes, causing the wave to change speed as it moves from one medium to another. The table here shows the wavelengths of visible light:

Color Wavelength
Violet 380-450 nm
Blue 450-495 nm
Green 495-570 nm
Yellow 570-590 nm
Orange 590-620 nm
Red 620-750 nm

Since white light is made up of all of the colors listed in the table, when white light enters a prism and the light bends, the white light spreads out into the different colors of the rainbow. This process is called dispersion.

The larger wavelengths have a greater speed (red and orange), and the smaller wavelengths have a lower speed (blue and violet). Since they have different speeds through the prism, they bend by different amounts as they exit the prism.The most spectacular example of dispersion is the rainbow. There are two major things that need to be present for a rainbow to happen: drops of rain and sunlight. The drops of rain act like tiny little prisms, and when the white light from the sun encounters the rain droplets, the white light spreads into the colors of the rainbow. There are many little droplets in the sky, causing the effect to be magnified into what we see as the rainbow.

Lesson Summary

White light is the combination of all of the colors of the visible spectrum of light (which have different wavelengths). A prism is a clear, triangular device made up of plastic or glass (or any transparent material). When white light goes through a prism, the light bends due to the phenomenon called refraction. Refraction is the process of bending light as light goes from one medium (like air) to another medium (like water or glass).The bending happens because light changes speed as it goes from one medium to another. Waves have a wavelength (the distance over which the shape of a wave repeats) and frequency (the number of oscillations or complete wavelengths per second). And when light refracts, the frequency remains constant but the wavelength changes.

Since white light is made up of a variety of different wavelengths, the light is dispersed through the prism, which separates the light into the different wavelengths (or colors). So, when white light passes through a prism, a rainbow emerges on the other side.

Key Points

White light enters a prism and emerges as a rainbow
A prism
  • White light is light that includes all the colors of the spectrum
  • Refraction is the process of bending white light as it travels between mediums due to the change in speed
  • Prisms function by refracting white light, which separates the colors, causing a rainbow

Learning Outcomes

When you are finished with this lesson, you should be able to:

  • Explain what a prism is and how it refracts white light
  • Recall Snell’s Law and how it is used
  • Recite why light refracts when it moves between two mediums

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