What is a Prism? - Definition & Refraction

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  • 0:00 What Is a Prism?
  • 0:44 Definition of Refraction
  • 1:12 Index of Refraction
  • 2:24 Snell's Law
  • 3:22 Properties of Waves
  • 4:58 Lesson Summary
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Lesson Transcript
Instructor: Richard Cardenas

Richard Cardenas has taught Physics for 15 years. He has a Ph.D. in Physics with a focus on Biological Physics.

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. 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.

A prism

Definition of Refraction

Have you ever noticed that people standing in a pool always look shorter than they really are or that a spoon in a glass of water seems to be bent? The reason why these phenomena occur is because of the concept of refraction, which is simply the bending of a light wave. In order to further understand what refraction is and why it occurs, we need to look at what happens to light when it goes from one medium to another (like from air to water or a vacuum to air).

Index of Refraction

The speed of light in a vacuum is denoted by the letter c and has a value of 300,000,000 meters per second (m/s). When light goes from a vacuum to another medium, light slows down. How much it slows down is dependent on something called the index of refraction, which is denoted by the letter n. The index of refraction is a property of a material (or medium). When light goes into a denser medium, it slows down more. The denser the material, the slower light will travel through that medium. The speed of light in a material, denoted by v, changes according to the following formula:

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 2.42

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).

Graph of light refraction

When light goes from a medium with a smaller index of refraction to one with a larger index of refraction, the angle gets smaller (it bends toward the normal). An example would be when light goes from air to water, and another example would be from air to glass. When light goes from a medium with a larger index of refraction to a medium with a smaller index of refraction, like from water to air, the angle gets larger (or bends away from the normal).

Refraction explains why light bends in a prism, but does it also explain why the colors of the rainbow emerge from a prism? Yes, it does. But in order to explain why the colors emerge, we need to look at two more properties of light waves.

Properties of Waves


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.

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