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Refraction of Light and SnellÂ’s Law

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Reflection and Refraction

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Refraction of Light and Snell’s Law


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Students examine the refraction of light when it travels through different mediums.

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Now You Know

After completing this tutorial, you will be able to complete the following:

  • Explain why light refracts and relate the properties of media to the speed of light.
  • Describe how the angle of refraction of light can be found using Snell’s law.
  • Observe that light cannot always pass from one medium to another, and describe the conditions for total internal reflection.

Everything You'll Have Covered

One of the fundamental properties of optical media is refraction, the apparent "bending" of light when it passes from one medium to another. Refraction occurs because light travels at different speeds through different media. Although the velocity of light in a vacuum is a physical constant (c), and the fastest speed at which anything can travel, light travels slower through air, water, and other substances. When light passes from one medium to another, refraction occurs because the speed of light changes.

Every medium has an index of refraction. When light crosses the boundary from one medium to another, it refracts toward or away from the normal. Willebrord Snellius, working in the seventeenth century, is best known for determining the relationship between the refraction indices of different media, the angle of incidence of the incident light ray, and the resulting angle of refraction. Snell's law states that the ratio between the indices of refraction of the internal (first) medium and the external (second) medium is equal to the ratio between the sine of the angle of refraction and the sine of the angle of incidence.

When light travels from a medium with a larger refractive index to one with a smaller refractive index, its speed increases and it refracts away from the normal. Objects appear closer to the observer, such as when objects in water appear closer to someone outside the water. In contrast, when light travels from a medium with a smaller refraction index to one with a larger refraction index, it slows and refracts closer to the normal and appears farther from the observer. In some situations, the angle of incidence is such that the angle of refraction would be greater than 90° and the light never passes into the second medium. For this total internal reflection to occur, the critical angle of incidence must be exceeded and the external medium must have a lower index of refraction than the internal medium. This phenomenon is essential to fiber optic technology.

Tutorial Details

Approximate Time 20 Minutes
Pre-requisite Concepts Students are familiar with reflection of light and properties of waves. They are familiar with ratios and trigonometric functions.
Course Physics
Type of Tutorial Concept Development
Key Vocabulary angle of incidence, angle of refraction, critical angle