How Fast Does Light Travel in Water vs. Air? Refraction Experiment (page 2)

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Author: Erin Bjornsson
Topics: Tenth Grade, Physics


Light will have the fastest velocity when it travels through the air. Light will have the slowest velocity when it travels through gelatin.


Light slows down when passing through different transparent materials. The more it slows down, the more it bends when it hits a medium made of that material. Snell’s Law of Refraction shows the relationship between incidence and refraction angles and the phase velocities of the materials involved. For this experiment, your laser beam traveled through an air phase before hitting the phase of whatever solid you chose. Snell’s law states that the ratio of the sine of the incidence to the refraction angles, θ, is equal to the ratio of the phase velocities, v.

Snell's Law

Another variation on Snell’s law includes the index of refraction, n. The previously stated Snell’s law is equal to the reciprocal of the ratio of the indices of refraction.

Snell's Law 2

The index of refraction is a dimensionless number, or a number without any units. Dimensionless numbers are used to be able to compare two different objects on the same parameters. The index of refraction describe how light travels through a medium.

Snell's Law

Where c is the speed of light in a vacuum (2.99 x 108 m/s) and v is the speed of light in the medium you are measuring in m/s.

Going Further

Try adding salt or sugar to the water in the container and perform the experiment again. What happens? Is the velocity different when you dissolve solids in the liquid? You can also try measuring other see-through liquids like clear soda or liquid soap. You can also try using different shaped objects like prisms to see how light is refracted differently.

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