Light in a vacuum always travels at a speed c = 2.99 x 10⁸ m/s. When any wave is incident on a boundary, like when light travels from the air onto water, some of the wave is reflected and some of it is transmitted. Experiment will show that light appears to slow down while traveling through the water as opposed to the air. This is called the effective speed ($v_{eff}$) of the light in the medium. The Index of Refraction (n) is a measure of the effective speed of light in a medium.

$n=\frac{c}{v_{eff}}$

Notice, that since nothing can travel faster than the speed of light, that the index of refraction is always greater than or equal to 1. This slowing down of light also causes the wavelength to change.

This is because the constant for a wave across a boundary is the frequency. with $v=f \lambda$, if the frequency is constant and the speed decreases, the wavelength must also decrease. You'll also notice the direction the light travels also changes. The index of refraction can be used to determine how much light bends when traveling from one medium to the next. See Snell's Law of Refraction for more about the bending of light.

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Index of Refraction (1min)



Index of Refraction (1min)

Section 25.3 of the Openstax text book introduces the Law of Refraction. 

The people at the Physics Classroom discuss Refraction in a more indepth way. Read this section to better understand the physics.

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Here is a good video on index of refraction,

This is an excellent video on the refractive index,

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(1) Light with a wavelength $\lambda = 88.7 MHz $ is broadcast through a medium with an index of refraction $ n = 3$. What is the wavespeed of the broadcast?

(2) An electromagnetic wave traveling through a medium with index of refract $n= 8$ has a wavelength $\lambda = 1.8 nm$. What is its wavelength in vacuum?

(3) You are floating in a pool. Your friend thinks he is such a fast swimmer that he can beat light in a race to the other end of the pool. (your friend is not smart). The pool is 100 m long and you are at one end of it. You have a red laser pointer ($\lambda_{vac} = 700 nm$). (a) If you shine the laser pointer, underwater, long will it take for light to travel down the length of the pool? Your friend swims at a rate of 5 m/s. (b) Who will win the race? Water has an index of refraction $n_w = 1.33$.  

Solutions found HERE

Short foundation building questions, often used as clicker questions, can be found in the clicker questions repository for this subject.

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