A thick slab of plastic lies at the bottom of a tank filled with water having an

Question

A thick slab of plastic lies at the bottom of a tank filled with water having an index of refraction of 1.33. A narrow laser beam is shone directly downward through the water on the plastic. You then gradually rotate the laser, thereby increasing the angle with respect to the normal that it strikes the plastic. You carefully note that at the instant the beam makes an angle of 66.5 degree with the normal in the water, no light the plastic, but it did enter for angles smaller than that. what is the speed of light in the plastic slab (c = 3.00 * 10^8 m/s)

Explanation / Answer

when theta_i = 66.5 degrees, theta_r = 90 degrees

n1 = 1.33

n2 = ? (for plastic)

apply Snell's law

sin(theta_i)/sin(theta_r) = n2/n1

sin(66.5)/sin(90) = n2/1.33

==> n2 = 1.33*sin(66.5)

= 1.22 <<<<<<<----------------Answer

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