Physics Review Question

Unit: Waves Year: 1996 Question#: 54
Question: Light (wavelength= 5.9 x 10-7 meters) travels through a solution. If the absolute index of refraction of the solution is increased, the critical angle will

(1 ) decrease
(2 ) increase
(3 ) remain the same


What is this question really asking?




Explanation by: Rachel M
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Answer 1
decrease

CORRECT
n= c / v n= absolute index of refraction
c= speed of light in a vacuum
v= speed of light in the medium

As n increases, v decreases because c is constant. If the speed in the medium decreases, the medium must be denser. The critical angle of a light ray decreases in denser mediums because the rays bend toward the normal as the medium gets denser and slower. The critical angle is what the ray makes with the normal when the refracted ray is at a right angle to the normal. return to top























Answer 2
increase

INCORRECT

The critical angle will increase in a faster, less dense, medium. But since the angle of refraction is increasing, this means the medium is becoming more dense, decreasing the critical angle. The reason we know the medium is becoming more dense is because n=c / v (n is the index of absolute refraction, v is the speed of light in the medium, c is the speed of light in a vacuum). If n is increasing, then v must be decreasing since c is constant. If the speed in that medium (v) decreases, it must be denser. Thus, the critical angle must be smaller as the light ray bends towards the normal. return to top



























Answer 3
remain the same

INCORRECT

The angle will not stay the same because as the density of a medium increases, the ray bends more towards the normal, changing all the angles involved. We know the density is increasing because n=c / v (n is the index of absolute refraction, v is the speed of light in the medium, c is the speed of light in a vacuum). If n is increasing, then v must be decreasing since c is constant. If the speed in that medium (v) decreases, it must be denser. Thus, the critical angle must be smaller as the light ray bends towards the normal. return to top



















































What's this question really asking?

How are the absolute index of refraction and the critical angle related? What is a critical angle? How does it increase and decrease in size? return to top

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