It is important to distinguish between the refractive index of a medium and the refractive index between two media. The refractive index of a medium is always greater than or equal to 1.
The refractive index between two media may be greater or less than 1 depending on the light ray’s route:
• When light travels from a rarer medium into a denser medium, the refractive index between media is greater than or equal to 1.
• When light travels from a denser medium into a rarer medium, the refractive index between media is less than or equal to 1.
If given the refractive index for light travelling from medium A into medium B, you can calculate the refractive index for light travelling from medium B into medium A by applying the reciprocal of the answer as shown below:
Refractive index for light travelling between media AnB = 1
______ BnA
QUESTIONS AND ANSWERS
1. A ray of light travels from air to glass and has an angle of incidence of 40° and angle of refraction of 25°. Calculate the refractive index of the glass.
Solution
n = sin i _____ sin r = sin 40° _________
sin 25° = 1.52
2. If the refractive index between air and diamond is 2.4, what is the refractive index between diamond and air?
Solution
When you reverse the direction of the path of light between two media, you can use the reciprocal of the refractive index:
So: air – diamond = 2.4 – diamond – air = 1 ÷ 2.4 = 0.417
You may now complete Exercise 14A (page 151).
You may now proceed with the mandatory experiment: Verification of Snell’s law of refraction (page 428).
LEAVING CERTIFICATE PHYSICS 143
3. If a ray of light entering glass of refractive index 1.5 has an angle of refraction of 30°. Calculate the corresponding angle of incidence that causes this refraction.
Solution
n = sin i ______ sin r
1.5 = sin i _________ sin 30°
1.5(sin 30°) = sin i 0.75 = sin i
sin–1(0.75) = i 48.59° = i
The refractive index of a medium,
n, is taken to be sin i _____ sin r
when the ray of light travels from a vacuum into the medium.
the value of sin i ______ sin r
The refractive index between two media, AnB, is
when the ray of light travels from medium A into medium B.
