# Homework Help: What is the refractive index?

1. Jan 20, 2016

### kent davidge

1. The problem statement, all variables and given/known data

Old photographic plates were made of glass with a lightsensitive emulsion on the front surface. This emulsion was somewhat transparent. When a bright point source is focused on the front of the plate, the developed photograph will show a halo around the image of the spot. If the glass plate is 3.10 mm thick and the halos have an inner radius of 5.34 mm, what is the index of refraction of the glass? (Hint: Light from the spot on the front surface is scattered in all directions by the emulsion. Some of it is then totally reﬂected at the back surface of the plate and returns to the front surface.)

2. Relevant equations

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3. The attempt at a solution

Is my sketch for the situation correct? I found n = 1.532 and the book answer is also 1.53.
Since the reflected ray is tangent to the upper end of halo, I assumed the ray reflecting half-way in the back surface. Is it correct?

2. Jan 21, 2016

### Rx7man

I think that's right.. the angle of incidence is always equal to the angle of reflection.