# [SPM Physics Light & Optics] How to solve this question?

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Hi Farhaini,

### SPM Physics Light & Optics

1. Figure 10 shows a ray is refracted through a prism.

Determine the refractive index of prism that is shown in below.
A 0.68
B 0.82
C 1.23
D 1.46

### SPM Physics Light & Optics Solution

This question is related to Snell’s Law:

Img source: Wikipedia Oleg Alexandrov

Formula

From figure above, we can find the incident angle by recognising the “c” pattern of 2 parallel lines. The x + y inside “c” must be equal to 180 degree.

Applying the formula,

Refractive index = sin\theta_1 / sin\theta_2

where \theta_1 is angle from less dense medium (air), \theta_2 is angle from denser medium glass prism.

Refractive index of prism = sin(70) / sin(40) = 1.46

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Thank you very much!

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Is it wrong if I use this formula?
n = sin r
--------
sin I

n= sin 70
-----------
sin 40

n=1.46

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You are correct, I made a mistake, sorry

Since we are finding refractive index of prism, the angle on top of fraction should be angle from the less dense medium, which is air, so should be sin(70) / sin(40).

Refractive index of glass prism normally should be greater than 1, because it is denser than air.

I have edited my solution to reflect this changes

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It’s okay. Thank you so much.