Hi Farhaini,

## SPM Physics Light & Optics

- 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

**Answer: D**

Thank you very much!

Is it wrong if I use this formula?

n = sin r

--------

sin I

n= sin 70

-----------

sin 40

n=1.46

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

It’s okay. Thank you so much.