# [SPM Physics Trial Penang 2019 P1Q27 Solution] How to answer this question?

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### SPM Physics Trial 2019 Penang P1 Q27

1. Diagram 21 shows a refraction of light ray \mathrm{R} when hitting point \mathrm{O} after entering glass prism. M is an angle from refracted light ray to normal line.
Rajah 21 menunjukkan pembiasan alur cahaya R apabila menghentam titik O selepas memasuki prisma kaca. M ialah sudut dari sinar biasan ke garis normal.

Diagram 21
Rajah 21
What happen to angle M when the incident angle, \theta is increased ? Apakah yang berlaku kepada sudut M apabila sudut tuju, \theta ditingkatkan ?
A. Increases.
Bertambah.
B. Unchanged.
Tidak berubah.
C. Decreases.
Berkurang.

### SPM Physics Trial 2019 Penang P1 Q27 Solution

Hi Farhaini,

First we need to identify the relevant Physics concept related to this question. Notice the keywords like “refracted light ray”, “prism”, “normal line”, we know that it is related to the concept of total internal reflection.

In diagram below, when we shine a light ray at a small incident angle (red line), the light ray will undergo normal refraction. As we gradually increases the incident angle (purple line), the light ray will be refracted at same line at the wall of the prism. This is known as the critical angle. If we increases the light ray incident angle to be larger than critical angle (green line), then total internal reflection will occur.