A beam of laser light is directed towards a narrow slit. After emerging from the other side of the slit, the light then falls on a screen.

Question 18

A beam of laser light is directed towards a narrow slit.

After emerging from the other side of the slit, the light then falls on a screen.

What is the pattern of light seen on the screen?

Reference: Past Exam Paper – November 2017 Paper 13 Q29

Solution:

Answer: B.

Diffraction occurs as the light enters the slit. [A and D are incorrect]

Consider the slit carefully.

The slit spreads the light horizontally.

The slit is narrow in the horizontal direction. So, the slit would spread the light horizontally, and not vertically. [C incorrect]

 

The grating is wide in the vertical direction so very little diffraction occurs in this direction. 

Remember that for diffraction to be significant, the width of the slit should be of the same order as the wavelength of the wave (light).