Question

A small circular disc is placed in the path of monochromatic light. The centre of the geometrical shadow is always
A. bright
B. dark
C. coloured
D. partly bright and partly dark

Answer 1

Hint: Monochromatic light is also known as one-color. When a disc is placed in the path of monochromatic light, it undergoes diffraction. Diffraction occurs from every corner of the disc and the light converges to form a constructive interference. Find the nature of constructively interfere light and that will be our answer.

Complete step by step answer:
We are given to find the nature of the centre of geometrical shadow formed when a small circular disc is placed in the path of monochromatic light.
We know that monochromatic light is a light where its optical spectrum has only a single frequency and a single wavelength.
So when a disc is placed in the path of a one-color light, light waves from every edge of the circular obstacle converge at the centre of the shadow or constructively interfere at the centre of its shadow.
This happens because when a light wave encounters an obstacle, light waves happen to bend around the corners of the obstacle into the geometrical shadow region of the obstacle. This phenomenon is called diffraction.
So here the obstacle is circular, so waves diffracted from the corners of the disc converge at the centre of the geometrical shadow region of the disc forming a constructive interference. The nature of light with constructive interference is bright.

Hence, the correct option is Option A, the centre of the geometrical shadow is always bright.

Note: Do not confuse bright light with dark light because bright light will be resulted due to constructive interference and dark light is resulted due to destructive interference. Constructive interference occurs when the phase difference is an even multiple of π between the waves and destructive interference occurs when the phase difference is an odd multiple of π between the waves.