Question

What do you mean by interference of light? Discuss necessary conditions for interference.

Answer 1

Hint: In this question use the concept that interference basically refers to the superimposition of two waves that are produced by the two or more coherent sources. Figure out the basics of additive interference and subtractive interference. This will help approaching the problem.

Complete step-by-step answer:
Interference of light
Interference of light is a special phenomenon that is produced by two or more same coherent sources and same frequency not necessarily the same amplitude.
There are two types of interference of light
$\left( i \right)$ Additive interference – In this the resultant wave has higher amplitude but the frequency remains same otherwise interference is not possible and also produced by same coherent sources.
For example: 2 sin (wt) and 3 sin (wt)
Here amplitudes of two signals are 2 and 3 respectively but the frequency is the same i.e. w.
Now the resultant signal due to interference of these two signals produces 5 sin (wt).
So the amplitude of the resultant signals goes up but the frequency remains the same so this is called additive interference.
$\left( {ii} \right)$ Subtractive or destructive interference – In this the resultant wave has smaller amplitude but the frequency remains same otherwise interference is not possible and also produced by same coherent sources.
For example: -2 sin (wt) and 3 sin (wt)
Here amplitudes of two signals are -2 and 3 respectively but the frequency is the same i.e. w.
Now the resultant signal due to interference of these two signals produces 1 sin (wt).
So the amplitude of the resultant signals goes down but the frequency remains the same so this is called as subtractive or destructive interference.

Necessary conditions for the interference
$\left( i \right)$ Signals produced by same coherent sources.
$\left( {ii} \right)$ There should be no phase difference in the signals as well as no time lapse in the signals.
$\left( {iii} \right)$ Frequencies of the signals always remain the same which is going to interfere.
So this is the required answer.


Note – Interference is simply observed in all types of electromagnetic waves like radio, light waves, acoustic waves and even gravity waves. The important fact about interference is that after interference the resultant waves thus formed the intensity of the resultant wave is dependent both upon the individual intensities of the waves along with the phase difference between them.