Interference and Young's Double Slit Experiment

Whatever we see around is a combined effect of many factors

For example, on a falling raindrop; there's a gravitational force, a drag force, etc.

When more than one stimuli are acting on a body, the resultant response is a vector sum of all stimuli

This is known as superposition

Similarly, when two light waves super-impose on each other, they produce a resultant wave

This superposition is known as Interference of Light Waves

This interference is essentially of two types

The first is Constructive Interference

When the crest of one wave falls on the crest of another wave, then the amplitude adds up

Conversely, the other is Destructive Interference

Now, when the crest of one wave falls on the trough of another wave, then the amplitude here is minimum

A classic example that showcases Interference of Light Waves is Young's Double Slit Experiment

In YDSE, we place a barrier in front of a source of light

In that barrier, we make two slits. The two slits should be placed very close to each other

Lastly, we place a screen at a large distance from the source

When the source is turned on, we observe a pattern on the screen

We see the formation of alternate dark and bright bands of light called fringes

However, we need to keep certain conditions in mind

Firstly, instead of using two sources we use one source and two slits

We do this to get coherent sources of light. Any change in the source will be equally reflected in the light waves coming out from the slit

Two sources are said to be coherent when they have same the frequency and maintain a constant phase difference throughout

Secondly, the light source used must be monochromatic (comprising one single wavelength)

The two slits should be very small (comparable with the wavelength of light used) and must be very close to each other

Lastly, the distance between the source and the screen should be large

If we carefully look at the fringes, we can observe constructive and destructive interference

At points on the screen, where the path difference between the two waves is $n\lambda$

We see bright fringes where constructive interference takes place

Alternatively, at points where the path difference between the two waves is $(2n+1)\frac{\lambda }{2}$

We see dark fringes where destructive interference takes place

Revision

Interference of light waves is when two or more light waves superimpose on each other to produce one resultant ray

Interference is of two types: $\bullet$Constructive Interference $\bullet$Destructive Interference

Thomas Young in 1801, performed an experiment to showcase Interference

With this experiment, he obtained alternative bright and dark fringes

Constructive Interference was observed with bright fringes

And, Destructive Interference was observed with dark fringes

The End