Interference of Light—Interference is the superposition of two light waves due to which non-uniform distribution of energy takes place in the medium. Due to interference of two light waves redistribution of energy takes place in the medium.
The points at which intensity of light is maximum is called constructive intereference while at some points in the medium at which intensity is minimum is called distructive interference. Constructive interference is also called Bright fringes. Destructive interference is also called Dark fringes.
The constructive pattern of constructive interference and destructive interference pattern of light. If interference pattern is permanent on the screen then it is called sustained interference pattern.
Following are the important conditions for sustained interference pattern—
1. The two light source emits light wave contiuously.
2. The two light source must be very-very close to each other.
3. The light wave should be of nearly same wave length.
4. The intensity of light of two source must be nearly same.
5. The two light source must be in same phase or constant phase.
6. The light source must be narrow.
CONDITIONS FOR CONSTRUCTIVE AND DESTRUCTIVE INTERFERENCE
Let us consider two light waves represented by the equations—
By definition of interference,
Let us consider,
Using in eqn (4)
eqn (7) is the general eqn for superposed wave which is periodic in nature. Taking square of eqn (5) and (6) and then adding.
Conditions for constructive interference or bright fringes—
For constructive interference, intensity must be maxm ,
By definition of periodic function.
eqn (15) is the required conditions for constructive interference.
Conditions for destructive interference or dark fringes—
For destructive interference, intensity must be minimum,
