Question

Two beams of light will not give rise to an interference pattern, if

• A. they are coherent
• B. they have the same wavelength
• C. they are linearly polarized perpendicular to each other
• D. they are not monochromatic

Hint:

For interference, light must be coherent and preferably monochromatic, otherwise fringes overlap and disappear.

Answer 1

The Correct Option is D

Step 1: Understand the condition for sustained interference.
For a stable and well-defined interference pattern, the two light sources must be coherent (constant phase difference) and preferably monochromatic (single wavelength).
Step 2: Why monochromatic light is necessary.
If the light is not monochromatic, it contains many wavelengths. Each wavelength produces its own fringe pattern, and these patterns overlap.
Step 3: Result of overlapping fringes.
Due to overlapping, the bright and dark fringes wash out, and a clear interference pattern is not observed.
Step 4: Conclude from options.
Thus, the interference pattern will not be formed properly when the beams are not monochromatic.
Final Answer:
\[ \boxed{\text{(D) they are not monochromatic}} \]