Question

In Young's experiment with a monochromatic source and two slits, one of the slits is covered with black opaque paper, the fringes will

• A. be darker
• B. be narrower
• C. be broader
• D. not be observed

Hint:

Interference is fundamentally a two-body phenomenon for waves. Single slit = diffraction only; Double slit = interference modulated by diffraction. Blocking one slit always destroys the interference fringes completely.

Answer 1

The Correct Option is D

Step 1: Understanding the Question:
The question asks about the effect on the interference pattern (fringes) in Young's double-slit experiment when one of the two slits is completely covered by an opaque material.

Step 2: Key Formula or Approach:
The formation of an interference pattern requires two coherent light sources. The resultant intensity $I$ at any point on the screen due to two waves is given by:
$$I = I_1 + I_2 + 2\sqrt{I_1 I_2}\cos\phi$$ where $I_1$ and $I_2$ are the intensities of the individual sources, and $\phi$ is their phase difference. If one source is completely blocked, no superposition can take place.

Step 3: Detailed Explanation:
When one of the two slits is covered with black opaque paper, light can no longer pass through it, reducing the number of active light sources to one ($I_2 = 0$). Substituting $I_2 = 0$ into our intensity equation leaves:
$$I = I_1$$ Because there is no second wave to overlap and interfere with the first, the sustained constructive and destructive interference conditions cannot occur. Instead of an alternating pattern of bright and dark bands, the single open slit will project a continuous, uniform diffraction pattern of light on the screen. Consequently, distinct interference fringes will disappear entirely.

Step 4: Final Answer:
The fringes will not be observed, which matches option (D).