Question

In Young's double slit experiment, the central maximum is bright in the interference pattern obtained on a screen. What will happen when one of the slits is closed ? Justify your answers.

Hint:

Interference = Multiple slits (superposition of multiple waves). Diffraction = Single slit (spreading of a single wavefront). Closing a slit transitions the experiment fundamentally from the former to the latter.

Answer 1

Concept:
• The phenomenon of interference explicitly requires the superposition of two or more coherent wave trains.
• In YDSE, these two coherent waves originate from the two separate slits, $S_1$ and $S_2$.
• Diffraction, on the other hand, is the bending and spreading of light as it passes through a single narrow aperture.

Step 1: Analyze the physical change to the setup
When one of the slits is completely closed, light can only pass through the single remaining open slit. Because only one source of light is present, there is no longer a second coherent wave available to superimpose with the first.

Step 2: Determine the resulting phenomenon
Without two superimposing waves, the sharp, evenly spaced interference fringes (alternating bright and dark bands of equal width) completely disappear. Instead, the light passing through the single remaining narrow slit undergoes diffraction. This results in a single-slit diffraction pattern being projected on the screen.

Step 3: Describe the visual outcome on the screen
The single-slit diffraction pattern is characterized by a very broad and extremely bright central maximum. This central maximum is flanked on both sides by alternating dark and bright secondary bands (maxima and minima). Crucially, these secondary diffraction maxima are much wider and their intensity decreases very rapidly as you move away from the center, completely unlike the uniform intensity fringes of interference.

Step 4: Conclusion
The sharp interference pattern will completely vanish, and it will be replaced by a single-slit diffraction pattern characterized by a broad, bright central maximum with rapidly fading secondary maxima.