In a Young's double slit experiment, the source is white light. One of the slits is covered by red filter and another by a green filter. In this case:

Solution

Physics sir · 4 steps

Fringes need the same frequency from both slits, and red light and green light can never share one.

1. What fringes need. A steady interference pattern needs coherent sources. The two waves must have the same frequency, so that their phase difference at each point on the screen stays fixed in time.
2. What each slit sends. The red filter lets only red light through one slit, and the green filter lets only green light through the other. So the two waves reaching the screen have different frequencies, ωᵣ ≠ ω_g.

   Δφ(t) = (ω_g - ωᵣ) t + φ₀

3. Phase keeps drifting. With unequal frequencies the phase difference changes very rapidly with time. Every point on the screen swings between bright and dark millions of times a second, so the eye sees only the average intensity, I₁ + I₂, with no bright or dark bands.
4. So. The screen is lit by an overlap of red and green light, but no interference fringes form. The answer is (d).

The idea to keep. Two slits give fringes only if the same wave reaches both. Change the colour through one slit and the light is no longer coherent, so all you get is the plain sum of intensities.

Answer from the NTA final answer key. Solution by PrepWiser.