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MCQEasyJEE 2025Interference (Young's Experiment)

JEE Physics 2025 Question with Solution

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

  • A

    There shall be alternate interference fringes of red and green.

  • B

    There shall be an interference pattern, where each fringe's pattern center is green and outer edges is red.

  • C

    There shall be an interference pattern for red distinct from that for green.

  • D

    There shall be no interference fringes.

Answer

Correct answer:D

Step-by-step solution

Standard Method

Given: In Young's double slit experiment, the source is white light. One slit is covered by a red filter and the other by a green filter.

Find: Whether interference fringes are formed.

Interference in YDSE requires the two waves reaching a point to be coherent and effectively of the same wavelength for a stable fringe pattern.

Here, one slit transmits only red light and the other transmits only green light. Thus, the waves coming from the two slits have different wavelengths and do not produce a stable mutual interference pattern.

Because of this, no distinct interference fringes are observed.

Therefore, the correct option is D: There shall be no interference fringes.

Conceptual Explanation

Given: White light is incident in Young's double slit experiment. One slit has a red filter and the other has a green filter.

Find: The nature of the pattern obtained on the screen.

  1. In Young's double slit experiment, interference occurs when light waves from two slits superpose.
  2. For visible and stable interference fringes, the two sources must be coherent and should contribute waves of the same wavelength.
  3. The red filter allows only red light to emerge from one slit, while the green filter allows only green light to emerge from the other slit.
  4. Since red and green lights have different wavelengths, they do not form a common stable interference pattern.
  5. Hence, the usual bright and dark interference fringes are not obtained.

The solution text also notes that although one may think of separate colour contributions, there is no combined interference fringe pattern between the red light from one slit and the green light from the other.

Therefore, the correct option is D.

Common mistakes

  • Assuming that any two light beams from two slits will always interfere. This is wrong because stable interference requires coherence and matching wavelength conditions. Check whether both slits transmit the same colour before applying YDSE fringe formulas.

  • Choosing the option about separate red and green patterns. That is misleading here because each colour is coming through only one slit, so there is no two-slit interference for a single colour. Do not confuse colour filtering with ordinary white-light YDSE.

  • Thinking that white light automatically guarantees coloured fringes. That happens when both slits receive the same white light spectrum. Here the filters make the two slits emit different colours, so the required mutual interference is not obtained.

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