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Physics Question 42 – JEE-MAIN 2025

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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

For a stable and observable interference pattern, the interfering light waves must be coherent, meaning they must maintain a constant phase relationship.

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Ninja Strategy•Coherence Principle

Recognize that for interference to occur, the light waves from the two slits must be coherent, which is not possible when they emit different colors (wavelengths).

Step 1: Identify Coherence Condition✦ Active

For a sustained interference pattern in Young's double-slit experiment, the light waves originating from the two slits must be coherent. This means they must have the same frequency (and thus wavelength) and a constant phase difference.

Step 2: Analyze Light from Slits○ Expand

In this setup, one slit is covered by a red filter, allowing only red light (λR) to pass. The other slit is covered by a green filter, allowing only green light (λG) to pass. Since red and green light have different wavelengths (λR≠λG), the light waves emerging from the two slits have different frequencies.

Step 3: Conclusion on Interference○ Expand

Because the light waves from the two slits are of different frequencies, they are incoherent. Therefore, they cannot produce a stable, observable interference pattern.

💡 Teacher's Secret Hint

Remember that coherence is a fundamental requirement for observing sustained interference.

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