In a Young's double slit experiment, the source is white light. One of the slits is covered by a red filter and the other by a green filter. What will be observed on the screen? (1) Alternate interference fringes of red and green. (2) An interference pattern for red distinct from that for green. (3) No interference fringes. (4) An interference pattern with green center and red edges for each fringe.

No interference pattern will be observed because red and green light have different wavelengths, making them incoherent sources. Coherent waves are essential for interference, and here, the filtered light from the two slits cannot interfere due to the difference in wavelength.

Young's Double Slit Red Green Filters-JEE Mains 2025 PYQ Explained

Young’s Double Slit Experiment with Red and Green Filters – JEE Mains 2025 PYQ Explained

This blog post explains a conceptual question from the JEE Mains 2025 Physics paper (8th April Shift 2), based on the Young’s Double Slit Experiment (YDSE) with red and green filters. Understanding this helps students appearing in JEE Mains 2026 and 2027 master the wave optics topic and avoid common pitfalls in interference-based questions.

Question

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

There shall be alternate interference fringes of red and green.
There shall be an interference pattern for red distinct from that for green.
There shall be no interference fringes.
There shall be an interference pattern, where each fringe’s pattern center is green and outer edges is red.

Detailed Step-by-Step Solution

Step 1: Understand the setup

In YDSE, interference occurs when waves from both slits are coherent – i.e., they have the same frequency and maintain a constant phase difference. Here, the source is white light, but red and green filters are used on different slits.

Step 2: Effect of the filters

The red filter allows only red light (≈700 nm) to pass through one slit, and the green filter allows green light (≈550 nm) through the other. These filtered lights have different wavelengths.

Step 3: Can red and green light interfere?

No. Interference requires waves of the same wavelength. Since red and green lights differ in wavelength, they are incoherent and do not produce a stable interference pattern.

Step 4: Final conclusion

Due to the lack of coherence between the red and green beams, no interference fringes will be observed on the screen.

✅ Correct Answer: (3) There shall be no interference fringes.

Watch the Full Video Solution

📺 Watch the detailed video explanation of this question below:

Conclusion

This question highlights an important concept: interference only occurs between waves of the same wavelength. Applying filters of different colors at the slits destroys the required coherence, and no fringe pattern is seen. Keep this in mind for wave optics questions in JEE Mains 2026 and 2027!

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Young’s Double Slit Red Green Filters-JEE Mains 2025 PYQ Explained

Young’s Double Slit Experiment with Red and Green Filters – JEE Mains 2025 PYQ Explained

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