What is the correct answer to the interference and diffraction NEET 2026 question?
Option 3 is correct: A is true and B is false. The interference and diffraction NEET 2026 question tests conservation of energy across a pattern and whether these wave effects belong to light alone.
The premise is that light energy becomes unevenly distributed: dark regions have reduced intensity, while bright regions have increased intensity. Judge these statements independently:
- A: This redistribution respects conservation of energy.
- B: Interference and diffraction occur exclusively in light waves.
The supplied choices are:
This is a medium-tier, statement-based Physics question from Optics. The question bank’s expected solving time is 45 seconds, not an official time limit.
Why is statement A true even when dark fringes form?
A is true because energy is redistributed, not destroyed. A dark fringe describes intensity in one region, not the energy balance of the complete pattern. The official solution follows these steps:
- Identify what changes: The distribution of intensity across the pattern changes.
- Separate local from overall: Lower intensity at one dark fringe does not establish an overall energy loss.
- Account for brighter regions: Reduced intensity in dark regions accompanies increased intensity in bright regions.
- Decide A: Energy is redistributed rather than destroyed, so A is true.
For an illustrative energy check, take a total of 10 joules over a fixed interval. If darker regions together receive 2 joules, the remaining regions receive:
The complete-pattern total is:
These are illustrative values, not question data. Conservation applies to the complete pattern; it does not require unchanged intensity at every point. This reasoning concerns the ideal situation in the question; real apparatus may absorb some light.
Why is “only light” false, and how does that give option 3?
B is false because sound waves also exhibit both interference and diffraction. This is the official counterexample. Step 2 is to test the word “only”, which excludes every non-light wave.
The sound examples show two distinct effects:
- Interference: Overlapping sound waves can reinforce or cancel, depending on their relative phase.
- Diffraction: Sound can spread around obstacles or through openings.
One valid non-light counterexample is enough to disprove an exclusivity claim. You do not need to list every kind of wave or prove that every wave behaves identically.
Step 3 is to combine the independently established results:
- A is true: Redistribution preserves the overall energy balance.
- B is false: These effects are not exclusive to light.
Final answer: option 3. Options 2 and 4 require A to be false, so both fail. Option 1 incorrectly accepts B, which the sound-wave counterexample disproves.
What reasoning mistake leads to option 1?
Option 1 can follow from confusing “light exhibits these effects” with “only light exhibits these effects.” The route is to accept energy conservation correctly, recall interference and diffraction in light, and then accept B without testing “only.”
The logical error is treating evidence that light shows an effect as proof that no other wave can show it. Familiar optical examples establish that light belongs in the category, not that it is the entire category.
Use this repeatable repair:
- Identify restrictive words such as “only.”
- Test an example outside the named category.
- Use sound, which shows both interference and diffraction.
- Assign each statement its own truth value before checking the option number.
This keeps the correct judgement on A separate from the rejection of B. A true statement does not make its neighbour true.
Can you apply the same ideas to three related Optics questions?
Test them through coherence, diffraction spreading and energy balance. These are original practice questions, not additional verified PYQs. Choose an option for each, then check the explanation immediately below it.
Which source pair is coherent?
Answer: option 1. Coherence requires the same frequency and a stable phase relationship. Equal frequency alone is insufficient if the phase difference changes randomly.
Equal intensities may affect interference contrast, but they do not establish coherence. Keep the phase condition separate from source brightness.
At fixed wavelength, how does narrowing a slit change the central diffraction maximum’s angular width?
Answer: option 1, increases. A narrower slit produces greater diffraction spreading, so the central single-slit maximum becomes angularly wider. The question asks about angular spread, not how much light passes through.
In an ideal interference pattern, what accompanies reduced intensity in dark fringes?
Answer: option 1. Increased intensity elsewhere preserves the overall energy balance. Local darkness does not mean energy destruction.
If coherence was your sticking point, practise the phase relationship next: Oscillations and Waves NEET 2026: Phase Difference.
Frequently asked questions
What is the correct answer to the interference and diffraction NEET 2026 question?
Option 3 is correct: A is true and B is false. Redistribution of intensity respects conservation of energy, but interference and diffraction are not exclusive to light.
Do dark fringes violate conservation of energy?
No. In an ideal interference pattern, reduced intensity in dark regions accompanies increased intensity elsewhere. Conservation applies to the complete pattern, not to unchanged intensity at every point.
Can sound waves show both interference and diffraction?
Yes, sound waves exhibit both effects. Overlapping sound waves can reinforce or cancel depending on their relative phase, while sound also spreads around obstacles or through openings. Sound therefore disproves the claim that these effects occur only in light.
What are the conditions for two sources to be coherent?
Coherent sources have the same frequency and a fixed phase difference. Equal frequency alone is insufficient if the phase difference changes randomly. Equal intensities do not establish coherence.
How does narrowing a slit affect the central diffraction maximum?
At fixed wavelength, narrowing a slit increases the angular width of the central diffraction maximum. This describes greater diffraction spreading, not how much light passes through the slit.