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Physics Question 40 – JEE-MAIN 2026

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A point light source emits E.M. waves in free space. A detector, placed at a distance of L m, measures the intensity as Io. The detector is now shifted to another location on the same spherical surface ensuring the angle between original location and new location as 45∘. The measured intensity at new location will be _______.

Understand how the intensity of electromagnetic waves from a point source depends on the distance from the source.

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Ninja Strategy•Constant Distance Insight

Recognize that moving on the 'same spherical surface' means the distance from the point source remains constant, thus the intensity must also remain constant.

Step 1: Understand the setup and intensity dependence✦ Active

A point light source emits electromagnetic waves. The intensity I at a distance r from a point source in free space is given by the inverse square law, where P is the power of the source:

I=P4πr2
💡 Teacher's Secret Hint

The key is to recognize that a 'spherical surface' centered at the source implies a constant radius.

Step 2: Analyze the initial condition○ Expand

Initially, the detector is placed at a distance L from the source, and the measured intensity is Io. So, we have:

Io=P4πL2
Step 3: Determine intensity at the new location○ Expand

The detector is now shifted to another location 'on the same spherical surface'. This means the new distance from the point source is still L. The angle of 45∘ between the original and new locations is irrelevant as long as the distance from the source remains constant.

Therefore, the intensity at the new location will be:

Inew=P4πL2=Io
💡 Teacher's Secret Hint

The angle is a distractor. Focus on the distance from the source.

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