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Physics Question 39 – JEE-MAIN 2026

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Given below are two statements : one is labelled as Assertion (A) and the other is labelled as Reason (R). Assertion (A) : The electromagnetic wave exerts pressure on the surface on which they are allowed to fall. Reason (R) : There is no mass associated with the electromagnetic waves. In the light of the above statements, choose the correct answer from the options given below :

Recall that electromagnetic waves carry momentum and can exert pressure on surfaces.

Step 1: Analyze Assertion (A)✦ Active

Assertion (A) states that electromagnetic waves exert pressure on the surface on which they fall. This is true. Electromagnetic waves carry momentum (p=E/c for photons). When these waves are absorbed or reflected by a surface, they transfer their momentum to the surface, resulting in a force and thus pressure, known as radiation pressure.

Step 2: Analyze Reason (R)○ Expand

Reason (R) states that there is no mass associated with electromagnetic waves. This is also true. Electromagnetic waves are composed of photons, which are massless particles (they have zero rest mass). They only possess energy and momentum.

Step 3: Evaluate Relationship between A and R○ Expand

Both Assertion (A) and Reason (R) are true statements. However, the absence of mass in electromagnetic waves (Reason R) does not directly explain *why* they exert pressure (Assertion A). The pressure is exerted because electromagnetic waves carry momentum, and this momentum is transferred to the surface. While photons are massless, they still possess momentum. Therefore, Reason (R) is not the correct explanation for Assertion (A).

💡 Teacher's Secret Hint

Distinguish between a property of EM waves (massless) and the direct cause of radiation pressure (momentum transfer).

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