Q4. Assertion: at fixed frequency and beam area, doubling the intensity of a beam of light doubles the number of photons per second but leaves each photon’s energy unchanged. Reason: a photon’s energy depends only on the frequency. Are both true, and does the reason explain the assertion?
Both are true, and the reason explains the assertion: E = hν, so only the frequency fixes it, and the extra intensity must come from more photons (NCERT page 283).
Q5. A 1.0 mW laser emits light of 500 nm. How many photons does it emit per second?
E = hc/λ ≈ (6.63 × 10−34 × 3 × 108)/
Q6. A laser beam of power 2.0 mW falls normally on a surface and is completely absorbed. What force does it exert?
Each second the beam delivers momentum N × h/λ = P/c, so F = P/c = (2.0 × 10−3)/(3 × 108) ≈ 6.7 × 10−12 N (reasoned from p = hν/c, NCERT page 283). A perfect mirror would feel twice this, about 1.3 × 10−11 N.
Q7. How many photons of wavelength 600 nm carry a total energy of 1.0 J?
E = hc/λ ≈ 3.3 × 10−19 J per photon, so 1.0/(3.3 × 10−19) ≈ 3.0 × 1018 photons (NCERT page 283).