Q4. Assertion: the wave picture predicts that a dim beam takes time to eject electrons. Reason: in the wave picture the energy is spread over the wavefront and absorbed continuously. Are both true, and does the reason explain the assertion?
Both are true, and the reason explains the assertion: each electron gets only a small share of the energy, so it would take a long time to collect enough (NCERT page 281). Experiment finds no such delay (NCERT page 280).
Q5. Repeat the estimate of Example 1 for bright light of 10 W/m2. Does the wave picture now agree with experiment?
Power per electron = 10 × 1 × 10−20 = 1 × 10−19 W, so the time is 3.2 × 10−19/(1 × 10−19) ≈ 3 s. That is still about 3 × 109 times longer than the observed 10−9 s, so no (reasoned from NCERT pages 280–281).
Q6. Which of these observations could the wave picture explain: (a) photocurrent proportional to intensity, (b) a threshold frequency, (c) stopping potential independent of intensity?
Only (a): more intense light could free more electrons. (b) and (c) contradict it (reasoned from NCERT page 281).
Q7. Seeing an object needs both pictures of light. Explain.
The focusing of light by the eye lens is described by the wave picture, while its absorption by the rods and cones of the retina needs the photon picture (NCERT page 285).