A2 Daily A Level Physics question
A lab source can emit monochromatic red light of wavelength 660 nm or blue light of 440 nm. The output is adjusted so the intensity at a photon-counting detector is identical in both cases. Which statement must be true when switching from the red to the blue setting?
Answer
The correct answer is C.
Correct: C — The photon count rate decreases to about 2/3 of the red rate because each blue photon carries about 1.5× more energy. A Higher photon energy at fixed intensity means fewer photons per second, not more, so the rate does not increase by 1.5×. B Equal intensity does not imply equal photon rate when photon energy changes; with higher-energy photons, the count must fall. C With the same intensity, photon rate is inversely proportional to energy per photon; since 660/440 ≈ 1.5, the blue count is about 1/1.5 ≈ 2/3 of the red. D A drop to 1/2 would require the photon energy to double (i.e., wavelength to halve), but 440 nm is 2/3 of 660 nm, giving a factor of about 2/3, not 1/2.