A2 Daily A Level Physics question
Two monochromatic LEDs each illuminate the same small photodiode with equal intensity (same power per unit area). LED G emits green light at wavelength 500 nm; LED V emits violet light at 400 nm. Which statement must be true about the photon arrival rates at the diode, and why?
Answer
The correct answer is C.
Correct: C — The 400 nm beam delivers about 20% fewer photons per second because each photon carries more energy at that wavelength. For equal intensity (same power), photon rate is inversely proportional to energy per photon; E ∝ 1/λ, so rate ∝ λ, giving R(400)/R(500) = 400/500 = 0.80, i.e. 20% fewer photons for 400 nm. A Equal intensity means equal power, not equal photon count; different photon energies at different wavelengths imply different photon rates. B Shorter wavelength means higher energy per photon, so for the same power the photon rate is lower, not higher; the correct change is about 20% fewer, not 25% more. C This follows from the inverse relation between photon rate and photon energy, with the 400/500 ratio giving 0.80. D In vacuum/air both wavelengths travel at essentially the same speed c; speed does not explain a photon-rate difference here.