A2 Daily A Level Physics question
In a dark lab, two monochromatic LED sources each emit 3.0 mW of light into identical beam areas. One is green at wavelength 500 nm, the other is red at 750 nm. A photon-counting detector with equal efficiency at both wavelengths measures the arrival rates. Which statement must be true?
Answer
The correct answer is D.
Correct: D — The red beam gives the higher photon count rate, about 1.5 times that of the green. For equal power, the photon arrival rate is inversely proportional to photon energy, so it is proportional to wavelength: 750 nm / 500 nm ≈ 1.5, hence red yields more photons per second. A misapplies the relationship, assuming higher frequency (green) means more photons for the same power. B confuses equal power with equal photon number per second, ignoring that higher-energy (shorter-wavelength) photons mean fewer photons for the same power. C inverts the ratio, giving two-thirds instead of the correct 1.5. D correctly identifies that longer wavelength at the same power gives a higher photon rate by a factor of about 1.5.