Let n_r and n_b be the number of photons emitted by a red bulb and a blue bulb, respectively, of equal power in a given time.
(a) n_r = n_b
(b) n_r < n_b
(c) n_r > n_b
(d) The information is insufficient to derive a relation between n_r and n_b.

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Solution

(c) n_r > n_b

The two bulbs are of equal power. It means that they consume equal amount of energy per unit time.
Now, as the frequency of blue light $(f_{b})$ is higher than the frequency of red light $(f_{r})$ , $h f_{b} > h f_{r}$ .
Hence, the energy of a photon of blue light is more than the energy of a photon of red light.
Thus, a photon of blue light requires more energy than a photon of red light to be emitted.
For the same energy given to the bulbs in a certain time, the number of photons of blue light will be less than that of red light.
$∴$ n_r > n_b (As the amount of energy emitted from the two bulb is same)

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