An electron in a hydrogen atom drops from energy level e3 to e2. what's the frequency of the emitted photon

For this problem we can use 2 equations.

(1) - E = mC²

E = Energy of photon (J)

m = Mass of photon (kg) (1.67x10⁻²⁷ kg)

C = speed of light (3 x 10⁸ m/s)

(2) - E = hf

E = Energy of photon (J)

h = plank's constant (6.63 * 10⁻³⁴ J s)

f = frequency of the photon (Hz)

(1) = (2)

hence, mC² = fh

by rearranging,

f = mC² / h

f = 1.67x10⁻²⁷ kg * (3 x 10⁸ m/s) ² / (6.63 * 10⁻³⁴J s)

f = 2.27 x 10²³ Hz