Which expression correctly relates energy to frequency and wavelength?

Which expression correctly relates energy to frequency and wavelength?

• A. E = h f
• B. E = h c / λ
• C. E = h f = h c / λ
• D. E = λ / (h f)

Answer: (C)

Energy of a photon is determined by its frequency, and frequency is related to wavelength by the speed of light. Planck’s relation gives E = h f. Because light in vacuum satisfies f = c/λ, you can also write E = h c / λ. These two forms describe the same energy, so the expression that shows both relationships at once is E = h f = h c / λ. This is best because it makes explicit how energy connects to either frequency or wavelength. The separate forms E = h f or E = h c / λ each capture one relation, while the last form is not correct.

Energy of a photon is determined by its frequency, and frequency is related to wavelength by the speed of light. Planck’s relation gives E = h f. Because light in vacuum satisfies f = c/λ, you can also write E = h c / λ. These two forms describe the same energy, so the expression that shows both relationships at once is E = h f = h c / λ. This is best because it makes explicit how energy connects to either frequency or wavelength. The separate forms E = h f or E = h c / λ each capture one relation, while the last form is not correct.