The energy (E) of a photon is directly proportional to its frequency (ν) and inversely proportional to its wavelength (λ), as given by the equation:
E = hν = hc/λ
where h is the Planck constant and c is the speed of light.
If the wavelength of a photon is doubled (2λ), we can see from the equation that the energy of the photon is inversely proportional to the wavelength. Therefore, if the wavelength is doubled, the energy of the photon is halved.
In other words, doubling the wavelength of a photon results in a decrease in its energy by a factor of two. This relationship is consistent with the inverse proportionality between energy and wavelength for photons.