For a photon, de Broglie wavelength, λ =

h

p

For an electromagnetic radiation of frequency υ and wavelength λ′(= c/υ),
Momentum, p =

E

c

=

hv

c

or p =

h

c

.

c

λ′

=

h

λ′

Then , λ' =

h

p

= λ
Thus the wavelength λ′ of the electromagnetic radiation is the same as the de Broglie wavelength λ of the photon.