The work function of material is given by ϕ=hvϕ=λhc ..(i) [∵v=λc] where, h= Planck's constant =6.63×10−34 J−s c= speed of length =3×108 ms−1 and λ= wavelength of light Here, ϕ=4eV=4×1.6×10−19 J Substituting the given values in Eq. (i), we get ⇒4×1.6×10−19=λ6.63×10−34×3×108 or λ=4×1.6×10−196.63×10−34×3×108 =3.108×10−7 m≃310 nm