Wavelength of incident light, λ = 5000 Aº = 5000 x 10⁻¹⁰ m
Speed of light, c = 3x 10^s m
Frequency of incident light is given by the relation,
ν =c/λ  =3x 10⁸ /5000 x 10⁻¹⁰ = 6 x 10¹⁴ Hz
The wavelength and frequency of incident light is the same as that of reflected ray. Hence, the wavelength of reflected light is 5000 A and its frequency is 6 x 10¹⁴ Hz. When reflected ray is normal to incident ray, the sum of the angle of incidence, i and angle of reflection, r is 90°.
According to the law of reflection, the angle of incidence is always equal to the angle of reflection. Hence, we can write the sum as:
<i + <r  = 90
<i + <i  = 90

2<i = 90

Therefore, <i = 90/2 = 45º

Therefore, the angle of incidence for the given condition is 45°.