Light of wavelength 5000 Aº falls on a plane reflecting surface. What are the wavelength and frequency of the reflected light? For what angle of incidence is the reflected ray normal to the incident ray?

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°.