During reflection at a surface, wavelength and frequency of light wave remain same, only phase of the wave changes by 180°.

Thus, wavelength of reflected light is 5000 Å itself.

We know c = ν λ

where c, ν, λ are velocity, frequency, wavelength respectively.

c = ν × 5000 × 10^-10

∴ ν = 3 × 10⁸/5000 × 10^-10

= 6 × 10¹⁴ Hz

Wavelength of reflected light is 5000 Å

and frequency is 6 × 10¹⁴ Hz

Angle between incident and refracted ray,

θ = I + R

Where ‘I’ is incident angle and ‘R’ is reflected angle.

We need to find angle of incidence for which angel between incident and reflected ray is 90°.

i.e., θ = 90°

I + R = 90°

From laws of reflection,

Angle of incidence = Angle of reflection

i.e., I = R

∴ I + I = 2I = 90°

I = 45°