Light of wavelength 5000 Armstrong 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, $[\lambda]=5000$ Armstrong $=5000 \times 10^{-10} \mathrm{~m}$
Speed of light, $\mathrm{c}=3 \times 10^{8} \mathrm{~m}$
Following is the relation for the frequency of incident light:
$\mathrm{v}=\frac{c}{\lambda}=\frac{3 \times 10^{8}}{5000 \times 10^{-10}}=6 \times 10^{14}$
The wavelength and frequency of incident light is equal to the reflected ray. Therefore, 5000 Armstrong and
$6 \times 10^{14} \mathrm{~Hz}$ is the wavelength and frequency of the reflected light. When reflected ray is normal to incident
ray, the sum of the angle of incidence, $\angle i$ and angle of reflection, $\angle r$ is $90^{\circ}$.
From laws of reflection we know that the angle of incidence is always equal to the angle of reflection
$\angle i+\angle r=90^{\circ}$
i.e. $\angle i+\angle i=90^{\circ}$
Therefore, 45° is the angle of incidence.