The focal length of a plane mirror is infinity as the image can be formed at an infinite distance inside the mirror. A plane mirror can be recognised as a spherical mirror of infinite radius of curvature. Focal length is half of the radius of curvature and hence it is infinity. Hence, the correct answer is A

 Total internal reflection refers to a phenomenon whereby there is a complete reflection of light as it moves from a denser to a less medium with the incident angle more than the critical angle.

 Violet light bends the most, as the refractive index of glass for it is greatest.

Dispersion of light is the phenomenon of splitting a beam of white light into its constituent colours on passing through a prism. The band of seven colours is called the spectrum. The dispersion of white light into seven different colours is because that all the seven rays of light of different colours travel at different speeds through the glass prism.

The degree of refraction estimates the bending of each light ray depending upon their individual speed while passing through the prism. All the different colours of light rays’ travel at different speeds in a glass prism and so the angle of bending or angle of refraction of each light ray is different.

The bending of light rays is inversely proportional to the wavelength of the light.

Different colours bend differently on passing through a prism as it depends upon their wavelength. Higher wavelengths bend less, whereas shorter wavelengths bend more.Hence, the correct answer is C

Explanation: The twinkling of a star is due to atmospheric refraction of starlight. When the light of a star enters the earth’s atmosphere, it approaches refraction due to the different optical densities of air at various altitudes. Light from star reaches our eyes varies continuously due to the atmospheric refraction which appears twinkling of star at night.

Hence the correct option is C.

More Details :

• Atmospheric reflection occurs when light bounces off an object when it falls on a smooth surface.

The apparent depth of a tank of water varies if it is viewed obliquely. It is due to the bending of light during the travelling from one medium to another. While viewing obliquely, the apparent depth of the tank is less than the near-normal viewing.