Light waves are incident on an air-glass boundary. Some of the light w

Light waves are incident on an air-glass boundary. Some of the light waves are reflected and some are refracted in the glass. Which one of the following properties is the same for the incident wave and the refracted wave?

Speed
Direction
Brightness
Frequency
This question was previously asked in
UPSC NDA-1 – 2021
The correct answer is (D) Frequency.
When light waves pass from one medium to another (like air to glass), their speed and wavelength change, but their frequency remains constant. The frequency of a wave is determined by the source emitting the light and does not change when the medium changes.
Speed of light changes because the refractive index of the medium changes (speed = speed of light in vacuum / refractive index). Wavelength changes proportionally to the speed (speed = frequency * wavelength). Direction changes due to refraction (bending of light), unless the incidence is normal to the surface. Brightness (intensity) changes due to partial reflection at the boundary, absorption, and scattering within the medium.
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