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?

The correct answer is D, Frequency. The frequency of a wave is set by its source, so it does not change when light passes from air into glass; only the speed and wavelength change. At the boundary, each crest arriving from the air produces one crest in the glass, so the number of waves passing per second must be the same on both sides. Because speed equals frequency multiplied by wavelength, a fall in speed inside glass is matched by a shorter wavelength. This is also why the colour of light, which depends on frequency, does not change on refraction. Option A is wrong because light slows down in glass, to about two-thirds of its speed in air, since glass has a refractive index of about 1.5. Option B is wrong because refraction itself means a change of direction; light entering obliquely bends towards the normal. Option C is wrong because part of the light is reflected at the surface and some is absorbed, so the refracted beam is less bright than the incident one. Exam tip: on refraction, frequency stays the same; speed and wavelength change, and direction changes unless the light falls along the normal.