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?
- (a)Speed
- (b)Direction
- (c)Brightness
- (d)Frequency
Correct — D, Frequency. When light passes from air into glass its frequency is fixed by the source and does not change on crossing the boundary; the number of wave crests arriving each second must equal the number leaving, or crests would pile up at the surface. What does change is the speed (light slows in glass) and, in step with it, the wavelength; the direction bends (refraction) and the brightness drops because some energy is reflected. Only the frequency is carried across unchanged.
- (a)Speed — Light travels slower in glass than in air (that is why it bends), so the speed of the refracted wave is not the same as that of the incident wave.
- (b)Direction — The refracted ray bends towards the normal on entering the denser glass, so its direction differs from the incident ray's.
- (c)Brightness — Because some light is reflected at the boundary, the refracted wave carries less energy and is dimmer than the incident wave.
Refraction occurs when a wave crosses into a medium where its speed differs. Frequency is set by the source and is conserved across the boundary; since wave speed v = f × λ, a change in speed forces a matching change in wavelength while frequency stays constant. The bending of the ray follows Snell's law.
Students often assume speed or wavelength is preserved. The reliable rule is that on refraction frequency is always conserved, while speed and wavelength change together. Colour (which the eye ties to frequency) therefore does not change on entering glass.
- On refraction, frequency stays constant while speed and wavelength both change.
- Wave speed relation: v = f × λ, so if v falls, λ falls in proportion.
- Light slows down in an optically denser medium such as glass and bends towards the normal.
- Some incident light is reflected at the boundary, so the refracted beam is dimmer.
Only the frequency is the same for the incident and refracted wave — option (d).
- Assuming speed or wavelength is conserved — it is frequency that stays constant.
- Forgetting that some light is reflected, so the refracted beam is dimmer.
Asked as 'which property stays the same on refraction' or through Snell's-law and refractive-index numericals.
Assertion (A): A diamond sparkles more than a glass imitation cut to the same shape. Reason (R): The refractive index of diamond is less than that of glass. In the context of the above two statements, which one of the following is correct?
- (a) Both A and R are true, and R is the correct explanation of A
- (b) Both A and R are true, but R is not a correct explanation of A
- (c) A is true, but R is false
- (d) A is false, but R is true
Answer(c) A is true, but R is false
Same concept — refractive index and refraction govern how light behaves crossing a boundary; a higher refractive index means light slows more and bends more, the same physics that keeps frequency fixed while speed and wavelength change.
The refractive index of fused quartz is 1.46 and that of sapphire is 1.77. If vq is the speed of light in quartz and vs is the speed of light in sapphire, then which one of the following relationships is correct?
- (a) vq > vs
- (b) vs > vq
- (c) vs = vq
- (d) vs = vq/2
Answer(a) vq > vs
Same concept — the higher a medium's refractive index the slower the light (quartz, n = 1.46, gives faster light than sapphire, n = 1.77); this speed change on refraction is exactly what accompanies the unchanged frequency.
A lemon kept in water in a glass tumbler appears to be larger than its actual size. It is because of
- (a) reflection of light
- (b) scattering of light
- (c) refraction of light
- (d) polarization of light
Answer(c) refraction of light
Same concept — the lemon looks larger because light bends (refracts) on leaving the water, the same air/denser-medium refraction described in this question.
- practice — not a real PYQ
When light travels from air into water, which quantity remains unchanged?
- (a)Speed
- (b)Wavelength
- (c)Frequency
- (d)Direction
Answer(c) Frequency — it is fixed by the source; speed and wavelength decrease in water.
- practice — not a real PYQ
The speed of light in a medium of refractive index 1.5 is (speed in vacuum = 3 × 10⁸ m/s)
- (a)3 × 10⁸ m/s
- (b)2 × 10⁸ m/s
- (c)4.5 × 10⁸ m/s
- (d)1.5 × 10⁸ m/s
Answer(b) 2 × 10⁸ m/s — v = c/n = (3 × 10⁸)/1.5.