When a ray of light enters a glass slab, then
- (a)only the frequency changes
- (b)frequency and velocity change
- (c)frequency does not change
- (d)frequency and wavelength change
Correct — C, frequency does not change. When light crosses from air into glass, three quantities are in play and they do not all behave alike. The frequency is set by the source that made the light and travels with it unchanged; the glass cannot alter it. There is a simple physical reason. The boundary is not a place where waves can accumulate or disappear, so however many crests arrive at the surface each second, exactly that many must leave into the glass each second — and that count is the frequency. What the glass does change is the speed: light slows to c divided by the refractive index, so in ordinary glass of index about 1.5 it travels at roughly two-thirds of its vacuum speed. Since speed equals frequency multiplied by wavelength, and the frequency is pinned, the wavelength must shorten in exactly the same proportion as the speed. The practical consequence worth carrying away is that colour, which depends on frequency, is unchanged — red light does not turn any other colour inside glass.
- (a)only the frequency changes — It states the exact opposite of the truth. Frequency is the one quantity that survives the crossing untouched, while speed and wavelength both change.
- (b)frequency and velocity change — Half right and therefore wrong. The velocity does fall, but including frequency in the list spoils it — no change of medium can alter the rate at which the source is oscillating.
- (d)frequency and wavelength change — The most tempting option, because the wavelength genuinely does shorten inside the glass. But it wrongly pairs that true fact with a change in frequency. Wavelength changes precisely because frequency does not, the two being tied together through the speed.
Refraction is the bending of light as it passes into a medium in which its speed is different. The refractive index of a medium is the ratio of the speed of light in vacuum to its speed in that medium, so a larger index means a slower wave. Across any boundary the frequency of the wave is conserved while its speed and wavelength both change, and the relation v equal to f times lambda keeps the three consistent.
The reliable way to think about it is to decide first which quantity belongs to the source and which belong to the medium. Frequency is the source's property — it is how fast the emitting atoms oscillate — and it rides along unchanged. Speed is the medium's property. Wavelength is whatever the other two require it to be. Once that hierarchy is fixed, options (a), (b) and (d) all fail on the same point, and the only surviving statement is the one that says frequency does not change. This also settles the familiar puzzle of why a beam of light does not change colour when it enters water or glass.
- The frequency of light is fixed by its source and does not change when the light enters a new medium.
- The speed in a medium equals the speed in vacuum divided by the refractive index; in glass of index 1.5 that is about 2 × 10^8 metres per second.
- Because v equals f times lambda and f is fixed, the wavelength shortens in the same proportion as the speed.
- Colour is determined by frequency, so light keeps its colour on entering glass or water.
- Light returns to its original speed and wavelength when it emerges back into air.
Only the source-determined quantity survives the crossing, which is why option (c) is the correct statement.
- Assuming that because the wave slows down, everything about it must change.
- Confusing colour with wavelength — colour tracks frequency, which is why a beam keeps its colour under water.
Refraction is among the most heavily repeated NDA GAT physics topics, asked as a concept statement, as a refractive-index ratio and as a lens calculation, so hold the relation between speed, frequency and wavelength ready.
When light waves pass from air to glass, the variables affected are
- (a) Wavelength, frequency and velocity
- (b) Velocity and frequency
- (c) Wavelength and frequency
- (d) Wavelength and velocity
Answer(d) Wavelength and velocity
The same question in almost the same words, and its key confirms the point exactly — wavelength and velocity change, frequency does not.
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
Turns on the same quantity read the other way — diamond's refractive index is much higher than glass, which slows light more sharply and produces the sparkle.
The refractive indices of two media are denoted by n₁ and n₂, and the velocities of light in these two media are respectively v₁ and v₂. If n₂/n₁ is 1·5, which one of the following statements is correct?
- (a) v₁ is 1·5 times v₂.
- (b) v₂ is 1·5 times v₁.
- (c) v₁ is equal to v₂.
- (d) v₁ is 3 times v₂.
Answer(a) v₁ is 1·5 times v₂.
Puts the speed half of this question into numbers — a higher refractive index means a proportionately lower speed, while the frequency stays out of it entirely.
- practice — not a real PYQ
When light passes from air into water, its wavelength
- (a)increases
- (b)decreases
- (c)remains the same
- (d)becomes zero
Answer(b) decreases — the speed falls while the frequency stays fixed, so the wavelength must shorten.
- practice — not a real PYQ
The refractive index of a medium is defined as the ratio of
- (a)the wavelength in the medium to that in vacuum
- (b)the speed of light in vacuum to the speed in the medium
- (c)the frequency in the medium to that in vacuum
- (d)the speed in the medium to the speed in vacuum
Answer(b) the speed of light in vacuum to the speed in the medium — so a higher index means a slower wave.