Which one of the following statements is not correct for light rays?
- (a)Light travels at different speeds in different media.
- (b)Light travels at almost 300 million metres per second in air.
- (c)Light speeds down as it leaves a water surface and enters the air.
- (d)Light speeds up as it leaves a glass surface and enters the air.
Correct — C, 'Light speeds down as it leaves a water surface and enters the air.' Light travels faster in an optically rarer medium. Air is rarer than water (refractive index of air ≈ 1.0 versus water ≈ 1.33), so light actually speeds UP when it passes from water into air, not down. This statement is therefore the incorrect one, which is what the question asks for.
- (a)Light travels at different speeds in different media. — This is correct — the speed of light depends on the medium, which is exactly why refraction occurs; so it is not the answer.
- (b)Light travels at almost 300 million metres per second in air. — This is correct — the speed of light in air is about 3 x 10^8 m/s (nearly its vacuum value); so it is a true statement, not the answer.
- (d)Light speeds up as it leaves a glass surface and enters the air. — This is correct — glass is denser than air, so light speeds up going from glass to air; a true statement and hence not the answer.
The speed of light is greatest in vacuum (≈ 3 x 10^8 m/s) and slows down in a material medium. The denser the medium (higher refractive index), the slower the light: v = c/n. So light is faster in air than in water or glass. When light passes into a rarer medium it speeds up; into a denser medium it slows down — and this change of speed is what bends (refracts) the ray.
Three of the statements are standard true facts; the odd one out reverses the correct direction of speed change. Going from water (denser) to air (rarer) must speed light up, so a statement saying it 'speeds down' is the incorrect one. Anchoring on 'denser medium = slower light' quickly exposes it.
- Speed of light in vacuum ≈ 3 x 10^8 m/s; in air it is almost the same.
- Refractive index n = c/v; higher n means a slower speed in that medium.
- Air (n ≈ 1.0) is rarer than water (n ≈ 1.33) and glass (n ≈ 1.5).
- Light speeds up entering a rarer medium and slows down entering a denser one — the basis of refraction.
Only statement (c) reverses the correct direction — light speeds up, not down, entering the rarer air from water.
- Reversing the rule — light slows in a denser medium and speeds up in a rarer one.
- Assuming the speed of light is the same in all media; it changes with refractive index.
Asked as 'which statement is not correct' about light speed, or by comparing speeds and refractive indices across air, water and glass.
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. 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 governs the speed and bending of light. Diamond has a HIGHER refractive index than glass (so R is false), the same 'denser medium = slower, more-bent light' idea behind the water-to-air speed change here.
- practice — not a real PYQ
Light travels fastest in
- (a)glass
- (b)water
- (c)vacuum
- (d)diamond
Answer(c) vacuum — the speed is greatest where there is no medium to slow it.
- practice — not a real PYQ
When light passes from air into water, its speed
- (a)increases
- (b)decreases
- (c)stays the same
- (d)becomes zero
Answer(b) decreases — water is optically denser than air, so light slows down.