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.
Answer
Why
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.
Why the others are wrong
- (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.
Concept
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.
Key facts
- 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.
Study next
Common traps
- 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.
Related PYQs
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
- 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.