CDS (II) 2021 General Knowledge · Question 21 of 120

Correct — C, change in speed of light. Refraction happens because light does not travel at the same speed in every transparent medium, and the textbook says so in one line: refraction is due to change in the speed of light as it enters from one transparent medium to another. Picture the wavefront rather than the ray and the mechanism becomes visible. When a broad wavefront meets the boundary at an angle, one edge of it crosses into the new medium while the rest is still in the old one; that edge immediately travels at the new speed, the wavefront pivots, and the direction of travel swings. Where the second medium is slower the pivot turns the ray towards the normal, and where it is faster the ray turns away. This is also why refractive index is defined as a ratio of speeds — the speed of light in vacuum divided by the speed in the medium — and why a ray entering along the normal changes speed without changing direction, since a wavefront meeting the boundary square-on has no edge that arrives first.

• (a)change in temperature of the media. — Temperature can alter a medium's refractive index slightly, and that small effect is behind mirages and the twinkling of stars. It is not why refraction occurs; a ray bends between two media held at the same temperature.
• (b)change in the amplitude of light. — Amplitude sets brightness, not direction. A dim beam and a bright one entering glass at the same angle bend by exactly the same amount.
• (d)internal property of light. — Too vague to be a mechanism, and wrong in substance. Light travelling through a single uniform medium goes perfectly straight; bending needs a boundary between two media with different speeds.

Light travels at about 3 × 10⁸ metres per second in vacuum and more slowly in every material medium. The refractive index of a medium is that vacuum speed divided by the speed within it, so it is always greater than one for a material medium with respect to air. When a ray crosses obliquely between two media the change of speed forces a change of direction, and Snell's law fixes the amount: the ratio of the sine of the angle of incidence to the sine of the angle of refraction is a constant for the pair of media.

Three of the four options name something that has an effect on light without being the cause of refraction, which is the standard shape of a why-does-this-happen item. The test that separates them is to hold each candidate fixed and ask whether refraction still occurs. Two media at identical temperature still refract; a beam of any brightness still refracts by the same angle; and light passing through one medium alone does not refract at all no matter what property it carries. Only a difference in speed survives that test. Temperature is worth a second look because it is not merely wrong — a temperature gradient in air produces a continuously varying refractive index, and that is the mechanism behind desert mirages and the shimmer above a hot road, both of which are refraction driven by changing speed through the gradient rather than by temperature acting directly.

• Refraction is due to the change in the speed of light as it enters from one transparent medium to another.
• Refractive index equals the speed of light in vacuum divided by the speed in the medium, and is greater than one for any material medium relative to air.
• A ray entering along the normal, at zero angle of incidence, changes speed without changing direction.
• Snell's law states that the ratio of the sine of the angle of incidence to the sine of the angle of refraction is constant for a given pair of media.
• Frequency does not change on refraction; the speed and the wavelength both do.

• Treating temperature as the cause rather than as one thing that can change a medium's refractive index.
• Assuming brightness or intensity has anything to do with how much a ray bends.
• Forgetting that a ray at normal incidence changes speed but not direction.

As a cause-of-refraction item like this one, as a refractive-index numerical using c divided by v, or as an application question on mirages, apparent depth or optical fibres.

• practice — not a real PYQ

  A ray of light strikes the surface of a glass slab along the normal. Which one of the following is true?

  • (a)Its speed and direction both change
  • (b)Its speed changes but its direction does not
  • (c)Its direction changes but its speed does not
  • (d)Neither its speed nor its direction changes

  Answer(b) Its speed changes but its direction does not — the whole wavefront enters together, so there is nothing to make it pivot.

• practice — not a real PYQ

  The twinkling of stars is caused mainly by

  • (a)reflection of starlight by clouds
  • (b)dispersion of starlight into colours
  • (c)atmospheric refraction through air layers of varying refractive index
  • (d)the varying brightness of the stars themselves

  Answer(c) atmospheric refraction through air layers of varying refractive index — the light's speed changes continuously as it passes through air of different densities.