CDS (II) 2021 General Knowledge · Question 3 of 120

Correct — B, slows down and bends towards the normal. Denser and rarer here are optical descriptions, not statements about mass per unit volume: of two media, the one with the larger refractive index is the optically denser. Light travels more slowly in the optically denser medium, and the refractive index is just the ratio of the two speeds. So a ray crossing from a rarer medium into a denser one loses speed at the boundary, and the textbook rule that follows is that it bends towards the normal. Going the other way, from denser to rarer, it speeds up and bends away from the normal. The reason the direction of bending tracks the change in speed is that the leading edge of the wavefront enters the new medium first and is slowed first, so the wavefront pivots; that pivot is what an observer sees as a kink in the ray. Snell's law puts numbers on it — n1 sin i = n2 sin r — and with n2 greater than n1 the angle of refraction must come out smaller than the angle of incidence, which is precisely what bending towards the normal means.

• (a)slows down and bends away from the normal. — Gets the speed right and the direction wrong. Slowing and bending towards the normal always travel together; they cannot be separated.
• (c)speeds up and bends away from the normal. — This is the reverse journey, from a denser medium into a rarer one — light emerging from water into air, for instance.
• (d)speeds up and bends towards the normal. — Physically impossible as a pair. Speeding up means entering a medium of lower refractive index, and Snell's law then forces the refracted angle to be larger, not smaller.

Refraction is the change in the direction of light when it crosses obliquely from one transparent medium into another, and it happens because the speed of light differs from medium to medium. The refractive index of a medium is the speed of light in vacuum divided by the speed in that medium, so a larger index means slower light. Comparing two media, the one with the larger index is called optically denser and the other optically rarer; the pairing of slower-plus-towards-the-normal and faster-plus-away-from-the-normal is fixed by Snell's law.

The item is a two-by-two grid of speed against bending direction, and only two of the four cells describe anything that can happen. Fixing one half fixes the other, so a candidate who is sure only that light slows in glass has already reduced the field to (a) and (b). The remaining hazard is the word 'denser', which invites a detour into mass density. Turpentine has a lower mass density than water yet a higher refractive index, so optical density and mass density are genuinely different properties. The safe anchor is the everyday observation that a stick pushed into water looks bent at the surface, and that a swimming pool looks shallower than it is: both are the air-to-water crossing, and in both the ray has swung towards the normal.

• Refractive index is the speed of light in vacuum divided by the speed in the medium, so a higher index corresponds to slower light.
• Of two media, the one with the larger refractive index is the optically denser; the speed of light is higher in the rarer medium.
• A ray going from rarer to denser slows down and bends towards the normal; from denser to rarer it speeds up and bends away from the normal.
• A ray striking the boundary along the normal, at an incidence angle of zero, passes straight through without bending, though its speed still changes.
• Snell's law, n1 sin i = n2 sin r, contains both rules; refractive index for a material medium with respect to air is always greater than one.

• Reading 'denser' as mass density. Turpentine is lighter than water but optically denser than it.
• Assuming a ray always bends at a boundary; at normal incidence it goes straight through, though its speed still changes.
• Thinking the frequency of the light changes on refraction. Frequency is set by the source; it is speed and wavelength that change.

As this pairing of speed with bending direction, as a refractive-index numerical using c divided by v, or as an application item on apparent depth, the bent-stick effect or optical fibres.

• practice — not a real PYQ

  A ray of light passes from water into air. Which one of the following happens at the boundary?

  • (a)It slows down and bends towards the normal
  • (b)It speeds up and bends away from the normal
  • (c)It speeds up and bends towards the normal
  • (d)Its speed and direction are both unchanged

  Answer(b) It speeds up and bends away from the normal — air is the optically rarer medium here.

• practice — not a real PYQ

  The refractive index of a medium is 1.5 and the speed of light in vacuum is 3 × 10⁸ m/s. The speed of light in the medium is

  • (a)4.5 × 10⁸ m/s
  • (b)3 × 10⁸ m/s
  • (c)2 × 10⁸ m/s
  • (d)1.5 × 10⁸ m/s

  Answer(c) 2 × 10⁸ m/s — speed in the medium equals the speed in vacuum divided by the refractive index.