CDS (II) 2020 General Knowledge · Question 103 of 120

Correct — C, 0°. An angle of incidence of zero means the ray strikes the boundary along the normal — head on, perpendicular to the surface. Snell's law gives the answer immediately: n₁ sin i = n₂ sin r, and with i = 0 the left-hand side is zero, so sin r must be zero and r = 0. Physically, refraction is a sideways swing produced because one edge of a wavefront enters the new medium before the other and slows down first. When the wavefront meets the surface square on, every part of it crosses at the same instant and slows by the same factor together, so there is nothing to turn the ray. The light does slow down on entering the glass — its speed and wavelength change and its frequency does not — but it carries straight on without bending. This holds for any pair of media in either direction, water to glass or glass to water alike.

• (a)90° — A refracted ray at 90 degrees would run along the boundary itself, never entering the second medium. That happens only at the critical angle when light travels from a denser to a rarer medium — the opposite situation to a normal-incidence ray going into a denser medium.
• (b)45° — There is no rule that produces 45 degrees here. The refraction angle depends on the incidence angle and on the two refractive indices; a zero incidence angle can only give a zero refraction angle.
• (d)The ray will not enter at all — Total internal reflection is what this option is reaching for, but that requires light to go from a denser medium to a rarer one and to strike at more than the critical angle. Here the ray goes from water into glass, which is the denser of the two, and hits at zero degrees — the case furthest from total internal reflection.

Refraction is the change in the direction of light when it crosses from one transparent medium into another, and it happens because the speed of light differs between the media. Snell's law states that n₁ sin i = n₂ sin r, where n is the refractive index of each medium, measured from the normal in both cases. Light bends towards the normal when it enters a denser medium and away from the normal when it enters a rarer one — except at normal incidence, where it does not bend at all.

Two things make this a one-line item once they are separated. First, the angles in optics are always measured from the normal, never from the surface, so 'incidence 0°' means straight in, not grazing along the surface — a candidate who reads it the other way will hunt for 90 degrees. Second, refraction and change of speed are different things. Light slows on entering glass whether or not it bends, and it is only the sideways swing that disappears at normal incidence. The typical Indian values put water at about 1·33 and ordinary crown glass at about 1·5, so glass is the denser medium here, but the answer would be the same for any pair.

• Snell's law: n₁ sin i = n₂ sin r, with both angles measured from the normal to the surface.
• At normal incidence, i = 0 gives r = 0 — the ray passes straight through without bending.
• The speed and wavelength of light change on entering a new medium, but the frequency does not.
• Refractive index of water is about 1·33 and of ordinary crown glass about 1·5, so glass is the optically denser of the two.
• Total internal reflection requires travel from a denser to a rarer medium at more than the critical angle, and cannot occur at normal incidence.

• Measuring the angle from the surface instead of the normal, which turns 0 degrees into 90.
• Believing that no bending means no change in speed; the speed does change.
• Invoking total internal reflection when the light is entering the denser medium, where it can never occur.

As this zero-angle special case, as a numerical Snell's law calculation, or as a critical-angle and total-internal-reflection item.

• practice — not a real PYQ

  When light passes from one medium into another, which one of the following remains unchanged?

  • (a)Speed
  • (b)Wavelength
  • (c)Frequency
  • (d)Direction

  Answer(c) Frequency — it is fixed by the source. Speed and wavelength both change, and the direction changes at any incidence other than zero.

• practice — not a real PYQ

  Total internal reflection of light can occur only when light travels

  • (a)from a rarer medium to a denser medium
  • (b)from a denser medium to a rarer medium at an angle greater than the critical angle
  • (c)along the normal to the boundary
  • (d)through a vacuum

  Answer(b) from a denser medium to a rarer medium at an angle greater than the critical angle — the principle on which optical fibres work.