If a ray of light enters from a rarer medium to a denser medium at zero angle of incidence, it would
- (a)reflect back.
- (b)go straight.
- (c)turn towards right.
- (d)bend at 45°.
Answer
Why
Correct — B, go straight. A ray striking a boundary at zero angle of incidence travels along the normal — straight into the surface. By Snell's law, if the angle of incidence is 0° then the angle of refraction is also 0°, so there is no bending: the ray continues straight into the denser medium (its speed drops, but its direction is unchanged). Bending occurs only when light meets the surface at an angle.
Why the others are wrong
- (a)reflect back. — A ray along the normal passes into the denser medium; it is not reflected straight back — that would require mirror-like or total reflection, which does not happen when going from rarer to denser.
- (c)turn towards right. — There is no sideways turn at normal incidence — refraction bends a ray towards the normal only when it strikes at a non-zero angle.
- (d)bend at 45°. — No bending occurs at 0° incidence; a 45° bend would require a specific oblique angle and refractive index, not normal incidence.
Concept
Refraction is governed by Snell's law, n₁ sin i = n₂ sin r. When the angle of incidence i = 0 (the ray travels along the normal), sin i = 0 forces sin r = 0, so r = 0 and the ray goes straight through undeviated — even though its speed decreases in the denser medium. The amount of bending increases with the angle of incidence.
The intuitive trap is to assume that entering a denser medium always bends the ray. It bends only at oblique incidence; at perfectly normal (0°) incidence the ray passes straight through. Its speed still changes, but its direction does not.
Key facts
- Snell's law: n₁ sin i = n₂ sin r; at i = 0°, r = 0° (no bending).
- At normal incidence a ray goes straight into the medium, undeviated.
- Its speed still decreases on entering the denser medium.
- Refraction bends light towards the normal (rarer → denser) only at oblique incidence.
Study next
Common traps
- Assuming light always bends on entering a denser medium — it does not at 0° incidence.
- Confusing 'no bending' with 'no change' — the speed still changes.
Asked as the behaviour at normal incidence, or via Snell's-law bending direction for oblique rays.
Related PYQs
A lemon kept in water in a glass tumbler appears to be larger than its actual size. It is because of
- (a) reflection of light
- (b) scattering of light
- (c) refraction of light
- (d) polarization of light
Answer(c) refraction of light
Same concept — everyday refraction of light at a water surface; the lemon looks larger because rays bend on crossing the boundary at oblique angles, the flip-side of the no-bending case at normal incidence.
Bottom of a tank containing water appears to be raised. It is due to
- (a) Reflection of light
- (b) Refraction of light
- (c) Interference of light
- (d) Diffraction of light
Answer(b) Refraction of light
Same concept — the apparent raising of a tank's bottom is refraction of light at the water surface, the same boundary-refraction physics tested here.
Practice
- practice — not a real PYQ
A ray of light passes from air into glass along the normal to the surface. It will
- (a)bend towards the normal
- (b)bend away from the normal
- (c)go straight without bending
- (d)be totally reflected
Answer(c) go straight without bending — at 0° incidence the angle of refraction is also 0°. - practice — not a real PYQ
Light bends towards the normal when it travels from
- (a)denser to rarer medium
- (b)rarer to denser medium
- (c)one medium to the same medium
- (d)vacuum to vacuum
Answer(b) rarer to denser medium — for example air to glass, at oblique incidence.