Which one of the following figures correctly shows the path of a ray of light through a glass prism?
- (a)Figure (a) — the ray bends at the first surface, travels through the glass, and bends again on leaving, the deviation at both surfaces being towards the base
- (b)Figure (b) — the ray crosses the first surface without bending and is deviated only as it leaves
- (c)Figure (c) — the ray passes through undeviated at entry and then bends away from the base, towards the apex
- (d)Figure (d) — the ray bends once on entering and then crosses the second surface in a straight line, without refracting again
Answer
Why
Correct — A. In the printed figures the prism is drawn as a right triangle: a horizontal base along the bottom, a vertical face on the right, and a sloping face running from the bottom-left corner up to the apex at the top. Light arrives horizontally from the left, meets the sloping face, and leaves through the vertical face. Those two faces are the refracting surfaces and the angle between them at the top is the refracting angle, so the base of the prism is the thick part at the bottom.
A ray crossing into a prism refracts twice, once at each surface. Entering glass from air it moves into a denser medium and bends towards the normal; leaving glass for air it bends away from the normal. Both bends turn the ray the same way, and the standard result is that the emergent ray is deviated towards the base of the prism — here, downwards.
Figure (a) is the only one that shows both features: a visible change of direction where the ray enters, a straight path through the glass, and a second change of direction where it leaves, with the emergent ray heading down and away from the apex.
Why the others are wrong
- (b)Figure (b) — the ray crosses the first surface without bending and is deviated only as it leaves — The ray is drawn passing straight through the sloping face. Light striking a surface obliquely must refract there; it continues undeviated only when it arrives along the normal, which is not the case for a horizontal ray meeting a slanted face.
- (c)Figure (c) — the ray passes through undeviated at entry and then bends away from the base, towards the apex — This deviates the ray the wrong way. A prism always turns light towards its base, never towards the apex, so an emergent ray rising towards the top of the figure is impossible for this geometry.
- (d)Figure (d) — the ray bends once on entering and then crosses the second surface in a straight line, without refracting again — It gets the first refraction right but drops the second. The ray is leaving glass for air at an oblique angle, so it must bend again — away from the normal — as it emerges.
Concept
Refraction at a plane boundary follows Snell's law, n₁ sin i = n₂ sin r. Passing from a rarer to a denser medium the ray bends towards the normal; from denser to rarer it bends away. A prism has two non-parallel refracting faces, so a ray refracts twice and the two deviations add rather than cancel — unlike a glass slab, whose parallel faces return the ray to its original direction, merely displaced.
Answering this is a matter of checking each diagram against two fixed rules rather than calculating anything. Rule one: there must be a bend at each surface, which removes (b) and (d) immediately. Rule two: the net deviation is towards the base, which removes (c). Only (a) survives. The reason a prism does not behave like a slab is that its faces are inclined to one another, so the second refraction reinforces the first instead of undoing it.
Key facts
- A ray refracts at BOTH faces of a prism — entering and leaving.
- Air to glass: bends towards the normal. Glass to air: bends away from it.
- The emergent ray is always deviated towards the base of the prism.
- A glass slab, with parallel faces, leaves the direction unchanged and only displaces the ray.
- Deviation depends on the refracting angle, the refractive index and the angle of incidence.
Two refractions, both turning the ray towards the base — only figure (a), option (a).
Study next
Common traps
- Forgetting the second refraction as the ray leaves the glass.
- Drawing the deviation towards the apex instead of the base.
- Treating a prism like a parallel-sided slab.
- Assuming a ray passes undeviated through a surface it meets obliquely.
NDA prints four ray paths through a prism and asks which is correct — check for a bend at each face, then check the emergent ray leans towards the base.
Related PYQs
When a light beam falls on a triangular glass prism, a band of colours is obtained. Which one of the following statements is correct in this regard?
- (a) Red light bends the most, as the refractive index of glass for red light is greatest.
- (b) Red light bends the most, as the refractive index of glass for red light is lowest.
- (c) Violet light bends the most, as the refractive index of glass for violet light is greatest.
- (d) Violet light bends the most, as the refractive index of glass for violet light is lowest.
Answer(c) Violet light bends the most, as the refractive index of glass for violet light is greatest.
The very next item in the same paper takes the same prism a step further — once the ray is deviated towards the base, the colours separate, and it asks which colour bends most and why.
Practice
- practice — not a real PYQ
A ray of light passes through a glass slab with parallel faces. The emergent ray is
- (a)deviated towards the base
- (b)parallel to the incident ray but laterally displaced
- (c)bent towards the normal
- (d)totally internally reflected
Answer(b) parallel to the incident ray but laterally displaced — the two parallel faces produce equal and opposite bends. - practice — not a real PYQ
When a ray of light travels from air into glass, it
- (a)bends away from the normal
- (b)bends towards the normal
- (c)is undeviated
- (d)is reflected back
Answer(b) bends towards the normal — glass is the denser medium, so the angle of refraction is smaller than the angle of incidence.