Refraction of light through a glass prism
A triangular glass prism has two slanting faces (refracting surfaces). The angle between them is the angle of the prism (A).
- At the first face light goes from air into glass (rarer → denser) and bends towards the normal.
- At the second face it goes from glass into air and bends away from the normal.
- Both bends turn the ray towards the base of the prism.
The angle between the incoming ray (extended) and the outgoing ray is the angle of deviation (D). Unlike a glass slab, the outgoing ray of a prism is not parallel to the incoming ray, because the two faces are not parallel.
Words to draw in a diagram
Incident ray, refracted ray, emergent ray, angle of incidence (i), angle of refraction (r), angle of emergence (e), angle of deviation (D).
Dispersion of white light
When a narrow beam of white light passes through a prism, a band of seven colours appears on a screen. This band is the spectrum and the splitting is called dispersion.
Order from the base upward: Violet, Indigo, Blue, Green, Yellow, Orange, Red (VIBGYOR).
Why do colours separate?
In a vacuum all colours travel at the same speed. In glass, violet light slows down the most and red the least, so glass has a slightly larger refractive index for violet. Larger refractive index → more bending. So violet deviates most, red least.
Recombination (Newton's experiment)
Isaac Newton placed a second, identical prism upside down after the first. The colours came out as white light again. This proved that the prism does not add colour; white light is itself a mixture of all colours.
How a rainbow forms
A rainbow is a natural spectrum in the sky after rain. Tiny water drops floating in the air act like small prisms:
- Refraction and dispersion as sunlight enters the drop.
- Internal reflection at the back of the drop.
- Refraction again as the light comes out, spreading the colours further.
Red reaches our eye from drops higher in the sky (about 42°) and violet from lower drops (about 40°), so red is on the outer edge and violet on the inner edge. A rainbow is always seen in the direction opposite the Sun: the Sun must be behind you. You can also make one with a water sprinkler on a sunny morning.
Board-exam focus
Common questions: draw the path of a ray through a prism and mark i, e and D (2–3 marks); why violet bends most; what dispersion and spectrum mean; Newton's recombination; the three steps in rainbow formation and why it is opposite the Sun.
Key formulas and definitions
- Dispersion: splitting of white light into its colours
- Spectrum order (least to most bent): R O Y G B I V
- Refractive index: n(violet) > n(red) in glass, so deviation D(violet) > D(red)
- Prism: ray bends towards the base at both faces
- Rainbow = refraction + dispersion + internal reflection + refraction
Worked examples
1. A ray of white light passes through a prism. Which colour appears nearest the base on the screen, and why?
Violet. Glass has the highest refractive index for violet, so violet is bent the most and ends up nearest the base.
2. Why does a glass slab not split white light into colours the way a prism does?
In a slab the two faces are parallel, so the bending at the second face exactly undoes the first. The colours come out parallel and overlap into white again (only a tiny sideways shift). In a prism the faces are inclined, so the bends add up and the colours separate.
3. How would you show that white light is made of seven colours?
Pass a narrow beam through a prism to get a spectrum. Then place an identical prism upside down in the path: the colours join back into white light. Splitting and rejoining shows white light is a mixture.
4. Why is a rainbow never seen at noon in the south sky in India?
A rainbow forms opposite the Sun at about 40–42° from the point directly opposite it. At noon the Sun is high, so that point is far below the horizon and the bow cannot be seen. Rainbows appear in the morning or late afternoon.
Common mistakes
- Writing that the prism creates the colours. The colours were already in white light; the prism only separates them.
- Saying red bends the most. Red bends the least; violet the most.
- Drawing the emergent ray bending towards the apex. At both faces the ray turns towards the base.
- Thinking a rainbow can be seen while facing the Sun. It is always opposite the Sun.