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NDA Physics · Light and Optics

Prisms and Dispersion

A prism refracts light twice and deviates it toward the base. White light splits into a spectrum because the glass refracts each colour by a different amount — violet (shortest wavelength, highest refractive index) bends most, red bends least. A rainbow is dispersion plus internal reflection inside water drops.

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Why this matters

Eight PYQs that cluster on one idea: the order of deviation in a prism. The recurring tests are which colour deviates most (violet/blue) and least (red), WHY (refractive index, not reflection), the standard ray-path figure, and the make-up of a rainbow. Almost every question is settled by the single rule 'violet bends most'.

Concept 1 of 3

Refraction through a prism and deviation

Intuition

A prism is a wedge of glass with two slanted faces. Light entering bends toward the normal at the first face and away from the normal at the second — both bends turn the ray the same way, toward the thick base of the prism. The total turning is the angle of deviation.

Definition

Light passing through a prism refracts at both faces and emerges deviated toward the base. The total turn is the angle of deviation δ\delta.

  • It depends on the angle of incidence, the prism angle AA, and the refractive index.
  • The deviation is a minimum at one symmetric angle (the angle of minimum deviation, δm\delta_m), where n=sin⁡ ⁣(A+δm2)sin⁡ ⁣(A2)n = \dfrac{\sin\!\left(\frac{A + \delta_m}{2}\right)}{\sin\!\left(\frac{A}{2}\right)}.

The bending is refraction (a speed change at each face), not reflection.

basewhite lightredvioletViolet bends most, red bends least — the glass refracts short wavelengths more

The prism refracts each colour by a different amount because the glass has a higher refractive index for shorter (violet) wavelengths, so violet deviates most and red least.

Worked example

Trace what happens to a single ray of light entering one slanted face of a glass prism. Which way does it ultimately turn?
  1. Entering the denser glass, the ray bends toward the normal at the first face.
  2. Leaving into air at the second face, it bends away from the normal.
  3. Both refractions turn the ray the same way — toward the base of the prism.
Answer:It refracts at both faces and deviates toward the base of the prism.
Practice this conceptself-check · 4 quick reps

Try it yourself

Is the deviation of a ray by a glass prism caused by reflection or refraction of light?

Practice — Level 1 (4 reps)

Quick reps to lock in the method. Try each, then check.

  1. 1.
    A prism deviates light toward its apex or its base?
  2. 2.
    How many times does a ray refract passing through a prism?
  3. 3.
    Prism deviation is caused by reflection or refraction?
  4. 4.
    At minimum deviation, the ray inside the prism is…

From the bank · past-year question

Example 1Light and OpticsMODERATE
Which one of the following figures correctly shows the path of a ray of light through a glass prism?

[Q146 · Sep · 2021]

Deviation is refraction, not reflection

A prism bends light because it refracts at each face. Options blaming 'reflection' for the deviation or the colour spread are wrong — both are refraction effects.
Drill 1 more on refraction through a prism and deviation

Concept 2 of 3

Dispersion — why violet bends most

Intuition

White light is a mix of colours, and the glass refracts each colour by a slightly different amount. Violet has the shortest wavelength and the highest refractive index in glass (so the slowest speed there), so it bends the most. Red has the longest wavelength, the lowest index, the highest speed in glass — so it bends the least. That spread of bend angles is dispersion.

Definition

Dispersion is the splitting of white light into its component colours (VIBGYOR) by a prism.

  • A medium's refractive index is highest for violet, lowest for red — so violet's speed in glass is the lowest and red's the highest.
  • Hence violet deviates the most, red the least.
  • The cause is refraction (wavelength-dependent index), not reflection.
  • Order of increasing deviation: Red < Orange < Yellow < Green < Blue < Violet.

Worked example

When white light passes through a glass prism, which colour deviates the most, and why?
  1. Violet has the shortest wavelength of visible light.
  2. Glass has its highest refractive index for violet (so violet travels slowest in glass).
  3. A higher refractive index means a larger bend, so violet deviates the most.
Answer:Violet — because the glass's refractive index is greatest (and its speed lowest) for violet.
Practice this conceptself-check · 4 quick reps

Try it yourself

In the dispersion of white light by a prism, which colour deviates the most and what is the correct reason about its speed in the prism?

Practice — Level 1 (4 reps)

Quick reps to lock in the method. Try each, then check.

  1. 1.
    Which colour deviates the most through a prism?
  2. 2.
    Which colour deviates the least?
  3. 3.
    Glass has its highest refractive index for which colour?
  4. 4.
    Which colour travels fastest inside the glass prism?

From the bank · past-year question

Example 2Light and OpticsMODERATE
In the dispersion of white light by a common glass prism, which one among the following is correct?

[Q148 · Apr · 2023]

Violet bends most because its speed in glass is LOWEST

The chain is: shortest wavelength → highest refractive index in glass → lowest speed in glass → greatest deviation. Distractors flip the speed ('highest speed') or swap red and violet. Red is the fast, least-bent one.
Drill 3 more on dispersion — why violet bends most

Concept 3 of 3

The rainbow

Intuition

A rainbow is the sky's prism: each raindrop refracts sunlight on the way in, reflects it once off the back, and refracts it again on the way out — splitting the light into colours. The primary rainbow uses one internal reflection and is the brighter, inner bow; the secondary uses two reflections, is dimmer, and sits outside with the colours reversed.

Definition

A rainbow forms by dispersion in water droplets:

  • Primary rainbow: sunlight undergoes refraction → one internal reflection → refraction inside each drop. It is the inner (lower) bow, brighter, with red on the outside.
  • Secondary rainbow: involves two internal reflections, is fainter, sits outside the primary, with colours reversed.

So the rainbow is fundamentally a dispersion phenomenon (refraction + internal reflection combined), not simple reflection.

Worked example

Which statements about the PRIMARY rainbow are correct? (1) It involves refraction and one internal reflection of sunlight. (2) It involves refraction of sunlight only. (3) It is the inner bow. (4) It may involve more than one internal reflection.
  1. Primary rainbow = refraction (entry) + ONE internal reflection + refraction (exit) → (1) is correct.
  2. It is NOT refraction only — internal reflection is involved → (2) is wrong.
  3. It is the inner/lower bow compared to the secondary → (3) is correct.
  4. More than one internal reflection describes the SECONDARY rainbow → (4) is wrong.
Answer:Statements 1 and 3 are correct.
Practice this conceptself-check · 4 quick reps

Try it yourself

A rainbow in the sky is produced primarily by which optical phenomenon?

Practice — Level 1 (4 reps)

Quick reps to lock in the method. Try each, then check.

  1. 1.
    A rainbow is produced by which phenomenon?
  2. 2.
    How many internal reflections form a primary rainbow?
  3. 3.
    Is the primary rainbow the inner or outer bow?
  4. 4.
    The secondary rainbow involves how many internal reflections?

From the bank · past-year question

Example 3Light and OpticsMODERATE
Which of the following statements with regard to the phenomenon of the primary rainbow formation by water droplets is/are correct ? 1. It involves refraction and one internal reflection of sunlight. 2. It involves refraction of sunlight only. 3. It is formed as the inner bow. 4. It may involve more than one internal reflection as well as refraction of sunlight. Select the answer using the code given below :

[Q129 · Apr · 2024]

Primary rainbow = ONE internal reflection (the inner bow)

The primary bow has exactly one internal reflection and is the inner bow; two reflections make the fainter secondary (outer) bow. 'Refraction only' is wrong — reflection is always part of it.
Drill 1 more on the rainbow

Summary — formulas & gotchas at a glance

A revision cheat-sheet for the formulas and gotchas above. Click any concept name to jump back to its full explanation.

Watch out for (3)

Mastery check — 5 interleaved questions

Try each one before clicking. Questions are interleaved across the concepts above, not grouped — interleaving sharpens transfer.

Example 1Light and OpticsHARD
A light ray falls on a face of an equilateral glass prism as shown (ray at 60° to the face normal). Which diagram correctly shows the complete path of the ray through the glass prism?

[Q73 · Apr · 2026]

Example 2Light and OpticsMODERATE
A glass prism splits white light into different colours. This phenomenon is called dispersion of light by prism. Which one of the following statements is correct ?

[Q76 · Apr · 2021]

Example 3Light and OpticsEASY
A rainbow is produced due to which one of the following phenomena?

[Q125 · Sep · 2017]

Example 4Light and OpticsMODERATE
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?

[Q147 · Sep · 2021]

Example 5Light and OpticsEASY
When a beam of white light passes through a glass prism, the colour of light beam that deviates the least is

[Q118 · Apr · 2019]

Drill every past-year question on this subtopic

8 questions from the bank — paginated, with cart and Word-export support.

Drill the 8 questions