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Prism, Deviation, Dispersion and Dispersive Power

A prism deviates light because refraction occurs at two inclined surfaces. Dispersion is the splitting of white light into its constituent colours because refractive index depends on wavelength.

In one line: In glass prism dispersion, violet is the most deviated and red is the least deviated.

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Understand it

  • When a ray passes through a prism, it bends towards the base inside the prism and emerges deviated from its original direction.
  • For a thin prism, deviation is approximately proportional to refractive index and prism angle.
  • Since violet light has a greater refractive index in glass than red light, violet deviates more than red.
  • Angular dispersion measures the separation between colours, while dispersive power compares dispersion with mean deviation.

Example

A glass prism forms a spectrum from white light, with violet deviated more than red. This is the basis for explaining dispersion by prisms in school optics.

Formula and rule

For a thin prism: delta = (μ - 1)A, where delta is deviation, μ is refractive index and A is prism angle.

Angular dispersion = deltaV - deltaR.

Dispersive power ω = (deltaV - deltaR)/deltaY or approximately (μV - μR)/(μY - 1).

Angles must be in the same unit, usually degrees or radians consistently.

See it worked out

Follow the example line by line, then use the same method on new questions.

Worked example

For a thin prism with A = 6 degrees and μ = 1.5, deviation delta = (μ - 1)A = 0.5 × 6 degrees = 3 degrees.

  1. 1If deltaV = 3.2 degrees and deltaR = 2.8 degrees, angular dispersion = 0.4 degrees.
  2. 2This means violet and red emerge separated by 0.4 degrees.

Method you can reuse

Identify whether thin prism approximation is allowed; write delta = (μ - 1)A; calculate deviation for given colour or mean ray; subtract red deviation from violet deviation for angular dispersion; use dispersive power formula if required.

Visual tip

Diagram should show triangular prism, incident ray, emergent ray, angle of prism A, angle of deviation delta, base, normal at both surfaces, and spectrum order with violet more deviated than red.

Check yourself

Answer the question in your head before opening the answer.

Try it yourself

Why does violet light deviate more than red light in a glass prism?

Think first, then show answer
In glass, violet light has a higher refractive index than red light, so it travels slower in glass and bends more at the prism surfaces. Therefore violet has greater deviation than red.

Common wrong answer

A common wrong answer is deltaR - deltaV for angular dispersion, which gives a negative value if violet deviation is larger.

Don't mix this up

Students often say colours split because different colours have different speeds in vacuum. In vacuum all colours of light travel with the same speed; dispersion in a prism occurs because their speeds differ in the material.

Exam tip

In prism diagrams, mark the base direction and remember that deviation is towards the base, while violet bends more than red in glass.

Write it in the exam

Each point is worth about one mark. Write point 1 for a 1-mark question and keep adding points as the marks go up.

How CBSE asks it

Questions test thin prism deviation, angular dispersion, dispersive power, colour order, and explanation of why prisms disperse white light.
  1. 1

    Enough for 1 mark

    A prism deviates light because refraction occurs at two inclined surfaces. Dispersion is the splitting of white light into its constituent colours because refractive index depends on wavelength.

  2. 2

    Add for 2 marks

    Example: A glass prism forms a spectrum from white light, with violet deviated more than red. This is the basis for explaining dispersion by prisms in school optics.

  3. 3

    Add for 3 marks

    When a ray passes through a prism, it bends towards the base inside the prism and emerges deviated from its original direction.

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