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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.

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Student-friendly explanation

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.

How to write this in exams

  1. 1

    Start with the exact idea

    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

    Then show how to use it

    Identify whether thin prism approximation is allowed; write delta = (mu - 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.

  3. 3

    Add one concrete 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.

  4. 4

    Avoid this incomplete answer

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

Definition

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.

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.

Rule to remember

For a thin prism: delta = (mu - 1)A, where delta is deviation, mu is refractive index and A is prism angle. Angular dispersion = delta_V - delta_R. Dispersive power omega = (delta_V - delta_R)/delta_Y or approximately (mu_V - mu_R)/(mu_Y - 1). Angles must be in the same unit, usually degrees or radians consistently.

Memory hook

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

Examples and method

Worked example

For a thin prism with A = 6 degrees and mu = 1.5, deviation delta = (mu - 1)A = 0.5 x 6 degrees = 3 degrees. If delta_V = 3.2 degrees and delta_R = 2.8 degrees, angular dispersion = 0.4 degrees. This means violet and red emerge separated by 0.4 degrees.

Method to apply

Identify whether thin prism approximation is allowed; write delta = (mu - 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.

Diagram support

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.

How CBSE asks it

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

Avoid common mistakes

Common confusion

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.

Common wrong answer

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

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.

Quick check

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

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.

Answer writing and exam use

1-mark answer

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-mark answer

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. For a thin prism: delta = (mu - 1)A, where delta is deviation, mu is refractive index and A is prism angle. Angular dispersion = delta_V - delta_R. Dispersive power omega = (delta_V - delta_R)/delta_Y or approximately (mu_V - mu_R)/(mu_Y - 1). Angles must be in the same unit, usually degrees or radians consistently. 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-mark answer

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. For a thin prism: delta = (mu - 1)A, where delta is deviation, mu is refractive index and A is prism angle. Angular dispersion = delta_V - delta_R. Dispersive power omega = (delta_V - delta_R)/delta_Y or approximately (mu_V - mu_R)/(mu_Y - 1). Angles must be in the same unit, usually degrees or radians consistently. For a thin prism with A = 6 degrees and mu = 1.5, deviation delta = (mu - 1)A = 0.5 x 6 degrees = 3 degrees. If delta_V = 3.2 degrees and delta_R = 2.8 degrees, angular dispersion = 0.4 degrees. This means violet and red emerge separated by 0.4 degrees. Questions test thin prism deviation, angular dispersion, dispersive power, colour order, and explanation of why prisms disperse white light. A common wrong answer is delta_R - delta_V for angular dispersion, which gives a negative value if violet deviation is larger.
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