LIWA

Spectrum

ICSE Class 10 · Physics · Chapter 6

What this chapter covers

Key definitions

Wavelength is the distance between two consecutive crests (or two consecutive troughs) of a wave, and it is written as λ.

Frequency is the number of complete waves that pass a point in one second, and it is written as f; its SI unit is the hertz (Hz).

Angle of the prism is the angle between the two refracting faces of a prism, written as A; for a common laboratory prism it is 60°.

Angle of deviation is the angle between the direction of the incident ray produced forward and the direction of the emergent ray, and it is written as δ.

Dispersion is the splitting of white light into its constituent colours when it passes through a transparent medium such as a glass prism.

A spectrum is the coloured band obtained on a screen when white light is dispersed, in which the colours are arranged in order of their wavelengths.

Monochromatic light is light of a single colour, that is, of a single wavelength or a very narrow range of wavelengths.

Polychromatic light is light made up of many colours (many wavelengths), and white light is the commonest example.

Electromagnetic waves are waves made of electric and magnetic fields that vary together, at right angles to each other and to the direction in which the wave travels, and that need no material medium for their propagation.

The electromagnetic spectrum is the complete range of electromagnetic radiations arranged in order of their wavelengths (or frequencies).

Infrared radiation is electromagnetic radiation of wavelength longer than that of red light, from about 800 nm to about 1 mm, which produces a heating effect when absorbed.

Ultraviolet radiation is electromagnetic radiation of wavelength shorter than that of violet light, from about 10 nm to about 400 nm, which has a strong chemical effect.

Key points

Questions answered

Define the angle of deviation produced by a prism.
The angle of deviation is the angle between the direction of the incident ray produced forward and the direction of the emergent ray.
State four factors on which the angle of deviation produced by a prism depends.
The angle of incidence, the material (refractive index) of the prism, the angle of the prism, and the colour (wavelength) of the light.
Why does a glass prism deviate violet light more than red light?
Violet light travels more slowly in glass than red light, so the refractive index of glass is greater for violet. A greater refractive index means greater bending at each face, so violet is deviated more.
What is dispersion? Name the colours of the visible spectrum in order of increasing wavelength.
Dispersion is the splitting of white light into its constituent colours when it passes through a medium such as a prism. In order of increasing wavelength the colours are violet, indigo, blue, green, yellow, orange and red.
Why does a rectangular glass block not produce a spectrum of white light?
Its two faces are parallel, so the second refraction exactly undoes the first. All colours emerge parallel to one another and to the incident ray, and they overlap to give white light.
A beam of green light is passed through a prism. Will it be dispersed? Give a reason.
No. Green light is monochromatic, containing only one colour, so the prism deviates it but cannot split it into other colours.
A ray enters a prism of angle 60° at i1 = 50° and emerges at i2 = 40°. Find the deviation.
δ = i1 + i2 - A = 50° + 40° - 60° = 30°.
Arrange the following in order of increasing wavelength: X-rays, radio waves, infrared, ultraviolet, gamma rays, microwaves, visible light.
Gamma rays, X-rays, ultraviolet, visible light, infrared, microwaves, radio waves.

Past board exam questions

ICSE 2027 sample paper · 1 mark

Assertion (A): Blue light scatters more than red light in the atmosphere. Reason (R): Intensity of scattered light is inversely proportional to the fourth power of wavelength.

  1. (A) is true and (R) is false.
  2. (A) is false and (R) is true.
  3. Both (A) and (R) are true, and (R) is the correct explanation of (A).
  4. Both (A) and (R) are true, but (R) is not the correct explanation of (A).
Show answer
Both (A) and (R) are true, and (R) is the correct explanation of (A).

ICSE 2026 board paper · 1 mark

When a blackened bulb thermometer is moved beyond the red region of the visible spectrum, there is a rapid rise in the temperature. This is due to the presence of:

  1. Infrared radiations
  2. Ultraviolet radiations
  3. X-rays
  4. Radio waves
Show answer
Infrared radiations

ICSE 2026 sample paper · 1 mark

Inside the prism, during the dispersion of white light, compared to blue, red light:

  1. slows down less and refracts more.
  2. slows down more and refracts less.
  3. slows down more and refracts more.
  4. slows down less and refracts less.
Show answer
slows down less and refracts less.

ICSE 2026 board paper · 1 mark

Assertion (A): Tiny air molecules scatter blue light more than red light. Reason (R): The refractive index of a medium is greater for blue light than red light.

  1. (A) is true but (R) is false.
  2. (A) is false but (R) is true.
  3. Both (A) and (R) are true and (R) is the correct explanation of (A).
  4. Both (A) and (R) are true but (R) is not the correct explanation of (A).
Show answer
Both (A) and (R) are true but (R) is not the correct explanation of (A).

ICSE 2026 sample paper · 1 mark

Assertion(A): Soldiers avoid firing at a target in foggy weather conditions. Reason(R): In foggy weather, light gets scattered by tiny water droplets, reducing visibility.

  1. (A) is true but (R) is false.
  2. (A) is false but (R) is true.
  3. Both (A) and (R) are true and (R) is the correct explanation of (A).
  4. Both (A) and (R) are true, but (R) is not the correct explanation of (A).
Show answer
Both (A) and (R) are true and (R) is the correct explanation of (A).

ICSE 2026 sample paper · 4 marks

(a) A mixture of red, blue, and green light rays, travelling parallel to the principal axis, is passed through a convex lens. State whether the ray passes through a single point or through different points on the principal axis after refraction. (b) Name the invisible radiation which is studied using the quartz prism. (c) State one use of the radiation mentioned by you in (b) above. (d) Name one type of radiation with a wavelength greater than that of the radiation mentioned by you in (b) above.

Show answer
(a) Through different points. The lens refracts blue the most and red the least (dispersion), so blue meets the principal axis closest to the lens, then green, then red farthest. (b) Ultraviolet radiation (glass absorbs it, quartz transmits it). (c) Sterilising water, surgical instruments etc. by killing germs (or: detecting the purity of gems/eggs; producing vitamin D in the skin). (d) Visible light (or infrared, microwaves, radio waves).

ICSE 2025 sample paper · 1 mark

White light is dispersed by a prism. Inside the prism, compared to the blue light, the red light

  1. slows down less and refracts more
  2. slows down more and refracts less
  3. slows down more and refracts more
  4. slows down less and refracts less
Show answer
slows down less and refracts less

ICSE 2025 board paper · 1 mark

Which one of the following combinations is the correct ascending order of electromagnetic waves in terms of wavelength?

  1. gamma-rays, visible light, microwaves
  2. microwaves, visible light, gamma-rays
  3. gamma-rays, microwaves, visible light
  4. microwaves, gamma-rays, visible light
Show answer
gamma-rays, visible light, microwaves