Spectrum and Scattering of Light ICSE Class 10: Dispersion, Electromagnetic Spectrum, IR, UV and the Blue Sky

What Spectrum and Scattering of Light is about

Weightage: CISCE publishes no chapter or unit weightage for ICSE Physics. Spectrum and Scattering of Light is in the 2. Light (iv) part of the 2027 syllabus and can be asked in both Section A and Section B of the 80-mark paper.Source: CISCE ICSE Physics (Science Paper 1) syllabus, examination year 2027

Key concepts

Dispersion of white light by a prism

When white light passes through a glass prism, it splits into its constituent colours: violet, indigo, blue, green, yellow, orange, red (VIBGYOR). This splitting is dispersion, and the band of colours is a spectrum.

Cause: the refractive index of glass is different for different wavelengths: largest for violet (shortest wavelength), smallest for red (longest). So violet travels slowest in glass and is deviated most; red is deviated least. In vacuum all colours travel at the same speed, $3\times 10^8$ m/s.

A parallel-sided glass slab does not produce a visible spectrum: the colours separate inside but emerge parallel to each other and overlap again.

Worked example

In the spectrum from a glass prism, which colour is (a) deviated most, (b) deviated least? Give the reason.

Try it first, then show the solutionSolution

1. (a) Violet: glass has its largest refractive index for violet (shortest wavelength).
2. (b) Red: smallest refractive index (longest wavelength).

Answer: (a) violet (b) red; μ depends on wavelength

Wavelength, frequency and colour

All light obeys $c=f\lambda$. The visible range runs from about 400 nm (violet) to about 700 nm (red); $1\text{ nm}=10^{-9}$ m. A longer wavelength means a lower frequency.

Colour is decided by frequency, which does not change on refraction; the wavelength in glass is shorter than in air.

Worked example

Find the frequency of yellow light of wavelength 600 nm in air.

Try it first, then show the solutionSolution

1. $f=\frac{c}{\lambda }=\frac{3\times 10^8}{600\times 10^{-9}}$.
2. $f=5\times 10^{14}$ Hz.

Answer: 5 × 10¹⁴ Hz

The electromagnetic spectrum

In order of increasing wavelength (decreasing frequency):
gamma rays → X-rays → ultraviolet → visible light → infrared → microwaves → radio waves.

Mnemonic: *Go X-ray Under Violet, In Many Rooms* (or any phrase you prefer).

Uses to remember: gamma rays (cancer treatment), X-rays (detecting bone fractures), ultraviolet (sterilising), visible (sight), infrared (remote controls, night vision), microwaves (radar, cooking, mobile phones), radio waves (radio and TV broadcasting).

Worked example

Arrange in increasing order of wavelength: microwaves, X-rays, infrared, gamma rays.

Try it first, then show the solutionSolution

1. Shortest: gamma rays, then X-rays.
2. Then infrared, then microwaves.

Answer: Gamma rays, X-rays, infrared, microwaves

Properties common to all electromagnetic radiations

• They travel through vacuum (no material medium needed) at the same speed, $c=3\times 10^8$ m/s.
• They are transverse waves.
• They obey $c=f\lambda$.
• They show reflection and refraction and obey their laws.
• They are not deflected by electric or magnetic fields (they carry no charge).

Do not mix these up with properties of α, β and γ radiations from radioactivity, or with sound (a mechanical wave that needs a medium).

Worked example

State two properties common to radio waves and microwaves.

Try it first, then show the solutionSolution

1. Both travel through vacuum at $3\times 10^8$ m/s.
2. Both are transverse electromagnetic waves, undeflected by electric and magnetic fields.

Answer: Speed 3 × 10⁸ m/s in vacuum; transverse, not deflected by fields

Infrared radiation: properties and uses

Infrared (IR) lies just beyond red, with longer wavelengths (about 700 nm to 1 mm).

Properties: emitted by all hot bodies (the Sun, heaters, our bodies); strong heating effect; detected by a thermometer with a blackened bulb or a thermopile; absorbed by glass, so a rock salt prism is used to study the IR spectrum; scattered less than visible light, so it passes through haze and fog better.

Uses: TV remote controls; night-vision cameras and thermal imaging; physiotherapy (heat treatment); drying paint and food; weather forecasting and remote sensing from satellites; signalling.

Worked example

Which radiation do remote controls and night-vision devices use, and why is a rock salt prism used to study it?

Try it first, then show the solutionSolution

1. Infrared radiation.
2. Ordinary glass absorbs much infrared; rock salt lets it pass.

Answer: Infrared; glass absorbs IR, rock salt transmits it

Ultraviolet radiation: properties and uses

Ultraviolet (UV) lies just beyond violet, with shorter wavelengths (about 10 nm to 400 nm).

Properties: produced by the Sun, electric arcs and mercury-vapour lamps; strong chemical effect (affects photographic film); causes fluorescence; kills germs; absorbed by ordinary glass, so a quartz prism is used to study the UV spectrum; harmful in excess to skin and eyes; mostly absorbed by the ozone layer.

Uses: sterilising water, food and surgical instruments; detecting forged notes and documents (fluorescence); helping the skin make vitamin D; in fluorescent lamps.

Do not confuse ultraviolet (electromagnetic) with ultrasonic (sound above 20 kHz), or infrared with infrasonic.

Worked example

Name the radiation used to sterilise drinking water and the prism material used to study its spectrum.

Try it first, then show the solutionSolution

1. Ultraviolet radiation (kills germs).
2. A quartz prism, since glass absorbs UV.

Answer: Ultraviolet; quartz

Scattering of light: blue sky, red sunset, white clouds

When light falls on particles much smaller than its wavelength (air molecules), it is scattered in all directions. The amount scattered is much greater for shorter wavelengths (it varies as $\frac{1}{{\lambda }^4}$).

• Blue sky: blue (short wavelength) is scattered far more than red, so scattered blue light reaches our eyes from every part of the sky. (Violet is scattered even more, but sunlight has less violet and our eyes are less sensitive to it.)
• Red Sun at sunrise and sunset: light travels a long path through the atmosphere; most blue is scattered away, leaving red and orange.
• Red danger signals: red is scattered least, so it travels farthest through air, fog and dust.
• White clouds: water droplets are larger than the wavelengths, so they scatter all colours nearly equally.
• Black sky in space: no air to scatter light.

Worked example

Compare the scattering of blue light (400 nm) with red light (700 nm) by air molecules.

Try it first, then show the solutionSolution

1. Scattering $\propto \frac{1}{{\lambda }^4}$.
2. Ratio $={\left(\frac{700}{400}\right)}^4\approx 9.4$.

Answer: Blue is scattered about 9.4 times as much as red.

Physics in real life: Spectrum and Scattering of Light

Each concept, met in an everyday situation. Read the situation, try the problem, then check the solution. The one-line hook is what to remember.

Topics in this chapter

• Electromagnetic Spectrum: Order, Properties, IR and UV
• Dispersion of White Light by a Prism
• Scattering of Light: Blue Sky and Red Sunset

Spectrum and Scattering of Light formula sheet

Wave equation for light

$c=f\lambda$

c = 3 × 10⁸ m/s in vacuum.

Visible range

$\lambda \approx 400\text{nm (violet)}\text{ to }700\text{nm (red)}$

1 nm = 10⁻⁹ m.

Scattering by small particles

$\text{intensity scattered}\propto \frac{1}{{\lambda }^4}$

Shorter wavelength, much more scattering.

Order of the electromagnetic spectrum

$\gamma <\text{X}<\text{UV}<\text{visible}<\text{IR}<\text{micro}<\text{radio}$

Increasing wavelength, decreasing frequency.

Spectrum and Scattering of Light: diagrams to practise

Practise each one on paper: draw it from the checklist, then tick off every label.

How the board asks Spectrum and Scattering of Light

The question patterns to expect, and the method that works for each.

Tricks to remember Spectrum and Scattering of Light

Short memory aids. Say each one aloud once, then use it on a question the same day.

• VIBGYOR, red is lazy

  Red bends least and scatters least; violet bends most and scatters most.

  Use it when: Dispersion and scattering.

• Go X-ray Under Violet, In Many Rooms

  Gamma, X-rays, UV, Visible, IR, Microwaves, Radio: increasing wavelength.

  Use it when: EM spectrum order.

• Ultra-violet vs ultra-sonic

  -violet is light (electromagnetic); -sonic is sound (mechanical).

  Use it when: Avoiding the common mix-up.

• Quartz for UV, rock salt for IR

  Q is near U in the alphabet's end; Rock salt for Red-side (infrared).

  Use it when: Prism materials.

Spectrum and Scattering of Light: board-level solved questions

Ordered from board level to HOTS. Cover the solution, write your own, then compare step by step.

Case-study practice (Section B style)

Read the situation, then answer every part. In the board paper this is a 10-mark question split into 3 + 3 + 4 marks.

Where the marks are

How a typical answer is split into step marks. Show every step: a right final answer alone rarely earns full marks.

Where students lose marks in Spectrum and Scattering of Light

The slips examiners see most often, and what to write instead.

• ✗Mistake: Wrong order of the spectrum

  CISCE examiners' comments, 2024 and 2025: candidates did not recall the order of radiations by wavelength, and in an assertion–reason item did not relate scattering to wavelength (UV has a shorter wavelength than microwaves).

  ✓Do this: Learn γ, X, UV, visible, IR, micro, radio (increasing λ); shorter λ is scattered more.

  Usually costs: 1 mark.

• ✗Mistake: Uses given instead of properties

  CISCE examiners' comments, 2024: asked for a property common to two radiations, candidates gave uses or vague answers such as 'long or short wavelength'.

  ✓Do this: Common properties: travel in vacuum at 3 × 10⁸ m/s, transverse, not deflected by electric or magnetic fields.

  Usually costs: 1 mark.

• ✗Mistake: Ultraviolet vs ultrasonic, infrared vs infrasonic

  CISCE examiners' comments, 2025: candidates confused ultraviolet with ultrasonic and infrared with infrasonic, and mixed up electromagnetic radiations with radioactive radiations.

  ✓Do this: -violet and -red are light (electromagnetic); -sonic is sound. α and β are particles, not electromagnetic waves.

  Usually costs: 1 mark each.

• ✗Mistake: Red signals explained by refraction

  CISCE examiners' comments, 2024: for red danger signals, candidates related red to refraction or just said it can be seen from far, instead of 'red is scattered least'.

  ✓Do this: Name the reason: least scattering because of the longest wavelength.

  Usually costs: 1 mark.

• ✗Mistake: Not naming the prism material

  The 2025 paper asked which radiation is studied with a quartz prism and which is used in remote sensing; the CISCE examiners reported that candidates confused ultraviolet with ultrasonic and infrared with infrasonic in that question.

  ✓Do this: UV → quartz; IR → rock salt; remote sensing uses infrared.

  Usually costs: 1 mark.

Practice questions on Spectrum and Scattering of Light

Write each answer on paper first. Open the hint only if you are stuck, and the answer only when you have finished.

1. Q1 · 1 mark · MCQBoard level

   Which of these has the longest wavelength?
   (a) X-rays
   (b) ultraviolet
   (c) infrared
   (d) radio waves

   Hint

   End of the spectrum.

   Answer

   (d)

2. Q2 · 1 mark · MCQBoard level

   The spectrum of ultraviolet radiation is studied using a prism made of:
   (a) glass
   (b) quartz
   (c) rock salt
   (d) plastic

   Hint

   Glass absorbs UV.

   Answer

   (b)

3. Q3 · 1 mark · MCQHarder board level

   The frequency of microwaves of wavelength 3 cm is:
   (a) $10^8$ Hz
   (b) $10^9$ Hz
   (c) $10^{10}$ Hz
   (d) $10^{12}$ Hz

   Hint

   f = c/λ with λ = 0.03 m.

   Answer

   (c)

4. Q4 · 1 mark · Assertion-reasonBoard level

   Assertion (A): Clouds appear white.
   Reason (R): Water droplets in clouds are much larger than the wavelengths of visible light, so they scatter all colours nearly equally.
   (a) (A) is true and (R) is false.
   (b) (A) is false and (R) is true.
   (c) Both (A) and (R) are true, and (R) is the correct explanation for (A).
   (d) Both (A) and (R) are true, but (R) is not the correct explanation for (A).

   Hint

   Particle size matters.

   Answer

   (c)

5. Q5 · 2 marks · Very short answerHarder board level

   Give two uses of infrared radiation.

   Hint

   Remote, night vision, therapy…

   Answer

   Any two: TV remote controls; night-vision and thermal cameras; heat therapy; remote sensing and weather forecasting; drying paint.

6. Q6 · 2 marks · Very short answerHarder board level

   Give two uses of ultraviolet radiation.

   Hint

   Germs and forgeries.

   Answer

   Any two: sterilising water and instruments; detecting forged notes; helping make vitamin D; fluorescent lamps.

7. Q7 · 3 marks · Long answerBoard level

   State three properties common to all electromagnetic radiations.

   Hint

   Speed, nature, fields.

   Answer

   They travel through vacuum at 3 × 10⁸ m/s; they are transverse waves; they are not deflected by electric or magnetic fields (also: they obey c = fλ and the laws of reflection and refraction).

8. Q8 · 3 marks · Long answerHarder board level

   Compare the scattering of light of wavelength 450 nm with that of 650 nm. Which colours are these roughly, and which is scattered more?

   Hint

   (650/450)⁴.

   Answer

   450 nm is blue, 650 nm is red. Blue is scattered more, about ${\left(\frac{650}{450}\right)}^4\approx 4.35$ times as much.

9. Q9 · 3 marks · Long answerHOTS / competency

   Explain why (a) danger signals are red, (b) a blue light would be a poor choice in fog, (c) the sky seen from the Moon is black.

   Hint

   Scattering depends on wavelength and on air being present.

   Answer

   (a) Red is scattered least, so it is visible from far. (b) Blue is scattered strongly by fog and dust, so the beam spreads out and fades quickly. (c) There is no air on the Moon to scatter sunlight, so the sky is dark.

10. Q10 · 3 marks · Long answerHarder board level

    Why does a triangular prism form a spectrum while a parallel-sided glass slab does not?

    Hint

    What happens at the second face?

    Answer

    In a prism the second face is inclined, so it deviates each colour further in the same direction and the colours spread more. In a slab the second face is parallel to the first, so each colour emerges parallel to the incident beam and the colours overlap, appearing white.

11. Q11 · 1 mark · MCQHOTS / competency

    Which has a higher frequency, X-rays or ultraviolet radiation?
    (a) X-rays
    (b) ultraviolet
    (c) both the same
    (d) depends on the source

    Hint

    Shorter wavelength, higher frequency.

    Answer

    (a)

12. Q12 · 4 marks · Long answerHOTS / competency

    Name the region of the electromagnetic spectrum for each wavelength and give one use: (a) $10^{-11}$ m, (b) $5\times 10^{-7}$ m, (c) $10^{-5}$ m, (d) 10 m.

    Hint

    Visible is about 4–7 × 10⁻⁷ m.

    Answer

    (a) Gamma rays or X-rays (very short): e.g. X-rays for bone imaging. (b) Visible light (green, 500 nm): sight. (c) Infrared: remote controls. (d) Radio waves: broadcasting.

Test your recall

Answer in your head or on paper first, then open the card. Pulling an answer out of memory makes it stick far better than re-reading.

1. 1.Define dispersion.Show answer

   Splitting of white light into its constituent colours.

2. 2.Which colour deviates least?Show answer

   Red.

3. 3.EM spectrum in increasing wavelength?Show answer

   Gamma, X-rays, UV, visible, IR, microwaves, radio waves.

4. 4.Prism used for the UV spectrum?Show answer

   Quartz.

5. 5.Prism used for the IR spectrum?Show answer

   Rock salt.

6. 6.Why is the sky blue?Show answer

   Air molecules scatter blue light much more than red.

7. 7.Speed of all EM waves in vacuum?Show answer

   3 × 10⁸ m/s

Quick revision checklist

The night before the exam: can you do each of these without looking?

• ☐Can you write VIBGYOR and say which deviates most?
• ☐Can you list the EM spectrum both ways?
• ☐Can you give three common properties?
• ☐Can you give two properties and two uses each of IR and UV?
• ☐Can you explain blue sky, red sunset, white clouds?
• ☐Do you avoid ultrasonic/ultraviolet mix-ups?

Spectrum and Scattering of Light: frequently asked questions