Spectrum Class 10 ICSE — Selina Chapter 6

The Spectrum class 10 ICSE chapter in Selina Concise Physics Chapter 6 explains how white light splits into its colours. It covers deviation by a prism, dispersion of white light, the full electromagnetic spectrum, and scattering of light. You will learn why violet bends more than red, how to use the wave relation c = f\lambda, and why the sky is blue. This chapter builds on the refraction ideas you met in earlier chapters.

Download Spectrum Class 10 ICSE Physics Chapter 6 PDF

Download the Spectrum Class 10 ICSE Physics chapter PDF from the Selina Concise textbook to revise offline or print for practice. It is the full chapter as printed in the book — 15 pages, 8 sections, 71 questions.

What is inside Chapter 6

15 pages · 8 sections · 71 exercise questions

• EXERCISE-6(A): 20 questions
• MULTIPLE CHOICE TYPE: 5 questions
• EXERCISE-6(B): 27 questions
• MULTIPLE CHOICE TYPE: 5 questions
• EXERCISE-6(C): 12 questions
• MULTIPLE CHOICE TYPE: 2 questions

6.1 DEVIATION PRODUCED BY A TRIANGULAR PRISM

When a ray of a single colour enters a triangular prism, it bends at both inclined faces. At the first surface it deviates by an angle \delta_1, and at the second surface by \delta_2. Both bends push the ray towards the base of the prism. The total deviation between the incident ray and the emergent ray is the sum of the two.

\delta = \delta_1 + \delta_2 \tag{6.1}

The total deviation depends on three factors. They are the angle of incidence i, the angle of the prism A, and the refractive index \mu of the prism material. The refractive index depends on the colour (or wavelength) of light. So the deviation depends on colour too. In glass, violet light has the least speed and red light the most. Therefore violet deviates the most and red the least.

Worked example pattern: suppose a ray deviates 18^\circ at the first surface and 24^\circ at the second surface. The total deviation is \delta = 18^\circ + 24^\circ = 42^\circ. That is the angle between the incident and emergent rays. This relation is the foundation of the chapter, because it shows that deviation depends on how each surface treats the ray. The refraction ideas here extend the work in Refraction through a Lens.

6.2 COLOURS IN WHITE LIGHT WITH THEIR WAVELENGTH AND FREQUENCY RANGE

White light is a mixture of light of different wavelengths. Each wavelength produces a different colour sensation on the retina. The seven prominent colours are violet, indigo, blue, green, yellow, orange and red. Wavelength is a characteristic of colour. Light of the same colour from different sources has the same wavelength.

The visible range runs from 4000 Å (violet) to 8000 Å (red). The unit Å means Angstrom, where 1\ \text{Å} = 10^{-10}\ \text{m}. Modern problems use nanometres: 1\ \text{nm} = 10^{-9}\ \text{m} = 10\ \text{Å}. So the visible range is 400 nm to 800 nm. Red has the longest wavelength and the lowest frequency, 3.75 \times 10^{14} Hz. Violet has the shortest wavelength and the highest frequency, 7.5 \times 10^{14} Hz.

Worked example pattern: find the frequency of green light of wavelength 540 nm. Use c = f\lambda, so f = c/\lambda = (3 \times 10^8)/(540 \times 10^{-9}) = 5.6 \times 10^{14} Hz. Green light oscillates about 5.6 \times 10^{14} times per second.

The refractive index of glass is \mu = \sin i / \sin r. Since \mu_{\text{violet}} \gt \mu_{\text{red}}, the angle of refraction satisfies r_{\text{red}} \gt r_{\text{violet}}. With deviation \delta = i - r, this gives \delta_{\text{violet}} \gt \delta_{\text{red}}. That is why the violet end bends more than the red end.

6.3 DISPERSION OF WHITE LIGHT THROUGH A PRISM AND FORMATION OF A SPECTRUM

Newton discovered dispersion while studying a lens image. He found the edges were coloured, even with a carefully polished lens. He concluded the fault lay in white light itself. In his experiment, sunlight entered a dark room through a small aperture and passed through a glass prism onto a white screen. A rainbow-like coloured patch appeared, which he called the spectrum.

Starting from the base of the prism, the colours appear in the order violet, indigo, blue, green, yellow, orange and red. The word VIBGYOR helps you remember this order. Each colour blends into the next; there is no sharp boundary. White light is polychromatic, meaning it contains many wavelengths.

Dispersion is the splitting of white light into its constituent colours. Spectrum is the band of colours seen on the screen. The cause is that speed in glass changes with wavelength. Dispersion occurs at the first surface of the prism. Deviation occurs at both surfaces. The prism does not produce colours; it only splits those already present in the light.

Three points commonly confuse students:

• Dispersion happens at the first surface only, not at the second.
• Deviation happens at both surfaces — dispersion and deviation are different phenomena.
• The prism never creates colour. It only separates colours already in the incident light.
• To convert between units, remember 1\ \text{nm} = 10^{-9}\ \text{m} = 10\ \text{Å}, so 400 nm is 4000 Å and 800 nm is 8000 Å.

With a parallel-sided glass slab, dispersion also occurs at the first surface. But at the second surface each colour bends back, so the emergent rays are parallel. They are so close that the light appears almost white again.

6.4 ELECTROMAGNETIC SPECTRUM

The visible spectrum is only a small part of the full electromagnetic spectrum. Beyond the red end lie infrared radiations, microwaves and radio waves, in increasing order of wavelength. Beyond the violet end lie ultraviolet rays, X-rays and gamma rays.

In increasing order of wavelength, the complete set is: gamma rays, X-rays, ultraviolet rays, visible light, infrared radiations, microwaves, radio waves. A mnemonic for this order is “Go, X-ray, Under Violet, Into My Room”. Take the first letter of each word: G, X, U, V, I, M, R.

All electromagnetic waves travel at the same speed in vacuum, c = 3 \times 10^8\ \text{m s}^{-1}. Their speed, frequency and wavelength are related by

c = f\lambda \tag{6.2}

Common properties of all electromagnetic waves:

• They need no material medium for propagation.
• In refraction, frequency stays unchanged, but speed and wavelength change.
• They are not deflected by electric or magnetic fields.
• They are transverse waves.
• The boundaries between regions overlap; there is no sharp cut-off.

Worked example pattern: an X-ray has wavelength 0.05 nm. Its frequency is f = c/\lambda = (3 \times 10^8)/(0.05 \times 10^{-9}) = 6 \times 10^{18} Hz. This is far above any visible frequency. For the full set of chapters, browse the Selina Concise Physics for Class 10 ICSE pages.

6.5 PROPERTIES AND USES OF DIFFERENT RADIATIONS OF ELECTROMAGNETIC SPECTRUM

Each radiation has its own source, properties and uses. The radiations are listed from shortest to longest wavelength. This order matters for questions that ask you to arrange radiations. The table below summarises the key points for quick reference.

The detectors matter for exams. Ultraviolet is detected by silver chloride solution and by its effect on photographic plates. Infrared is detected by a blackened-bulb thermometer or a thermopile. Ultraviolet needs a quartz prism because glass absorbs it. Infrared needs a rock-salt prism because glass absorbs it too.

Harmful effects: ultraviolet can cause skin cancer. High doses of infrared can burn the skin. These two hazards are frequently tested in multiple choice questions.

6.6 DISTINCTION BETWEEN THE ULTRAVIOLET, VISIBLE AND INFRARED RADIATIONS

This section compares ultraviolet, visible and infrared radiations point by point. The table below is the core revision aid for the section.

These three radiations are the ones students confuse most often. Ultraviolet and infrared are both invisible, but they differ in heating, photographic effect and the material they pass through. Note the three detectors again. Ultraviolet is detected by silver chloride solution. Infrared is detected by a blackened-bulb thermometer or a thermopile. These detection methods appear in several multiple choice questions in this ICSE chapter.

6.7 SCATTERING OF LIGHT

Scattering is the absorption and re-emission of light energy. It was first studied by the scientist Rayleigh. When sunlight enters the atmosphere, air molecules smaller than the wavelength absorb the light and re-emit it without changing its wavelength.

The intensity of scattered light is inversely proportional to the fourth power of the wavelength:

I \propto \frac{1}{\lambda^4}

Violet has the shortest visible wavelength, about 4000 Å, and red the longest, about 8000 Å. So violet is scattered most and red is scattered least.

Worked example pattern: compare the scattering of violet (4000 Å) and red (8000 Å). The ratio is (8000/4000)^4 = 2^4 = 16. Violet is scattered about 16 times more than red. This is why the blue end of sunlight is lost as it passes through the atmosphere.

The inverse fourth-power law holds only when the scattering particles are smaller than the wavelength. Larger molecules scatter all colours equally. That contrast explains the applications in the next section.

6.8 SOME APPLICATIONS OF SCATTERING

Scattering of sunlight by the atmosphere explains six everyday observations.

1. Red sun at sunrise and sunset: light travels the longest path through the atmosphere. Blue is scattered away, so red (white − blue) reaches the eye.
2. White sky at noon: the sun is overhead, the path is shortest, and little scattering occurs.
3. Blue sky: scattered blue light reaches the eye from directions other than the sun.
4. Black sky without atmosphere: no scattering means no scattered light. On the moon, or from a spacecraft above the atmosphere, the sky appears black.
5. White clouds: cloud particles are larger than the wavelength and scatter all colours equally.
6. Red danger signal: red has the longest visible wavelength, so it is scattered least. It can be seen from the farthest distance, even in fog.

The “white = blue + red” logic appears in the sunrise case. When blue is removed by scattering, the remaining light is red. For clouds, all colours are scattered equally, so the sum stays white.

What to practise from Spectrum Class 10 ICSE

The chapter has three exercise blocks and three multiple choice blocks. Work through them in the order they appear.

• EXERCISE-6(A): 20 questions
• MULTIPLE CHOICE TYPE: 5 questions
• EXERCISE-6(B): 27 questions
• MULTIPLE CHOICE TYPE: 5 questions
• EXERCISE-6(C): 12 questions
• MULTIPLE CHOICE TYPE: 2 questions

The numericals in Exercise 6(A) and 6(B) all use the relation c = f\lambda. Diagram-completion questions appear in Exercise 6(A). In the multiple choice blocks, watch for questions on detection methods and the order of the electromagnetic spectrum. For example, a thermopile detects infrared, and silver chloride solution detects ultraviolet. The spectrum order from shortest to longest wavelength runs gamma, X-ray, ultraviolet, visible, infrared, microwave, radio. Check the official CISCE website for syllabus and exam information.

Spectrum Class 10 ICSE frequently asked questions

Why does violet light deviate more than red in a prism in Class 10 ICSE Physics?

In glass, the refractive index for violet is higher than for red. From \mu = \sin i / \sin r, a higher \mu means a smaller angle of refraction r. Since deviation \delta = i - r, the violet ray bends more.

What is the difference between dispersion and deviation in the Class 10 ICSE spectrum chapter?

Dispersion is the splitting of white light into its constituent colours. It happens at the first surface of the prism. Deviation is the bending of a ray, and it happens at both surfaces. The two are often tested together in multiple choice questions.

Why is the sky blue but the sun appears red at sunrise in this ICSE chapter on scattering?

Blue light has a short wavelength, so it is scattered more by air molecules. At sunrise and sunset, sunlight travels the longest path, so blue is lost. Only red (white − blue) reaches the eye. The sky appears blue because scattered blue light reaches us from other directions.

Which radiation is detected by a thermopile and which by silver chloride solution in Class 10 ICSE Physics?

A thermopile detects infrared radiations through their heating effect. Silver chloride solution detects ultraviolet radiations through their chemical action. Both detection methods appear in the multiple choice questions.

How do I find the wavelength of visible light using c = fλ in the spectrum class 10 icse chapter?

Use c = f\lambda, with c = 3 \times 10^8\ \text{m s}^{-1}. For green light at 6 \times 10^{14} Hz, \lambda = (3 \times 10^8)/(6 \times 10^{14}) = 5 \times 10^{-7}\ \text{m} = 500\ \text{nm}. The full visible range runs from 400 nm to 800 nm, as the table in section 6.2 gives.

Reference: ICSE Class 10 Physics textbook material, chapter 6: Spectrum — CISCE-aligned.

More from this book

• Chapter 1: Force
• Chapter 2: Work Energy and Power
• Chapter 3: Machines
• Chapter 4: Refraction of Light At Plane Surfaces
• Chapter 5: Refraction Through a Lens
• Chapter 7: Sound
• Chapter 8: Current Electricity
• Chapter 9: Household Circuits
• Chapter 10: Electro Magnetism
• Chapter 11: Calorimetry
• Chapter 12: Radioactivity