Worksheet Solutions: Spectrum | Physics Class 10 ICSE PDF Download

Part A — Multiple Choice Questions 

Q1. In a glass prism, which colour of visible light is deviated the least?

• (a) Violet

• (b) Blue

• (c) Green

• (d) Red
  Answer: Red
  Explanation: Red light has the longest wavelength in visible light, so its refractive index in glass is the smallest and it bends the least.

Q2. The total angle of deviation produced by a triangular prism depends on which of the following?

• (a) Angle of incidence at the first face

• (b) Angle of the prism

• (c) Refractive index of the prism material

• (d) All of the above
  Answer: All of the above
  Explanation: Deviation changes with how the light enters (angle of incidence), with the prism’s geometry (angle of the prism), and with the material (refractive index).

Q3. The band of colours obtained when white light passes through a prism is called:

• (a) Reflection

• (b) Spectrum

• (c) Shadow

• (d) Mirage
  Answer: Spectrum
  Explanation: A prism splits white light into seven colours, forming a spectrum.

Q4. Which statement about recombination of colours is correct?

• (a) A single prism creates colours from nothing

• (b) Two properly aligned prisms can recombine the colours to give white light again

• (c) Colours cannot be recombined

• (d) Red turns into violet after the prism
  Answer: Two properly aligned prisms can recombine the colours to give white light again
  Explanation: The first prism disperses; the second prism undoes that spreading, giving white light.

Q5. Which part of the electromagnetic spectrum has the longest wavelength?

• (a) Ultraviolet rays

• (b) X-rays

• (c) Radio waves

• (d) Visible light
  Answer: Radio waves
  Explanation: Radio waves have the longest wavelength and the lowest frequency among these options.

Part B — Short Answer Questions 

Q1. What is dispersion? What is a spectrum? Give one example.
Answer: Dispersion is the splitting of white light into its seven colours by a transparent medium like a prism. A spectrum is the coloured band formed (Violet, Indigo, Blue, Green, Yellow, Orange, Red). Example: Sunlight through a glass prism produces a spectrum on a screen.

Q2. Why does the deviation in a prism depend on the colour of light?
Answer: Different colours travel at different speeds inside glass. Shorter wavelengths (like violet) slow down more, giving a larger refractive index and bending more. Longer wavelengths (like red) slow down less and bend less. Therefore, deviation changes with colour.

Q3. Write the order of colours you see on a screen when white light passes through a prism. Which colour deviates the most and which the least?
Answer: Order from the base side of the prism on the screen: Violet, Indigo, Blue, Green, Yellow, Orange, Red (VIBGYOR). Violet deviates the most; Red deviates the least.

Q4. A ray is made to enter the same prism with a larger angle of incidence than before. What generally happens to the total deviation?
Answer: For the same prism and colour, increasing the angle of incidence (away from the angle for minimum deviation) generally increases the total deviation. Deviation depends on how the ray meets the first face.

Q5. The speed of electromagnetic waves in air is 3 × 10^8 metres per second. If the wavelength of a visible light is 600 nanometres, find its frequency.
Answer: Convert 600 nanometres to metres: 600 nanometres = 600 × 10^-9 metres = 6.0 × 10^-7 metres.
Use speed = frequency × wavelength.
Frequency = speed ÷ wavelength = (3 × 10^8) ÷ (6.0 × 10^-7) = 0.5 × 10^15 = 5 × 10^14 hertz.

Part C — Long Answer Questions 

Q1. Describe step by step how a glass prism produces the spectrum of white light. Explain why red bends least and violet bends most.
Solution (stepwise):

1. Incident white light: A narrow beam of white light is directed onto the first face of a glass prism.

2. Refraction at first face: Light enters glass from air and bends towards the normal. Each colour slows by a different amount inside glass.

3. Different speeds: Shorter wavelengths (violet) slow more; longer wavelengths (red) slow less.

4. Separation begins: Because of different slowing, each colour bends by a different amount. The beam starts to spread into colours.

5. Refraction at second face: As the colours leave glass to air, they bend again. The differences in bending increase the spread.

6. Spectrum on screen: The emerging beam shows seven colours in order VIBGYOR.

7. Reason for least and most deviation: Red has the longest wavelength, the smallest refractive index in glass, and bends the least. Violet has the shortest wavelength, the largest refractive index, and bends the most.

Answer: A prism disperses white light because different colours travel at different speeds in glass, so they refract by different amounts, producing a spectrum with red least deviated and violet most deviated.

Q2. White light is dispersed by a prism into seven colours. Explain how a second prism can be used to recombine these colours back into white light. What does this show about the nature of white light and prisms?
Solution (stepwise):

1. First prism (dispersion): Place a glass prism in a beam of white light. A spectrum (VIBGYOR) appears.

2. Second prism placement: Align a second identical prism inverted relative to the first, so that the spread-out colours enter it.

3. Opposite refraction: The second prism bends each colour in the opposite way, reducing their spread.

4. Colours overlap: The colours come together to form white light again on a screen.

5. Conclusion about white light: White light is a mixture of seven colours; the prism does not create colours.

6. Conclusion about prisms: The first prism separates the colours (dispersion); the second prism recombines them. Together they behave like a parallel-sided glass slab, giving no net dispersion.

Answer: Two properly aligned prisms first separate and then recombine the colours, proving white light is a mixture of colours and that prisms only spread and recombine them.

Q3. Answer the following questions
(a) Arrange these in order of increasing wavelength: Ultraviolet rays, Visible light, Infrared radiations, Radio waves.
(b) State two properties and one use each of Ultraviolet rays and Infrared radiations.
(c) A microwave signal has a frequency of 3 × 10^9 hertz. Find its wavelength in metres. (Use speed of light = 3 × 10^8 metres per second.)
Solution (stepwise):
(a) Increasing wavelength: Ultraviolet rays < Visible light < Infrared radiations < Radio waves.
(b)
• Ultraviolet rays:
Property 1: Travel in straight lines at the speed of light in air.
Property 2: Chemically active and can cause fluorescence; long exposure can harm skin.
Use: Sterilising medical instruments or water.
• Infrared radiations:
Property 1: Cause heating and can be detected by their heating effect.
Property 2: Scatter less in mist and fog than visible light.
Use: Night photography or remote control signals.
(c) Wavelength calculation:
Wavelength = speed ÷ frequency = (3 × 10^8) ÷ (3 × 10^9) = 0.1 metre.