Give reasons : (i) The extent of deviation of a ray of light on passing through a glass prism depends on its colour. (ii) Lights of red colour are used for danger signals.
Give reasons : (i) The extent of deviation of a ray of light on passing through a glass prism depends on its colour. (ii) Lights of red colour are used for danger signals.
(i) The extent of deviation of a ray of light on passing through a glass prism depends on its colour because different colours of light have different wavelengths. As they pass through a glass prism, each colour bends through a different angle with respect to the incident ray. The red light (longest wavelength) bends the least, while the violet light (shortest wavelength) bends the most. Hence, the deviation produced by the prism is different for different colours of light, causing white light to split into its seven constituent colours.\n\n(ii) Lights of red colour are used for danger signals because red light has the longest wavelength in the visible spectrum. It is scattered the least by the atmospheric particles such as smoke, fog and dust. Therefore, red light can travel longer distances through the atmosphere without being scattered, and can be seen clearly from a far distance even in conditions of poor visibility. This makes red light the most suitable colour for danger and stop signals.
Marking Scheme
- 1Part (i) - 1 mark: Mentioning that different colours have different wavelengths, so they bend through different angles in a prism, with red bending the least and violet the most.
- 2Part (ii) - 1 mark: Stating that red light has the longest wavelength, is scattered the least by atmospheric particles, and hence can be seen from a far distance even in fog or smoke.
Hint
Think about the relationship between wavelength of light and the angle of bending through a prism. Also, recall which colour is scattered the least by the atmosphere.
Quick Oral Answer
Q: Why do we see a spectrum when white light passes through a prism?\nA: White light is a mixture of seven colours with different wavelengths. The refractive index of the prism material is different for each colour, so each colour bends through a different angle. The colours therefore separate out and form a spectrum on the screen.
Analysis & Explanation
(i) The dependence of deviation on colour is the basis of dispersion. The refractive index of glass for violet light is higher than for red light, so violet bends more than red. The angle of deviation for violet is about at minimum deviation, while for red it is about . This is why we can see the seven colours separately after passing through a prism.\n\n(ii) The reason is rooted in the inverse-fourth-power law of scattering (). Since red has nearly twice the wavelength of blue, its scattering is times less. This is the same reason why the sun appears red at sunrise and sunset, and why the sky is blue.
Common Mistakes
- 1Writing 'different speeds' instead of 'different wavelengths' in part (i).
- 2Saying red bends the most and violet the least - this is the opposite of the truth.
- 3In part (ii), saying red light is used because it is bright, without mentioning its least-scattering property.
- 4Confusing scattering of light with reflection of light in part (ii).
Interesting Facts
The sequence VIBGYOR is a useful mnemonic to remember the order of seven colours from shortest to longest wavelength.
Isaac Newton was the first to use a glass prism to obtain the spectrum of sunlight and to show that white light is made of seven colours.
A rainbow's red colour is on the outer edge because red light bends the least and reaches the observer at a higher angle than violet light.
Ambulances, fire engines and traffic stop signals are also red for the same reason - least scattering and longest visibility.
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Frequently Asked Questions
How many marks does this question carry in CBSE Class 10 Science 2011?
This question carries 2 marks in the CBSE Class 10 Science 2011 examination.
Which chapter does this question come from in Science?
This question is from the chapter "The Human Eye and the Colourful World" in the CBSE Class 10 Science syllabus.
What topic does this question cover in Science?
This question covers the topic "Dispersion and Scattering of Light" from CBSE Class 10 Science.
What type of question is this in the CBSE Class 10 Science 2011 paper?
This is a Short Answer question from Section A in the CBSE Class 10 Science 2011 paper.