The Human Eye And The Colourful World

Very Short Answer Questions
Q.1. What is spectrum ?
Ans. "The pattern of colours obtained by dispersion of white light is called spectrum".

Q.2. What is telescope ?
Ans. Telescope is an optical instrument which is used to see far distance object (heavenly bodies or the objects situated for away on earth).

Q.3. State the name of composite colours. 
Ans. (i) Yellow, (ii) Magenta, (iii) Peacock blue.

Q.4. What are pigments? 
Ans. The coloured substance which are used in paints are known as pigments.

Q.5. What are the complementary colours ? Give example. (mp 2014)
Ans. The two colours of light by superposition of which white light is obtained are called complementary colours of light.
Example: Yellow+Blue=White
Peacock blue+Red=White
obviously yellow, peacock blue and magenta are complementary colours of blue, red and green colours respectively.

Short Answer Type QuestionS
Q.1. Write the names of those three primary colours which when mixed in suitable ratio produce white light.(mp 2016)
Ans. Red,Green and Blue are called primary colours which when mixed suitable ratio produce white light. Apart from this, these primary colours when mixed with their complementary colour produce white light.

Q.2. Write down uses of lenses.(mp 2010, 17) 
Ans. (A) Uses of convex lenses: (i) These lenses are used as microscope, telescopes and camera etc.
(ii) These lenses are used to correct the long sightedness  and presbyopia
(iii) They are used as magnifying glass by watch maker.
(iv) They are used as reading lenses.
(v) They are used by palmist and fingerprint experts.
(B) Uses of concave lenses: (i) In Galilean telescope.
(ii) These lenses are used to correct the short sightedness.
(C) Uses of cylindrical lenses: (i) To correct the astigmatism.

Q.3. What is short sightedness and long sightedness ? Explain by diagram that how it can be removed. (mp 2014, 17 )
Ans. Short sightedness: Person cannot see the distant objects clearly. It can be removed by using concave lens.
Long sightedness: Person cannot see the nearby objects clearly. It can be removed by using convex lens.

Q.4. Write the differnces between denser medium and rarer medium.
Ans. Differences between denser medium and rarer medium:

Long Answer Type Questions
Q.1. Explain why the planets do not twinkle.
Ans. The planets are much closer to the earth and are thus, seen as extended source if we consider as a collection of large number point sized sources of light the total variation in the amount of light entering our eye from all the individuals nullifying the twinkling of effect.

Q.2. Does the sky appear dark instead of blue to an astronaut ?
Ans. The blue colour of sky is due to scattering of blue colour of sunlight. The scattering of light takes place due to small atoms or molecules of air present in the atmosphere. When astronauts are in space then there is no  atmosphere and hence, there is no scatterer. Therefore, sunlight does not scatter and hence, sky appears dark instead of blue.