Human Eye and Colourful World: Important Questions for Board Exams

CHECK YOUR KNOWLEDGE

VERY SHORT ANSWER TYPE QUESTIONS

1. What type of lens is used to correct (a) Hypermetropia, (b) Myopia ?

2. Name the defect of vision in which the eye-lens loses its power of accommodation due to old age.

3. What is the far point of a person suffering from myopia ?

4. What is the other name of old age hypermetropia ?

5. You friend can read a book perfectly well but cannot read the writing on black-board unless she sits on the front row in class. Is she short-sighted or long-sighted ?

SHORT ANSWER TYPE QUESTIONS

1. Differentiate between Hypermetropia and Myopia.

2. What is presbyopia ? Write two causes of this defect. Name the type of lens which can be used to correct presbyopia.

3. The near point of a person suffering from hypermetropia is at 50 cm from his eye. What is the nature and power of the lens needed to correct this defect ?

4. How is the amount of light entering the eye controlled ?

LONG ANSWER TYPE QUESTIONS

1. (a) What happens to the size of pupil of our eye in

(i) dim light, (ii) bright light ?
(b) Name the cells on the retina sensitive to

(i) bright light, (ii) dim light.

2. (a) Draw a simple diagram of the human eye and label clearly the cornea, iris, pupil, ciliary muscles, eye lens, retina and optic nerve.
(b) Describe the working of the human eye with the help of the above diagram.

3. What is short sightedness ? State the two causes of short-sightedness. With the help of ray diagrams, show :
(a) the eye defect short sightedness.
(b) correction of short sightedness by using a lens.

 

Hints to Long Answer Type Questions

1. (a) (i) Increases (ii) Decreases
(b) (i) Cones (ii) Rods

2. Labelled diagram of eye

3. A person can see nearby objects clearly but cannot see distinct objects clearly.
Reason :
(a) Elongation of eye ball.
(b) Excessive curvature of eye lens.

Diagram of myopic eye and correction using concave lens.

Prism : It is a pyramidal piece of glass with two triangular bases and three rectangular lateral surfaces.

Angle of Prism : The angle between two adjoining lateral surfaces.

Q. Why spectrum is formed when white light is passed through a glass prism ?
Ans. Each colour has a definite wavelength and for each wavelength the angle of deviation differs. Red is the least deviated and violet is the most deviated colour so different colours deviate at different angles to form spectrum.

Q. What is referred as white light ?
Ans. Any light that forms a spectrum similar to that of sunlight is referred as white light.

 

Q. Why do planets not twinkle ?

Ans. The planets are much closer to the earth and are thus seen as extended source. If we consider a planet as a collection of a large number of point-sized sources of light, the total variation in the amount of light entering our eye from all individual point sized sources will average out to zero and will nullify the twinkling effect.
Scattering effect : Spreading of light in various directions by colloid particles.

Tyndall effect : When light passes through a colloid its path becomes visible.
This is called Tyndall effect.E.g.,

 

(i) Path of light becomes visible when light enters a dark and dusty room through a slit or ventilator.
(ii) Path of light becomes visible when light passes through dense canopy of trees in a forest.

The colour of scattered light depends on the size of scattering particles

(i) If particles are very fine, they scatter mainly the blue colour of light (shorter wavelength).
(ii) Medium sized particles scatter mainly the red colour (longer wavelength).
(iii) Even larger particles scatter all the colours of light that is why it appears white.
• Wavelength of red light is about 1.8 times to that of blue light.

Q. Why danger signs are made in red colour ?
Ans. Red is the least scattered colour. It is least scattered by fog and smoke and can be seen in the same colour over a long distance. So, danger signs are made in red colour.

Q. Why the colour of sky appears blue on a clear day ?
Ans. The upper layer of atmosphere contains very fine particles of water vapours and gases. These particles are more effective in scattering of light of shorter wavelength mainly blue than larger wavelength. So, the sky appears blue.

Q. How does the sky appear to an astronaut in the space or to a passenger of jet plane flying at high altitude ?
Ans. The sky would appear dark to an astronaut in the space as scattering is not very prominent at such high altitude due to absence of particles.

Q. Why clouds appear white ?
Ans. Clouds are formed by water vapours. Water vapours condense to form water droplets due to larger size of droplets, all colours of light are scattered and clouds appear white.

Q. Why colour of sun appear red during sunrise and sunset ?
Ans. While sunset and sunrise, the colour of the sun and its surrounding appear red. During sunset and sunrise, the sun is near horizon and therefore the sunlight has to travel larger distance in atmosphere. Due to this most of the
blue light (shorter wavelength) are scattered away by the particles. The light of longer wavelength (red colour) will reach our eye. This is why sun appear red in colour.

QUESTIONS
VERY SHORT ANSWER TYPE QUESTIONS

 

1. Which of the two is scattered more easily – light of shorter wavelength or light of longer wavelength ?

2. What is the near and far point of a normal eye ?

3. State two effects produced by the scattering of light by the atmosphere.

4. What is tyndall effect ?

5. Which light has longer wavelength – red light or blue light ?

6. What do you understand by dispersion of light ?

7. As light rays pass from air into a glass prism, are they refracted towards or away from the normal ?

 

SHORT ANSWER TYPE QUESTIONS

 

1. Why do stars twinkle at night ?

2. Describe the formation of rainbow in the sky with the help of a diagram.

3. Why the sun appear red while sunset and sunrise ? Explain.

4. Why do stars seem higher than they actually are ? Illustrate your answer with the help of a diagram.
LONG ANSWER TYPE QUESTIONS
1. What is atmospheric refraction ? What causes atmospheric refraction ?

2. Draw a neat and labelled diagram of the experimental set-up for observing the scattering of light in a colloidal solution of sulphur to show how the sky appears blue and the sun appears red at sunrise and sunset.

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