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# NCERT solutions Science Class 10 chapter 11 Human Eye and Colourful World

## Chapter 11 - Human Eye and Colourful World

#### Pages 168 - 190

What is meant by power of accommodation of the eye?

Q 1 | Page 190

Draw ray diagrams to show the principal focus of a concave mirror.

Q 1 | Page 168

A person with a myopic eye cannot see objects beyond 1.2 m distinctly. What should be the type of the corrective lens used to restore proper vision?

Q 2 | Page 190

The radius of curvature of a spherical mirror is 20 cm. What is its focal length?

Q 2 | Page 168

What is the far point and near point of the human eye with normal vision?

Q 3 | Page 190

Name the mirror that can give an erect and enlarged image of an object.

Q 3 | Page 168

Why do we prefer a convex mirror as a rear-view mirror in vehicles?

Q 4 | Page 168

A student is unable to see clearly the words written on the blackboard placed at a distance of approximately 4 m from him. Name the defect of vision the boy is suffering from. Explain the method of correcting this defect.

Draw ray diagram for the:-

(i) defect of vision and also

(ii) for its correction.

Q 4 | Page 190

#### Pages 171 - 198

Find the focal length of a convex mirror whose radius of curvature is 32 cm.

Q 1 | Page 171

The human eye can focus objects at different distances by adjusting the focal length of the eye lens. This is due to
(a) presbyopia
(b) accommodation
(c) near-sightedness
(d) far-sightedness

Q 1 | Page 198

A concave mirror produces three times magnified (enlarged) real image of object placed at 10 cm in front of it. Where is the image located?

Q 2 | Page 172

The human eye forms the image of an object at its
(a) cornea

(b) iris

(c) pupil

(d) retina

Q 2 | Page 198

The least distance of distinct vision for a young adult with normal vision is about
(a) 25 m
(b) 2.5 cm
(c) 25 cm
(d) 2.5 m

Q 3 | Page 198

The change in focal length of an eye lens is caused by the action of the
(a) pupil
(b) retina
(c) ciliary muscles
(d) iris

Q 4 | Page 198

A person needs a lens of power -5.5 dioptres for correcting his distant vision. For correcting his near vision he needs a lens of power +1.5 dioptre. What is the focal length of the lens required for correcting (i) distant vision, and (ii) near vision?

Q 5 | Page 198

The far point of a myopic person is 80 cm in front of the eye. What is the nature and power of the lens required to correct the problem?

Q 6 | Page 198

Make a diagram to show how hypermetropia is corrected. The near point of a hypermetropic eye is 1 m. What is the power of the lens required to correct this defect? Assume that the near point of the normal eye is 25 cm.

Q 7 | Page 198

Why is a normal eye not able to see clearly the objects placed closer than 25 cm?

Q 8 | Page 198

What happens to the image distance in the eye when we increase the distance of an object from the eye?

Q 9 | Page 198

Explain why the planets do not twinkle?

Q 10 | Page 198

What is atmospheric refraction? Use this phenomenon to explain the following natural events:

Twinkling of stars

Q 11 | Page 198

Why does the sun appear reddish early in the morning? Will this phenomenon be observed by an observer on the moon? Justify your answer with a reason.

Q 12 | Page 198

Explain giving reason why the sky appears blue to an observer from the surface of the earth? What will the colour of the sky be for an astronaut staying in the international space station orbiting the earth? Justify your answer giving reason.

Q 13 | Page 198

#### Page 176

Find out, from Table, the medium having highest optical density. Also find the medium with lowest optical density.

 Material   medium Refractive index Material medium Refractive   index Air 1.0003 Canada Balsam 1.53 Ice 1.31 – – Water 1.33 Rock salt 1.54 Alcohol 1.36 – – Kerosene 1.44 Carbon disulphide 1.63 Fused   quartz 1.46 Dense   flint glass 1.65 Turpentine oil 1.47 Ruby 1.71 Benzene 1.50 Sapphire 1.77 Crown   glass 1.52 Diamond 2.42
Q 3 | Page 176

You are given kerosene, turpentine and water. In which of these does the light travel fastest? Use the information given in Table.

 Materialmedium Refractive index Material medium Refractive   index Air 1.0003 Canada Balsam 1.53 Ice 1.31 – – Water 1.33 Rock salt 1.54 Alcohol 1.36 – – Kerosene 1.44 Carbon disulphide 1.63 Fusedquartz 1.46 Denseflint glass 1.65 Turpentine oil 1.47 Ruby 1.71 Benzene 1.50 Sapphire 1.77 Crownglass 1.52 Diamond 2.42
Q 4 | Page 176

The refractive index of diamond is 2.42. What is the meaning of this statement?

Q 5 | Page 176

#### Page 184

Define 1 dioptre of power of a lens.

Q 1 | Page 184

A convex lens forms a real and inverted image of a needle at a distance of 50 cm from it. Where is the needle placed in front of the convex lens if the image is equal to the size of the object? Also, find the power of the lens.

Q 2 | Page 184

Find the power of a concave lens of focal length 2 m.

Q 3 | Page 184

#### Pages 185 - 186

The image formed by a concave mirror is observed to be virtual, erect and larger than the object. Where should be the position of the object?
(a) Between the principal focus and the centre of curvature
(b) At the centre of curvature
(c) Beyond the centre of curvature
(d) Between the pole of the mirror and its principal focus.

Q 1 | Page 185

A spherical mirror and a thin spherical lens have each a focal length of -15 cm. The mirror and the lens are likely to be
(a) both concave
(b) both convex
(c) the mirror is concave and the lens is convex
(d) the mirror is convex, but the lens is concave

Q 4 | Page 185

No matter how far you stand from a mirror, your image appears erect. The mirror is likely to be

(a) plane
(b) concave
(c) convex
(d) either plane or convex

Q 5 | Page 186

Which of the following lenses would you prefer to use while reading small letters found in a dictionary?

(a) A convex lens of focal length 50 cm
(b) A concave lens of focal length 50 cm
(c) A convex lens of focal length 5 cm
(d) A concave lens of focal length 5 cm

Q 6 | Page 186

We wish to obtain an erect image of an object, using a concave mirror of focal length 15 cm. What should be the range of distance of the object from the mirror? What is the nature of the image? Is the image larger or smaller than the object? Draw a ray diagram to show the image formation in this case.

Q 7 | Page 186

Name the type of mirror used in the following situations.
(b) Side/rear-view mirror of a vehicle
(c) Solar furnace

Q 8 | Page 186

One-half of a convex lens is covered with a black paper. Will this lens produce a complete image of the object? Verify your answer experimentally. Explain your observations.

Q 9 | Page 186

An object 5 cm in length is held 25 cm away from a converging lens of focal length 10 cm. Draw the ray diagram and find the position, size and the nature of the image formed.

Q 10 | Page 186

A concave lens of focal length 15 cm forms an image 10 cm from the lens. How far is the object placed from the lens? Draw the ray diagram.

Q 11 | Page 186

An object is placed at a distance of 10 cm from aconvex mirror of focal length 15 cm. Find the position and nature of the image.

Q 12 | Page 186

What is the magnification of the images formed by plane mirrors and why?

Q 13 | Page 186

An object 5 cm is placed at a distance of 20 cm in front of a convex mirror of radius of curvature 30 cm. Find the position, nature and size of the image.

Q 14 | Page 186

An object of size 7.0 cm is placed at 27 cm in front of a concave mirror of focal length 18 cm. At what distance from the mirror should a screen be placed, so that a sharp focused image can be obtained? Find the size and the nature of the image.

Q 15 | Page 186

Find the focal length of a lens of power -2.0 D. What type of lens is this?

Q 16 | Page 186

A doctor has prescribed a corrective lens of power +1.5 D. Find the focal length of the lens. Is the prescribed lens diverging or converging?

Q 17 | Page 186

#### Extra questions

The velocity of light in a medium is 1.5 x 108 m/s. What is the refractive index of the medium with respect to air, if the velocity in air is 3 x 108 m/s?

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