NCERT Solutions for Class 10th Science Chapter 10 Light Reflection and Refraction

Page 168

Question 1.  Define the principal focus of a concave mirror?
Answer: Light rays that are parallel to the principal axis of a concave mirror converge at a specific point on its principal axis after reflecting from the mirror. This point is known as the principal focus of the concave mirror.

Question 2.  The radius of curvature of a spherical mirror is 20 cm. What is its focal length?
Radius of curvature of a spherical mirror = 2 x Focal length (f)
f = R/2 = 20/2 =10cm

Question 3. Name the mirror that can give an erect and enlarged image of an object.
Answer: When an object is placed between the pole and the principal focus of a concave mirror, the image formed is virtual, erect, and enlarged.

Question 4. Why do we prefer a convex mirror as a rear-view mirror in vehicles?
Answer: Convex mirrors give a virtual, erect, and diminished image of the objects placed in front of them. They are preferred as a rear-view mirror in vehicles because they give a wider field of view, which allows the driver to see most of the traffic behind him.

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Question 1. Find the focal length of a convex mirror whose radius of curvature is 32 cm.
Radius of curvature = 2 x Focal length (f)
R = 2f
f = R/2 = 32/2 = 16cm
Hence, the focal length of the given convex mirror is 16 cm.

Question 2. 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?
Answer: Given, u = – 10 cm
Since image is real inverted so, m = -3
m = -v / u
=> -3 = -v/ -10
v= – 30 cm
Negative sign indicates the image will be real and image is formed at 30 cm in front of the mirror.

Page: 176

Question 1. A ray of light travelling in air enters obliquely into water. Does the light ray bend towards the normal or away from the normal? Why?
Answer: The light ray bends towards the normal. When a ray of light travels from an optically rarer medium to an optically denser medium, it gets bent towards the normal. Since water is optically denser than air, a ray of light travelling from air into the water will bend towards the normal.

Question 2. Light enters from air to glass having refractive index 1.50. What is the speed of light in the glass? The speed of light in vacuum is 3 Ã— 108 m/s.
Answer: Refractive index of a medium nm is given by, Question 3. Find out, from Table, the medium having highest optical density. Also find the medium with lowest optical density. Highest optical density = Diamond
Lowest optical density = Air
Optical density of a medium is directly related with the refractive index of that medium. A medium which has the highest refractive index will have the highest optical density and vice-versa.
It can be observed from table 10.3 that diamond and air respectively have the highest and lowest refractive index. Therefore, diamond has the highest optical density and air has the lowest optical density

Question 4. You are given kerosene, turpentine and water. In which of these does the light travel fastest? The light can travel fast through water.

Question 5. The refractive index of diamond is 2.42. What is the meaning of this statement?
Answer: Refractive index of a medium nm is related to the speed of light in that medium v by the relation: Where, c is the speed of light in vacuum/air
The refractive index of diamond is 2.42. This suggests that the speed of light in diamond will reduce by a factor 2.42 compared to its speed in air.

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Question 1. Define one dioptre of power of a lens?
Answer: One dioptre is the power Of a lens Of focal length 1m.
Power of lens is defined as the reciprocal of its focal length. If P is the power of a lens of focal length F in metres, then
P =1/ f (in meters)
The S.I. unit of power of a lens is Dioptre. It is denoted by D.
1 dioptre is defined as the power of a lens of focal length 1 metre.
1 D = 1 mâˆ’1

Question 2. 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 lens if the image is equal to the size of the object? Also find the power of the lens.
Answer:   v = + 50 cm
Since image is real and of same size. The position of image should be double the focal length.
Hence, the object should be at 2f.
V = 2f = 50, f = 25 cm.
Power = 1/f = 100/25 = 4D

Question 3. Find the power of a concave lens of focal length 2 m. Question 1. Which one of the following materials cannot be used to make a lens?
(a) Water (b) Glass
(c) Plastic (d) Clay

Question 2. The ¡mage 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.
Answer: (d) Between the pole of the mirror and its principal focus.

Question 3. Where should an object b. placed In front of a convex lens to get a real
image of the size of the object?
(a) At the principal focus of the lens
(b) At twice the focal length
(c) At infinity
(d) Between the optical centre of the lens and its principal focus
Answer: (b) At twice the focal length

Question 4. 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, but the lens is convex
(d) the mirror is convex, but the lens is concave

Question 5. 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
Answer: (d) Either plane or convex.

Question 6. Which of the following lenses would you prefer to use while reading small letters found ¡n a dictionary?
(a) A convex lens of focal length 50cm
(b) A concave lens of focal length 50cm
(c) A convex lens of focal length 5 cm
(d) A concave lens of focal length 5 cm.
Answer: (c) A convex lens of focal length 5 cm.

Question 7. 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.
We are given the focal length cf the concave mirror as f = -15cm.
For getting an erect image using a concave mirror, the object should be placed at a distance less than the focal length.
i.e. 15 cm from the pole. The image formed will be virtual, enlarged and erect. Question 8. Name the type of mirror used in the following situations.
(b) Side/rear-view mirror of a vehicle
(c) Solar furnace
Answer: (a) Concave mirror, to get powerful and parallel beams of light.
(b) Convex mirror because it always gives an erect image and enables the driver to view much larger area.
(c) Concave or parabolic mirror because it can concentrate sunlight at the focus to produce heat in the solar furnace.

Question 9. 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.
Answer: Yes, even when one half of the lens is covered with a black paper, complete image of the object will be formed. Take a convex lens and focus the light from a distant object onto a screen. As expected an image (sharp) is formed at a distance equal to the focal length Cover the lower or the upper half of the lens and focus the light from the same object onto the same screen. You will be able to get a sharp image again; however the brightness of the image will be less in the second case. The same effect w,ll be seen even if the lens is half covered with black strips.

Question 10. An object 5cm in length is held 25cm away from a converging lens of focal length 10 cm. Draw a ray diagram and find the position, size and the nature of the image formed.  Therefore, the mage ¡s formed between F2 and 2F2 on the other side of the lens. It is real and inverted, and smaller in size than the object.

Question 11. A concave lens of focal length 15 cm forms an ¡mage 10 cm from the lens. How far is the object placed from the Pens? Draw the ray diagram. Question 12. An object is placed at a distance of 10 cm from a convex mirror of focal length 15 cm. Find the position and nature of the ¡mage.
Answer: f = +15 cm. u = -1o cm
For mirror, we have The image must be virtual and erect.

Question 13. The magnification produced by a plane mirror is +1. What does this mean?
Answer: This means that size of the image is equal to the size of the object.

Question 14. An object 5.0 cm in length Is placedat a distanc, of 20 cm in front of a convex mirror of radius of curvature 30 cm. Find the position of the image nature and size. Question 15. 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.   