# NCERT Solutions for class 10th Science Chapter 10 Light-Reflection and Refraction

##### NCERT INTEXT QUESTIONS

Question 1. Define the principal focus of a concave mirror.

Sol. The point on principal axis where beam of rays parallel to principal axis meet after reflection from the mirror is called principal focus.

Question 2. The radius of curvature of a spherical mirror is 20 cm. What is its focal length?

Sol. Radius of curvature (R) of a spherical mirror= 20 cm
Radius of curvature (R) = 2 x Focal length (f)
So, Focal length (f) = R/2 =20/2
f=l0cm
The focal length of a spherical mirror will be 10 cm
Note: Cartesian sign convensions should be followed for spherical mirrors and lenses.

Question 3. Name a mirror that can give an erect and enlarged image of an object.

Sol. Concave mirror gives an erect and enlarged image of an object.
Note:When an object (AB) is placed between pole (P) and the principal focus (F) of a concave mirror, then the image formed is virtual, erect and enlarged.

Question 4. Why do we prefer a convex mirror as a rear-view mirror in vehicles?

Sol. Concave mirrors are used as a rear-view mirror in vehicles due to the following two reasons :
(i) A concave mirror produces an erect, virtual and diminished image of an object.
(ii) As concave mirror is curved outwards so it has a wider field of view which enables the driver to view much larger area.

Question 5. Find the focal length of a convex mirror whose radius of curvature is 32 cm.

Sol. Radius of curvature (R) of a convex mirror= 32 cm
Radius of curvature (R)=2 x Focal length (f)
So, Focal length (f) = R/2 = 32/2
f = 16 cm
The focal length of a convex mirror will be 16 cm.

Question 6. A concave mirror produces three times magnified (enlarged) real image of an object placed at 10 cm in front of it. Where is the image located?

Sol. Distance of an object from concave mirror (u)= -10 cm
As the image formed is real, so the magnification (m) will be negative. So, magnification (m)=-3
Magnification produced by spherical mirror is given by the relation,

Question 7. 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?

Sol. 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.
Note: If i is angle of incidence and r is angle of refraction, then sini/sinr= constant

Question 8. 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 vaccum is 3 x 108 m/sec.

Question 9. Find out, from Table 10.3 (NCERT Book) the medium having highest optical density. Also find the medium with lowest optical density.

Sol. Table 10.3: Absolute refractive index of some material media

From the given table, the medium having highest optical density is diamond which is 2.42 and the medium having lowest optical density is air which is 1.003.
Note: The medium with the large refractive index is optically denser medium than the other.

Question 10. You are given kerosene, turpentine and water. In which of these does the light travel fastest? Use the information given in Table 10.3. (NCERT)

Sol. Using the information given in table, the refractive index of kerosene is 1.44, that of turpentine is 1.47 and that of water is 1.33. Clearly, water having lower refractive index 1.33, is optically rarer than kerosene and turpentine. Therefore the light travels fastest in water because of its lower optical density.

Question 11. The refractive index of diamond is 2.42. What is the meaning of this
statement?

Sol. Refractive index of a medium (nm) can be given by the relation:

The refractive index of diamond is 2.42 which means that the speed oflight in diamond will reduce by a factor 2.42 as compared to the speed in air

Question 12. Define 1 dioptre of power of a lens.

Sol. When the focal length of a lens is one metre then the power of lens is called one dioptre.

Question 13. 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.

So f, focal length of lens = 25 cm

Question 14. Find the power of a concave lens of focal length 2 m.

#### Exercise

Question 1. Which one of the following materials cannot be used to make a lens?
(a) Water (b) Glass (c) Plastic (d) Clay

Sol. (d) Lens is made of transparent material whereas clay material is opaque for light so reflection as well as refraction both is not possible in clay.
Note: An opaque object is neither transparent nor translucent. When light strikes an inteiface between two substances, some may reflected, some absorbed, some scattered, and the rest transmitted.

Question 2. 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.

Sol. (d) A virtual and erect image is formed only when object is placed only between the pole of the mirror and its principal focus. Apart from this by placing object at all other possible positions, a real image is formed by a concave mirror

Question 3. Where should an object be 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

Sol. (b) When object is placed at 2F” i.e., at twice of the focal length (centre of curvature), an inverted, equal sized and real image is formed at 2F2 by a convex lens.

Note: If the object is placed at focus, image is formed at infinity and vice versa.

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 and the lens is convex.
(d) the mirror is convex but the lens is concave.

Sol. (a) According to sign convention, focal length is negative for concave mirror and concave lens. So, both mirror and the lens are concave.
Note: Focal length of convex mirror and convex lens is positive.

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 planer or convex

Sol. (d) A plane mirror and a convex lens always form a virtual and erect image, so the image appears always erect no matter what will be the position of the object.

Question 6. 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

Sol. (c) A convex lens gives a magnified image ofan object when it is placed between radius of curvature and focal length. Also, magnification for convex lens is more for convex lens having shorter focal length. Therefore, for reading small letters, a convex lens of focal length 5 cm should be used.

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.

Solution:

• A concave mirror forms a virtual, enlarged and erect image only when object is placed between its principal focus and pole. Thus, in given situation of a concave mirror having focal length equal to 15cm, the object must be placed less than 15cm from pole of the mirror to get an erect image.
• Thus, range of distance of the object from the mirror= less than 15cm
• Nature of image: Virtual and erect
• Size of image: Larger than the object
• Ray diagram to show image formation:

Question 8. Name the type of mirror used in the following situations.
(b) Side/rear-view mirror of a vehicle.
(c) Solar furnace.

Sol. (a) Headlights of a car: Concave mirror is used.
When object is placed at principal focus of a concave mirror, rays coming from object direct parallel to the principal axis forms a highly enlarged, real and inverted image at infinity. Thus, in headlights of a car, by placing bulb (source of light) at principal focus of a concave mirror, a parallel beam of light is obtained which goes up to infinity. This facilitates better vision up to a long distance to the driver.
(b) Side/rear view mirror of a vehicle: A convex mirror is used.
A concave mirror is used as side/rear view mirror of a vehicle because a convex mirror gives an erect and diminished image. A convex mirror has wider field of view as it is curved outwards which enable a driver to view larger area than would be possible with a plane mirror and enables the driver to see traffic behind him.
(c) Solar furnace: A concave mirror of large aperture is used.
A concave mirror converges light rays coming parallel to its principal axis from infinity at its principal focus. By converging rays of light at a point, all heat of light gets focussed at one point and produce enormous heat. Thus, in a solar furnace, a concave mirror having large size of aperture is used.

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.

Sol. Yes, this lens will produce a complete image of the object. The nature, size and location of the image will not change but the intensity of the image will become half

Experiment : An object is placed in front of it, say between FI and 2F1. It produces real and inverted image beyond 2F2. Experiment is repeated by placing object at different positions. In all cases image is formed. Since, rays coming from object passes through the upper part and optical centre of the lens, so a half portion covered convex lens with black paper produces complete image easily.

Question 10. 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.

Sol. Size of the object= 5 cm
Distance of the object (u) = -25 cm
Focal length of a convex lens (f) = 10 cm
Using Lens formula for converging lens,

Hence, the image is inverted and diminished in size.
Thus, the position of the image is at 16.66 cm on opposite side of the lens and is real and inverted. Size of the image is +3.3 cm.
The position, size and nature of the image are shown in the following ray diagram:

Question 11. 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.

Note: A concave lens always forms virtual and erect image on the same side of the object.

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 image.

Since distance of image is positive, this shows that image is formed at the other side of the mirror. And magnification is also positive this shows that image is erect.
Thus, position of image = 6cm (behind the mirror and the image is virtual and erect).

Question 13. The magnification produced by a plane mirror is+ 1. What does this mean?

Sol. The magnification of a plane mirror is+ 1, it means that the image is of same size of the object. Also, it is erect and virtual.

Question 14. An object 5 cm in length is placed at a distance of 20 cm in front of a convex mirror of radius of curvature 30 cm. Find the position of the image, its nature and size.

Sol. According to mirror formula,
Radius of curvature (R) = + 30 cm Distance of object (u) = -20 cm Height of object (h’) = 5 cm

Question 15. An object of size 7 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 focussed image can be obained ? Find the size and nature of the image.

The negative value of image height indicates that the image formed is inverted.

Question 16. Find the focal length of a lens of power-2D. What type of lens is this?

Question 17. 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?

Note: The power of lens is positive for converging lens and negative for diverging lens