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Karnataka State Syllabus Class 10 Science Chapter 10 Light Reflection and Refraction
KSEEB Class 10 Science Light Reflection and Refraction Textbook Exercise Questions and Answers
Question 1.
Define the principal focus of a concave mirror.
Answer:
It is a point on the principal axis at which a beam of light incident parallel to the principal axis converges after reflection from the concave mirror.
Question 2.
The radius of curvature of a spherical mirror is 20 cm. What is its focal length?
Answer:
Focal length = \(\frac{1}{2}\) x Radius of curvature
= \(\frac{1}{2}\) x 20cm
= 10 cm
Question 3.
Name a mirror that can give an erect and enlarged image of an object.
Answer:
Concave mirror.
Question 4.
Why do we prefer a convex mirror as a rearview mirror in vehicles?
Answer:
A convex mirror is preferred as a rearview mirror because.
- It always forms an erect, virtual and diminished image of an object placed anywhere infront of it.
- It has wider field of view.
Question 5.
Find the focal length of a convex mirror whose radius of curvature is 32 cm.
Answer:
Here R = +32 [convex mirror] f = ?
\(\mathrm{f}=\frac{\mathrm{R}}{2}=\frac{32}{2}=+16 \mathrm{cm}\)
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?
Answer:
Here, in case of concave mirror
u = -10 cm [u is always negative]
v = ? m = -3 [real image]
Now magnification formula is
\(m=-\frac{V}{u}\)
\(-3=-\left(\frac{V}{-10}\right)\) ∴ V = -30cm
Image will be formed at 30 cm from the mirror on the same side of object.
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?
Answer:
Light travels faster in rarer medium air and slower in denser medium water. As a ray of light travelling in air enters obliquely into water, it slows down and bends towards the normal.
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 vacuum is 3 x 108 m /sec.
Answer:
Given n= 1.50
Speed of light in vacuum C = 3 x 108 m/sec
Refractive index is given by \(n=\frac{C}{V}\)
where, C is the speed of light in vacuum and V is the speed of light in the given medium (glass)
Question 9.
Find out, from Table 10.3, the medium having highest optical density. Also, find the medium with lowest optical density.
Answer:
From table 10.3 Diamond has highest refractive index (= 2.42), so it has largest optical density.
Air has lowest refractive index (= 1.0003)
so it has lowest optical density.
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.
Answer:
For kerosene, n – 1.44
For turpentine oil, n = 1.47
For water, n = 1.33
As water has lowest refractive index light travels faster in water.
Question 11.
The refractive index of diamond is 2.42. What is the meaning of this statement?
Answer:
The refractive index of diamond is 2.42. It indicates that the ratio of speed of light in air to that in diamond is 2.42.
Question 12.
Define 1 dioptre of power of a lens.
Answer:
1 diopter is the power of a lens whose focal length is 1 meter.
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.
Answer:
Here,
V = +50 cm [V is +ve for real image]
As the real image is of the same size as the object, so
Question 14.
Find the power of a concave lens of focal length 2 m.
Answer:
Given f = -2m
Power \(P=\frac{1}{f}=\frac{1}{-2}=-0.5 \text { dioptre }\)
∴ Power of lens is 0.5 D
Question 15.
Which one of the following materials cannot be used to make a lens?
(a) Water
(b) Glass
(c) Plastic
(d) Clay
Answer:
(d) Clay
Question 16.
The image formed by a concave mirror is observed to be virtual, erect and larger then 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 17.
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.
Answer:
(b) At twice .the focal length
Question 18.
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.
Answer:
(a) Both concave
Question 19.
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 20.
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.
Answer:
(c) A convex lens of focal length 5 cm.
Question 21.
We wish to obtain an erect image of an object, using a concave mirror of focal lehgth 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.
Answer:
The object should be placed between the focus F and the pole P of the concave mirror i.e., between 0 and 15 cm from the mirror. Then the image will be virtual, erect and larger than the object.
Question 22.
Name the type of mirror used in the following situations.
(a) Headlights of a car.
(b) Side/rear-view mirror of a vehicle.
(c) Solar furnace.
Support your answer with reason.
Answer:
(a) Concave mirror: Because to get powerful parallel beams of light.
(b) Convex mirror: Because these mirrors are fitted on the sides of the vehicle, enabling the driver to see traffic.
(c) Concave Mirror: Because to get powerful parallel beams of light.
Question 23.
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:
As shown in the above figure when the lower half of the convex lens is covered with a black paper, it still forms the complete image of the object as that with uncovered lens. But the intensity of the image is reduced.
Question 24.
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.
Answer:
For a converging [convex] lens : Here
u = -25 [u is always negative]
f = +10 cm [convex lens]
from the lens formulae
This image is formed at a distance 16.67 cm behind the lens.
Also, magnification
Thus, image is inverted, real anc of size 3.33 cm.
Question 25.
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.
Answer:
Here, focal length of the concave [diverging] lens is
h = 5 cm, V = ? and h = ?
V = -10 cm [image same side in concave lens]
u = ?
The object is placed at a distance of 30 cm away from the lens.
Question 26.
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.
Answer:
Object distance, u = -10 cm
Focal length, f = +15cm [F is +ve for a convex mirror]
Image distance, V = ?
Image distance, V = + 6 cm
As V is + ve, so a virtual, erect image is formed at a distance 6 cm behind the mirror.
Question 27.
The magnification produced by a plane mirror is +1. What does this mean?
Answer:
The magnification is positive means image formed in plane mirror is virtual and erect. Size is as size of object because magnification is +1.
Question 28.
An object 5.0 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.
Answer:
Object size, h = +5 cm
Object distance, u = – 20 cm
Radius of curvature, R = + 3.0 cm
[R is +ve for a convex mirror]
A virtual erect image of height 2.2 cm is formed behind the mirror at a distance of 8-.6 cm from the mirror.
Question 29.
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 be placed,so that a sharp focussed image can be obtained? Find the size and the nature of the image.
Answer:
Object size, h = +7.0 cm; Focal length
f = -18 cm; Image size, h = ?
Object distance, u = -27 cm
Image distance, V = ?
The screen should be placed at a distance of 54 cm on the object side of the mirror to obtain sharp image.
The image is real inverted and enlarged in size.
Question 30.
Find the focal length of a lens of power – 2.0 D. What type of lens is this?
Answer:
Here, P = -2.0 D
\(f=\frac{1}{P}=\frac{1}{-2.0 D}=-0.5 \mathrm{m}\)
The type of lens is concave.
Question 31.
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?
Answer:
Here, P = +1.5D
\({ f }=\frac { 1 }{ { P } } =\frac { 1 }{ +1.5{ D } } =\frac { 10 }{ +15 } { m }\)
= +66.67 cm.
As the focal length is positive, the prescribed lens is converging.
KSEEB Class 10 Science Light Reflection and Refraction Additional Questions and Answers
Multiple Choice Questions:
Question 1.
The object u, image distance v and focal length f for a spherical mirror are related as …………
(a) \(\frac{1}{v}-\frac{1}{u}=\frac{1}{f}\)
(b) \(\frac{1}{u}+\frac{1}{v}=\frac{1}{f}\)
(c) v – u = f
(d) v + u = f
Answer:
(b) \(\frac{1}{u}+\frac{1}{v}=\frac{1}{f}\)
Question 2.
The image formed by a concave mirror ……….
(a) is always real
(b) is always virtual
(c) can be both real and virtual
(d) none of these
Answer:
(c) can be both real and virtual
Question 3.
The mirror which has a wide field of view must be ………..
(a) concave
(b) convex
(c) plane
(d) none of these
Answer:
(b) convex
Question 4.
The driver’s mirror used in automobiles is ___________
(a) convex
(b) concave
(c) plane
(d) none of these
Answer:
(a) convex
Question 5.
A concave mirror always forms real and inverted image except when the object is placed
(a) at infinity
(b) between F and C
(c) at F
(d) between F and Pole
Answer:
(b) between F and C
Question 6.
The ratio of the size of the image to the size of the object is called
(a) Magnification
(b) Power
(c) transformation ratio
(d) focal length
Answer:
(a) Magnification
Question 7.
A real inverted and enlarged image is formed by a convex lens when the object is placed
(a) at F
(b) between F and 2F
(c) at 2F
(d) beyond 2F
Answer:
(b) between F and 2F
Question 8.
The image formed by a concave lens is
(a) always virtual and diminished
(b) always virtual and enlarged
(c) always real
(d) sometimes real and sometimes virtual.
Answer:
(a) always virtual and diminished
Question 9.
An object is placed 20 cm from a convex lens of focal length 10 cm. The image must be
(a) real and diminished
(b) real and of same size
(c) real and enlarged
(d) virtual and enlarged
Answer:
(b) real and of same size
Question 10.
For using convex lens as a magnifying lens the object should be placed
(a) beyond 2F
(b) between F and 2F
(c) at F
(d) between F and the
Answer:
(d) between F and the
Question 11.
The power of a lens is given by
(a) \(\mathrm{D}=\frac{1}{\mathrm{fin} \mathrm{cm}}\)
(b) \(\mathrm{D}=\frac{100}{\mathrm{f in cm}}\)
(c) \(\mathrm{D}=\frac{1000}{\mathrm{f in cm}}\)
(d) \(\mathrm{D}=\frac{100}{\mathrm{f in m}}\)
Answer:
(b) \(\mathrm{D}=\frac{100}{\mathrm{fincm}}\)
Question 12.
A convex lens has a focal length of 50 cm. Its power is …..
(a) 2D
(b) 5D
(c) 0.5D
(d) 0.02D
Answer:
(a) 2D
Question 13.
Write the differences between real and virtual images.
Answer:
| Real image | Virtual image |
| 1. Here the rays actually meet at the image point. 2. It can be taken on a screen. 3. It is always inverted. |
1. Here the rays appear to diverge from the image point. 2. It cannot be taken on a screen. 3. It is always erect. |
Question 14.
With the help of ray diagram, state and explain the laws of reflection of light at a plane mirror.
Answer:
As shown in Fig. 10.2. When a ray of light is incident on a mirror, it gets reflected in accordance with the following laws of reflection.
First law: The incident ray, the reflected ray and the normal at the point of incidence all lie in the same plane. Second law: The angle of incidence (i) is equal to the angle of reflection (r) i.e., \(\lfloor i=\lfloor r\)
Question 15.
List four properties of the image formed by a plane mirror.
Answer:
Properties of images formed by a plane mirrors:
- The image formed by a plane mirror is virtual and erect.
- It is of the same size as the object.
- The image is formed as far behind the mirror as the object is in front of it i.e./ u = v.
- It is laterally inverted i.e., image is inverted sideways with respect to the object.
Question 16.
What are the values of angle of incidence (i) and that of reflection (r) for a normal incidence?
Answer:
For normal incidence,\(\lfloor i=\lfloor r={ 0 }^{ 0 }\)
Question 17.
What is the number of images of an object held between two plane parallel mirrors?
Answer:
Infinity.
Question 18.
What is a spherical mirror? What are the two different types of spherical mirrors?
Answer:
Spherical mirror: A spherical mirror is a mirror whose reflecting surface is a part of a hollow sphere.
In fig. 10.6(a), S is a hollow glass sphere being cut by an plane AB. The section LPM of this spherical shell cut by the plane, forms a part of the sphere and is known as spherical surface. If either side of this spherical surface is silvered, we get a spherical mirror.
Spherical mirrors are of two types:
- Concave mirror: A spherical mirror is concave if it is silvered on the outer bulged surface and reflects lights from the inner hollow surface.
- Convex mirror: A spherical mirror is convex if it silvered on the inner hollow surface and reflects light from the outer bulged surface.
Question 19.
What is the minimum number of rays for locating the image formed by a concave mirror for an object. Draw a ray diagram to show the formation?
Answer:
A minimum of two rays are required to locate the image formed by a concave mirror. Take help of fig. 10.24.
Question 20.
List the four properties of the image formed by a concave mirror when object is placed between focus and pole of the mirrors.
Answer:
The image formed by the concave mirror is
- virtual
- erect
- enlarged, and
- lies behind the mirror.
Question 21.
Which type of mirror is used to give an erect and enlarged image of an object?
Answer:
Concave mirror, because it forms erect and enlarged image when the object is placed between the focus and the mirror.
Question 22.
Explain why a ray of light passing through the centre of curvature of a concave mirror gets reflected along the same path.
Answer:
A ray of light passing through the centre of curvature of the concave mirror falls normally \(\lfloor i=\lfloor r={ 0 }^{ 0 }\) on the mirror and so it is reflected back along the same path.
Question 23.
Between which two points of a concave mirror should an object be placed to obtain a magnification – 3?
Answer:
Between F and C of the concave mirror.
Question 24.
What happens to the size of the image formed by a convex mirror, when an object is gradually moved towards the mirror?
Answer:
As shown in Fig. 10.28, when the object is at position A, B, its virtual image is at a1b2
When the object is at position A2B2 virtual image at a2b2. So, when an object is gradually moved towards the pole and gradually increases in size till it has a size almost equal to that of the object. However, the image is always formed between F and P.
Question 25.
You are given three mirrors of equal size – concave, convex and plane. How will you identify them without touching their surfaces? OR
How to distinguish between a plane, concave and convex mirrors without touching them?
Answer:
We look our face in each mirror, turn by turn. First, we keep our face quite close to the mirror and then move it slowly away form the mirror.
- If the image formed is of same size as our face but laterally inverted (i.e., left face looks right and vice-versa) for all positions, then it is a plane mirror.
- If the image formed is erect and enlarged initially but gets inverted as the face is moved away, then it is a concave mirror.
- If the image formed is erect and smaller in size for all positions, then it is a convex mirror.
Question 26.
State the mirror formula. Is the same formula application to both concave and convex mirrors?
Answer:
Mirror formula: The mirror formula is a mathematical relation between the object distance u, image distance V and focal length f of a spherical mirror. This relation is
This formula is applicable to all concave and convex mirror.
Question 27.
A spherical mirror produces an image of magnification -1 on a screen placed at a distance of 50 cm from the mirror.
(a) Write the type of mirror.
(b) Find the distance of the image from the object.
(c) What is the focal length of the mirror.
(d) Draw the ray diagram to show the image formation in this case.
Answer:
(a) A concave mirror should be used.
(b)
∴ u = -50cm , V= -50cm
Distance of the image from the object = 0
(c)
Question 28.
Compare any two similarities and one dissimilarity between an image formed by a plane mirror and convex mirror in tabular form.
Answer:
| Plane Mirror | Convex Mirror |
| Similarities: 1. It forms erect image. 2. Image is virtual. Dissimilarities: 1. Image is of same size as the object. 2. Image is at same distance from the mirror as the object it is in front of it. |
1. It forms erect image. 2. Image is virtual. 1. Image is smaller than the object. 2. Image distance may be different from the object distance. |
Question 29.
Write three differences between concave and convex mirrors.
Answer:
| Concave mirror | Convex mirror |
| 1. It converges a parallel beam of light. | 1. It diverges a parallel beam of light. |
| 2. Image may be real or virtual. | 2. Image is virtual. |
| 3. | M | > | or | m | <1. | 3. | m | < 1. |