# Worksheets Chapter 10 Light Reflection and Refraction Class 10 Science

Students should refer to Worksheets Class 10 Science Light Reflection and Refraction Chapter 10 provided below with important questions and answers. These important questions with solutions for Chapter 10 Light Reflection and Refraction have been prepared by expert teachers for Class 10 Science based on the expected pattern of questions in the class 10 exams. We have provided Worksheets for Class 10 Science for all chapters on our website. You should carefully learn all the important examinations questions provided below as they will help you to get better marks in your class tests and exams.

## Light Reflection and RefractionWorksheets Class 10 Science

Question. The unit of power of lens is
(a) metre
(b) centimeter
(c) diopter
(d) m–1

C

Question. Four students showed the following traces of the path of a ray of light passing through a rectangular glass slab.

The trace most likely to be correctly is that of student
(a) A
(b) B
(c) D
(d) C

B

Question. If f1 and f2 are the focal lengths of two lenses, what is the relation for equivalent focal length?

B

Question. A student is to find the focal length of a
(i) concave mirror,
(ii) convex lens by using a distant object.
He will observe that the screen is on the same side as the object
(a) in both the cases
(b) in case (ii) but not in case (i)
(c) in case (i) but not in case (ii)
(d) in neither of the two cases

C

Question. The two dots P1 and P2 shown in each of the following diagrams, I, II, III and IV denote the position of two pins in respect of distance and direction for performing an experiment on tracing the path of a ray of light passing through a rectangular glass slag. In which one of the four cases, one is likely to get best result ?

(a) II
(b) III
(c) I
(d) IV

B

Question. Which of the following diagrams give a correct picture?

A

Question. The radius of curvature of a mirror is 20 cm the focal length is
(a) 20 cm
(b) 10 cm
(c) 40 cm
(d) 5 cm

B

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

D

Question. 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

D

Questions. When light enters from air to glass, which of the following changes:
A. Wavelength
B. Velocity
C. Frequency
D. Amplitude
(a) A and D
(b) B and C
(c) A and C
(d) A, B and D

D

Questions. A spherical mirror and thin spherical lens – each have a focal length of -15 cm. It is likely that:
(a) The mirror is convex, but the lens is concave.
(b) Both are concave
(c) Both are convex
(d) The mirror is concave, but the lens is convex.

B

Questions. A student has to do the experiment, on finding the focal length of a given concave mirror, by using a distant object. Out of the following set ups (A, B, C, D) available to her A. a screen, a mirror holder and a scaleB. a mirror holder, a screen holder and a scaleC. a screen holder and a scaleD. a mirror holder and a screen holder
(a) D
(b) C
(c) A
(d) B

D

Questions. Match the following with the correct response:

(a) 1-B, 2-D, 3-A, 4-C
(b) 1-C, 2-B, 3-D, 4-A
(c) 1-A, 2-C, 3-B, 4-D
(d) 1-D, 2-A, 3-C, 4-B

A

Questions. A ray passing through the focus and falling on a convex lens will:
(a) retrace its path
(b) will emerge parallel to principal axis
(c) will emerge through focus on other side
(d) will emerge perpendicular to principal axis

B

Question. 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

B

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

A

Question. 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.

D

Question. 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.

C

Question. All the distances in case of spherical mirror are measured in relation to
(a) object to image
(b) the pole of the mirror
(c) the focus of the mirror
(d) the image to the object.

B

Question. The diameter of the reflecting surface of a spherical mirror is called its
(a) Centre of curvature
(b) R=2f
(c) Aperture
(d) Principal focus.

C

Question. If a ray of light that is incident on a convex lens is parallel to its principal axis, the refracted ray passes through
(a) F2
(b) 2F2
(c) O
(d) Principal axis.

A

Question. A convex mirror used for the rear view on an automobile has a focal length of 2.5m. A car is located at a distance of 4m from the mirror.
In the above scenario the sign that should be assigned to f and u is
(a) u positive, f negative
(b) f positive, u negative
(c) f and u are positive
(d) f and u negative

B

Question. The magnification (m) of a lens can be calculated by using the formula
(a) m=-v/u
(b) m=v/u
(c) m=h1/h
(d) both b and c

D

Question. In a concave mirror an erect and virtual image is formed when the object is placed
(a) Between C and F
(b) Beyond C
(c) Between P and F
(d) At C

C

Question. Assertion (A).The value of F in a concave mirror is taken as –ve and in a convex mirror is taken as +ve.
Reason(R). All distances measured to the right of the origin are taken as +ve and those measured along the left of the origin are taken as –ve.
(a) Both A and R are true and R is the correct explanation of A
(b) Both A and R are true but R is not the correct explanation of A
(c) A is true but R is false
(d) A is false but R is true

A

Question. Assertion (A). 1.33 is the absolute refractive index of water.
Reason (R). Air is optically denser than water.
(a) Both A and R are true and R is the correct explanation of A
(b) Both A and R are true but R is not the correct explanation of A
(c) A is true but R is false
(d) A is false but R is true

C

Question. The radius of curvature and focal length of a concave mirror are
(a) positive
(b) negative
(c) both
(d) none of these

B

Question. The object distance in both concave as well as convex mirror is
(a) negative
(b) positive
(c) zero
(d) none of these

A

Question. The ratio of the speed of light in vacuum to that in a medium is known as
(a) magnification
(b) refraction
(c) refractive index
(d) Snell’s law

C

Question. Focal length of plane mirror is
(a) at infinity
(b) zero
(c) negative
(d) none of these

A

Question. The power of a lens is + 1.6 D. The nature of lens is
(a) Convex lens
(b) Concave lens
(c) both concave an convex
(d) none of these

A

Question. An incident ray makes 60° angle with the surface of the plane mirror, the angle of its refraction is
(a) 60°
(b) 90°
(c) 30°
(d) 0°

C

Question. Power of a lens is –40, its focal length is
(a) 4 m
(b) – 40 cm
(c) – 0.25 m
(d) – 25 m.

C

Question. In optics an object which has higher refractive index is called
(a) optically rarer
(b) optically denser
(c) optical density
(d) refractive index

B

Question. A mirror that has very wide field view is
(a) concave
(b) convex
(c) plane
(d) none of these

B

Question. If the object is placed at focus of a concave mirror, the image is formed at
(a) infinity
(b) focus
(c) centre of curvature
(d) between F and O.

A

Question. A concave mirror gives virtual, erect and enlarged image if the object is placed:
(a) at infinity
(b) between F and C
(c) between P and F
(d) at F.

C

Question. The mirror that always gives virtual and erect image of the object but image of smaller size than the size of the object is
(a) Plane mirror
(b) Concave mirror
(c) Convex mirror
(d) none of these

C

Question. The optical phenomena, twinkling of stars, is due to
(a) atmospheric reflection
(b) total reflection
(c) atmospheric refraction
(d) total refraction

C

Question. Convex lens focus a real, point sized image at focus, the object is placed
(a) at focus
(b) between F and 2f
(c) at infinity
(d) at 2f

C

Question. Image formed by plane mirror is
(a) real and erect
(b) real an inverted
(c) virtual and erect
(d) virtual and inverted

C

Question. A concave mirror gives, real, inverted and same size image if the object is placed
(a) at F
(b) at infinity
(c) at C
(d) beyond C

C

Question. The refractive indices of some media are given below:

In which of these is the speed of light minimum and maximum, respectively.
(a) X-minimum, W-maximum
(b) Z-minimum, W-maximum
(c) W-minimum, X-maximum
(d) X-minimum, Z-maximum

C

Question. The angle of reflection in the given figure is

(a) 90°
(b) 180°
(c) 0°
(d) 30°

C

Question. 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

D

Question. 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.

B

Question. 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

C

Question. 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.

D

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

D

Question. 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

A

Question. When a lemon kept in water in a bowl is viewed from outside, it appears ___________ than its actual size.
(a) None of these
(b) Smaller
(c) Larger
(d) Same

C

Question. As incident angle is increased for a given pair of medium, the refraction angle will
(a) decrease
(b) always remains same
(c) may increase or decrease
(d) increase

D

Question. An object move a distance ‘f’ between 2f and f of a concave mirror. The image would have travelled a distance of
(a) f/2
(b) ∞
(c) 2f
(d) f

B

Question. A student performs an experiment on finding the focal length of a convex lens by keeping a lighted candle on one end of laboratory table, a screen on its other end and the lens between them as shown in the figure. The positions of the three are adjusted to get a sharp image of the candle flame on the screen.If now the candle flame were to the replaced by a distant lamp on a far away electric pole, the student would be able to get a sharp image of this distant lamp on the screen by moving

(a) the screen in the direction of the lens or the lens away from the screen
(b) neither the screen nor the lens
(c) the screen in the direction of the lens or the lens in the direction of the screen
(d) the screen away fro the lens or the lens in the direction of the screen

C

Question. Four students set up the glass slab experiment as shown below:

The correct fixing of the pins P1 and P2 , for ‘defining the incident ray’, and of pins P3 and P4, for finding the emergent ray’, has been done by student:
(a) A
(b) C
(c) B
(d) D

C

Question. Give the cartesian sign convention for:
a. height of a real image, and
b. height of a virtual image.
Answer: (a) – ve, and (b) + ve.

Question. Define the term power of a lens. Give its SI unit. State whether the power of a converging lens is positive or negative.
Answer: Power of a lens is defined as the reciprocal of its focal length f (in metres).
P = 1/f(in m)

Question. Find out, from Table (10.3), the medium having highest optical density. Also, find the medium with lowest optical density.
Answer: As per table, diamond has highest optical density (2.42). Medium with lowest optical density is air (1.0003)

Question. An object 5 cm high is placed at a distance of 10 cm from a convex mirror of radius of curvature 30 cm. Find the nature, position and size of the image.
Answer: Here u = – 10 cm (u is always negative), v = ?
f = r/2 = +30/2 or f = + 15cm
f and r for convex mirror are always positive.
Using the formula 1/f = 1/v + 1/u we have 1/15 = 1/v – 1/10 or
1/v = 1/15 + 1/10 = 5/30 v
or ∴ Image is 6 cm behind the mirror (right or mirror).
It is virtual and errect.
Magnification m = – v/u = -(+6)/10 = 0.6
Hence image is 0.6 times the size of the object.
Again m = h2/h1 where h2 is the size of the image and h1 the size of the object.
∴ h2 = mh1= 0.6(5)= 3 cm.
Hence image is 3 cm. high.

Question. What is magnification of lens?
Answer: Magnification of a lens is to the ratio of the height of the image formed by lens to the actual height of object.
If h is the height of the object and h’ the height of the image formed by lens, then magnification m = h’/h
If u and v are the distances of object and image
m = h’/h = v/u

Question. Find the position, nature and size of the image formed by a convex lens of focal length 12 cm of an object 5 cm high placed at a distance 20 cm from it.
h = 5 cm
v = -20 cm

Question. What is refractive index ?
Answer: The ratio between the sine of angle of incidence (in air) to the sine of angle of refraction (in a denser medium) is called refractive index.

Question A mirror has magnification 0.4, what type of the mirror is that? and what type of the image is formed ?
Answer: It is a convex mirror since the magnification is positive as well as less than one. Image is diminished and erect.

Question. i. Draw a ray diagram to show the formation of image by a convex lens when an object is placed in front of the lens between its optical centre and principal focus.
ii. In the above ray diagram mark the object distance (u) and the image distance (v) with their proper signs (+ ve or – ve as per the new cartesian sign convention) and state how these distances are related to the focal length (f) of the convex lens in this case.
iii. Find the power of convex lens which forms a real and inverted image of magnification -1 of an object placed at a distance of 20 cm from its optical centre.

ii. U is -ve,V is -ve. By lens formula:

This is a required relation between u, v and f in the case when object is placed between optical centre and principal focus of convex lens

Question. How is focal length related to radius of curvature of the mirror ?
Answer: R = 2 f or f = R/2

Question. 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.

Questions. What is the unit of refractive index?
Answer: Refractive index has no units as it is a ratio of two similar physical quantities.

Questions. A student wants to project the image of a candle flame on the walls of the school laboratory by using a mirror.
i. Which type of mirror should he use and why?
ii. At what distance, in terms of focal length f of the mirror, should he place the candle flame to get the magnified image on the wall?
iii. Draw a ray diagram to show the formation of the image in this case.
iv. Can he use this mirror to project a diminished image of the candle flame on the same all State ‘how’ , if your answer is ‘yes’ and why not’, if your answer is ‘no’.
Answer: i. The student should use a Concave mirror because a concave mirror produces real images.
ii. To get magnified image, the student should put the candle flame between f and 2f.
iii. The ray diagram will be as follows:

iv. Yes, concave mirror can be used to obtain a diminished image. When the object is placed beyond 2f, then the image formed will be diminished one.

Questions. A convex 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: f = -15 cm, v = -10 cm
1/v – 1/u = 1/f
1/u = 1/15 – 1/10 = -1/30
u = -30 cm
Ray diagram as follows:

Questions. A rod of length 10 cm lies along the principal axis of a concave mirror of 10 cm in such a way that the end closer to the pole is 20 cm away from it. Find the length of image?
Answer: By mirror formula we have,

Questions. It is desired to obtain an erect image of an object, using concave mirror of focal length of 12 cm.
i. What should be the range of distance of an object placed in front of the mirror?
ii. Will the image be smaller or larger than the object? Draw ray diagram to show the formation of image in this case.
iii. Where will the image of this object be, if it is placed 24 cm in front of the mirror?
Draw ray diagram for this situation also to justify your answer. Show the positions of pole, principal focus and the centre of curvature in the above ray diagrams.
Answer:  i. In concave mirror erect image is formed only when the object is placed between pole and focus. Thus, range to obtain erect image is 0 < u < 12.
ii. The image would be larger than the object

iii. If the object distance is 24 cm which is the approx position of centre of curvature,
then the image will be formed at the same position. Here, f = -12 cm, u =-24 cm, v=?
By using mirror formula,

Questions. Name the type of mirror which always forms a virtual and diminished image.

Questions. If the magnification of a body of size 1 m is 2. What is the size of the image?
Answer: Height of object = h = 1 m
Height of image = h’ = ?
Magnification = m = 2
we know that,
m = h’/h
2 = h’/1
h’ = 2 m
Height/size of image is 2 m

Questions. How can you show that if a ray enters a rectangular glass slab obliquely and emerges from the opposite face, the emergent ray is parallel to the incident ray.

Questions. Name the kind of surfaces that
i. Reflect
ii. Refract most of the light falling on them.
ii. Transparent surfaces.

Questions. How would you decide the medium having highest optical density and medium with lowest optical density?
Answer: Optical density depends upon refractive index. Higher the refractive index, higher the optical density and vice versa.
Diamond (n = 2.42) is having maximum optical density and air (n = 1.0003) is having least optical density.

Question. 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. Magnification produced by a spherical mirror is given by the relation,

Let the height of the object, ho = h
Then, height of the image, hI = −3h (Image formed is real)

Object distance, u = −10 cm
v = 3 × (−10) = −30 cm
Here, the negative sign indicates that an inverted image is formed at a distance of 30 cm in front of the given concave mirror.

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

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

Answer. Speed of light in a medium is given by the relation for refractive index (nm). The relation is given as

It can be inferred from the relation that light will travel the slowest in the material which has the highest refractive index and travel the fastest in the material which has the lowest refractive index.
It can be observed from table 10.3 that the refractive indices of kerosene, turpentine, and water are 1.44, 1.47, and 1.33 respectively. Therefore, light travels the fastest in water.

Question. 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. 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.

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

Question. 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 × 108 m s−1.
Answer. Refractive index of a medium nm is given by,

Question. 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.

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

Question. 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. The convex lens will form complete image of an object, even if its one half is covered with black paper. It can be understood by the following two cases.
Case I : When the upper half of the lens is covered In this case, a ray of light coming from the object will be refracted by the lower half of the lens. These rays meet at the other side of the lens to form the image of the given object, as shown in the following figure.

Case II : When the lower half of the lens is covered In this case, a ray of light coming from the object is refracted by the upper half of the lens. These rays meet at the other side of the lens to form the image of the given object, as shown in the following figure.

Question. 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 distance, u = −20 cm
Object height, h = 5 cm
Radius of curvature, R = 30 cm
Radius of curvature = 2 × Focal length
R = 2f
f = 15 cm
According to the mirror formula,

The positive value of v indicates that the image is formed behind the mirror.

The positive value of image height indicates that the image formed is erect.
Therefore, the image formed is virtual, erect, and smaller in size.

Question. 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.
Answer. Range of object distance = 0 cm to15 cm
A concave mirror gives an erect image when an object is placed between its pole (P) and the principal focus (F).
Hence, to obtain an erect image of an object from a concave mirror of focal length 15 cm, the object must be placed anywhere between the pole and the focus. The image formed will be virtual, erect, and magnified in nature, as shown in the given figure.

Question. 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. Object distance, u = −25 cm
Object height, ho = 5 cm
Focal length, f = +10 cm
According to the lens formula,

The negative value of image height indicates that the image formed is inverted.
The position, size, and nature of image are shown in the following ray diagram.

Question. The magnification produced by a plane mirror is +1. What does this mean?
Answer. Magnification produced by a mirror is given by the relation

The magnification produced by a plane mirror is +1. It shows that the image formed by the plane mirror is of the same size as that of the object. The positive sign shows that the image formed is virtual and erect .

Question. Find the power of a concave lens of focal length 2 m.
Answer. Focal length of concave lens, f = 2 m
Power of a lens , P = 1 / f (in meters) = 1 / (-50) = -0.5D
Here, negative sign arises due to the divergent nature of concave lens.
Hence, the power of the given concave lens is −0.5 D.

Question. 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. When an object is placed at the centre of curvature, 2F1, of a convex lens, its image is  formed at the centre of curvature, 2F2, on the other side of the lens. The image formed is inverted and of the same size as the object, as shown in the given figure.

It is given that the image of the needle is formed at a distance of 50 cm from the convex lens. Hence, the needle is placed in front of the lens at a distance of 50 cm.
Object distance, u = −50 cm
Image distance, v = 50 cm
Focal length = f
According to the lens formula,

Question. 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. Focal length of convex mirror, f = +15 cm
Object distance, u = −10 cm
According to the mirror formula,

The positive value of v indicates that the image is formed behind the mirror.

The positive value of magnification indicates that the image formed is virtual and erect.

Question. 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?

A convex lens has a positive focal length. Hence, it is a convex lens or a converging lens.

Question. Name the type of mirror used in the following situations.
(b) Side/rear-view mirror of a vehicle
(c) Solar furnace
(a) Concave mirror is used in the headlights of a car. This is because concave mirrors can produce powerful parallel beam of light when the light source is placed at their principal focus.
(b) Convex mirror is used in side/rear view mirror of a vehicle. Convex mirrors give a virtual, erect, and diminished image of the objects placed in front of it. Because of this, they have a wide field of view. It enables the driver to see most of the traffic behind him/her.
(c) Concave mirrors are convergent mirrors. That is why they are used to construct solar furnaces. Concave mirrors converge the light incident on them at a single point known as principal focus. Hence, they can be used to produce a large amount of heat at that point.

Question. 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.
Answer. Object distance, u = −27 cm
Object height, h = 7 cm
Focal length, f = −18 cm
According to the mirror formula,

The screen should be placed at a distance of 54 cm in front of the given mirror.

The negative value of magnification indicates that the image formed is real.

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

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