# Assignments Class 10 Science Light Reflection and Refraction

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## Light Reflection and Refraction Assignments Class 10 Science

Question. Define absolute refractive index.
Answer : When medium 1 is vacuum, then refractive index of medium 2 is considered with respect to vacuum. This is called absolute refractive index.

Question. What is the unit of refractive index?
Answer : It has no unit.

Question. Define incident ray, reflected ray, normal ray, angle of incidence and reflection.
Answer : Incident ray – light which falls on the mirror/ polished surface is called incident ray.
Reflected ray – ray of light which goes back in the same medium after striking the surface is called reflected ray.
Normal – the perpendicular drawn to the reflecting surface is called normal at that point.
Angle of incidence – the angle between the incident ray and the normal is known angle of incidence.
Angle of reflection – the angle between reflected ray and the normal is known angle of reflection.

Question. Define optical density.
Answer : The ability of the medium to refract light is called optical density.

Question. What is the relation between optical density, refractive index and speed of light?
Answer : The medium with higher refractive index in which speed of light is less is known as optically denser medium and the medium with lower refractive index in which the speed of light is more is known as optically rarer medium.

Question. State lens formula and write it mathematically.
Answer : The relationship between object distance (u), image distance (v), and focal length of lens is known as lens formula. It is given by 1/v – 1/u = 1/f

Question. Define magnification of lens.
Answer : Magnification (m) = Height of image (h’) /Height of object (h) = ν/u
For convex lens ‘m’ can be more than, less than or equal to one.
For concave lens ‘m’ is less than one.

Question. Define a lens.
Answer : A transparent material bounded by two surfaces of which one or both surfaces are spherical forms a lens.

Question. Define the principal focus of a concave mirror.
Answer : It is a point on the principal axis where the rays of light parallel to principal axis meet.

Question. The radius of curvature of a spherical mirror is 20 cm. What is its focal length?
Answer :  R = 20 cm
R = 2f
f = R/2 ,
f = 20/2 = 10 cm.

Focal length is 10 cm.

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

Question. Why do we prefer a convex mirror as a rear-view mirror in vehicles?
Answer : Convex mirror can cover a wider range and give erect and diminished image. Hence convex mirror is used as a rear-view mirror to get wider field of view.

Question. Find the focal length of a convex mirror whose radius of curvature is 32 cm.
R = 32 cm
R = 2f
∴ f = R /2 =  32/2 = 16 cm.

Question. 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 : Object distance u = –10 cm (concave mirror)
Magnification m = –3
m = −v /u
∴ ν= –mu = –(–3) (–10) = –30 cm.
Image formed ν = 30 cm in front of the concave mirror.

Question. A ray of light travelling in air enters obliquely into water. Does the light ray bend towards the normal or away form the normal? Why?
Answer : The light bends towards the normal on entry into water because water is optically
denser than air.

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.
Answer : The speed of light in vacuum is = 3 × 108 m/s
Refractive index of glass ng = 1.50
ng = c /v
Speed of light in glass νg
= c/ng = 3 × 108 /1.50
= 2 × 108 m/s.

Question. Find out from following table the medium having highest optical density. Also find the medium with lowest optical density.

Answer : Diamond has highest optical density i.e., 2.42 and air has the lowest optical density i.e., 1.0003.

Question. You are given kerosene, turpentine and water. In which of these does the light travel fastest? Use the information given in table above.
Answer : Refractive index of kerosene = 1.44
Refractive index of turpentine = 1.47
Refractive index of water = 1.33
Lower the refractive index faster is the speed of light in that medium. Hence light will travel fastest in water.

Question. The refractive index of diamond is 2.42. What is the meaning of this statement?
Answer : As refractive index = speed of light in air /speed of light in diamond
This means the ratio of the speed of light in the air and the speed of light in diamond is equal to 2.42.

Question. Define 1 dioptre of power of a lens.
Answer : 1 dioptre is the power of a lens whose focal length is 1 metre.
1 D = 1 m–1

Question. A convex lens forms a real and inverted image of a needle at a distance of 50 cm from it.
Answer : 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.
1/v – 1/u = 1/f
v = + 50 cm. Convex lens as image is of same size hence
u = – 50 cm

1 /f = 1/(50 cm) – 1/(-50 cm)
= 1/50 + 1/50 = 1/25 cm

Question. Find the power of a concave lens of focal length 2 m.
Answer : Focal length of concave lens f = –2 m
Power of concave lens P = 1/f
P = 1/-2 = –0.5 D.

Question. Define power of a lens.
Answer : The degree of convergence or divergence of light rays achieved by lens is expressed in terms of power. It is given by
P = 1/f                f = focal length in metre

Question. What is the S.I. unit of power? Define it.
Answer : The S.I. unit of power is dioptre denoted by “D” 1 dioptre is the power of a lens whose focal length is 1 metre.
Power of a convex lens is +ve
Power of a concave lens –ve

Question. What is the magnification of a plane mirror?

Question. What is the radius of curvature of plane mirror?

Question. Which lens bends a light ray more or less with a shorter or with longer focal length?
Answer : The lens with the shorter focal length bends the light more.

Question. If a convex lens is used to focus sunlight on a paper, where the paper should be placed so that it catches fire.
Answer : At the Principal focus.

Question. What happens if a light falls on a glass slab making 90° at its surface?
Answer : It undergoes normal refraction that is there is no deviation in the light.

Question. Write nature, position and relative size of image formed by convex lens.
Answer : Table: Nature, position and relative size of the image formed by a convex lens for various positions of the object

Question. Write nature, position and relative size of image formed by cancave lens.
Answer : Table: Nature, position and relative size of the image formed by a concave lens for various positions of the object

Question. Give sign conventions for spherical lenses.

Question. Give the uses of concave mirrors.
Answer : (i) Concave mirror are used in torches, search-lights and vehicle headlights to get powerful parallel beams of light.
(ii) They are also used as shaving mirrors to see a large image of the face.
(iii) Dentists use concave mirrors to see large images of the teeth of patients.
(iv) Large concave mirrors are used in making solar furnaces, solar cookers etc.

Question. Give the uses of convex mirrors.
Answer : Convex mirrors are commonly used as rear-view mirrors in vehicles. As these mirrors can give an erect image, wider field of view, these mirrors are used in vehicles to see the traffic behind.

Question. With the help of a ray diagram show how a pencil appears when dipped in water.

A ray of light (as we see pencil in air passing into water) travels from rarer to denser medium i.e., from air to water, it bends towards the normal, hence the pencil appears to be bent in water as shown in the diagram.

Question. State the mirror formula, lens formula and power of lens.
1/v + 1/u = 1/f
(ν = image distance, u = object distance, f = focal length)
Lens formula      1/v + 1/u = 1/f
Power of lens  P = 1/f

Question. Draw the following diagram in your answer book and show the formation of image of the object AB with the help of suitable rays.

Question. Draw ray diagrams to represent the nature, position and relative size of the image formed by a convex lens for the object placed:
(a) At 2F
(b) Between F1 and the optical centre O of lens

(b)

Question. Which kind of mirrors are used in the headlights of a motor-car and why?
Answer : Concave mirror, to get the parallel beam of light.

Question. Give the laws of refraction of light.
Answer : The laws of refraction of light are
(i) The incident ray, the refracted ray and the normal to the interface of two transparent media at the point of incidence all lie in the same plane.
(ii) The ratio of sine of angle of incidence to the sine of angle of refraction is constant, for the light of a given colour and for the given pair of media. This law is also known as Snell’s law of refraction.

Question. What is refractive index?
Answer : If ‘i’ is the angle of incidence and ‘r’ is the angle of refraction then
sini / sinr = Constant This constant value is called the refractive index of the second medium with respect to the first.

Question. What is absolute refractive index of the medium?
Answer : When the refractive index of medium 2 is considered with respect to vacuum. This is called the absolute refractive index of the medium.
n/2 = Speed of light in 2 /Speed of light in 1

Question. Two medium with refractive index 1.31 and 1.50 are given. In which case
(i) bending of light is more?
(ii) speed of light is more?
Answer : (i) Bending of light is more in the medium where refractive index is 1.50.
(ii) Speed of light is more in the medium with refractive index 1.31

Question. When a ray of light entering from air is incident on the surface of a glass slab at an angle of 90°, what will be the measure of angle of refraction. Why does a ray change its path when it passes from one medium to another medium?
Answer : The angle of refraction will be zero. A light ray changes its path when it passes from one medium to another medium.

Question.Refractive index of kerosene oil is 1.44 and that of water is 1.33. A ray of light enters from kerosene oil to water. Where would light ray bend and why?
Answer : A ray of light enters from kerosene oil to water i.e., refractive index 1.44 to 1.33 i.e., from denser to rarer medium. Hence the ray of light bends away from the normal.

Question. Which is optically denser out of the two medium M1 = 1.71 (refractive index) and M2 = 1.36 (refractive index). How does speed of light change when it travels from optically rarer to denser medium.
Answer : Medium M1 with refractive index 1.71 is optically denser than the other medium M2.
Speed of light decreases when it travels from rarer to denser medium.

Question.Draw a ray diagram of image formed when an object is placed in front of convex lens (i) beyond 2 f and (ii) between f and 2f.

Question. Comment on the size, position of the image formed by a concave mirror of focal length
18 cm when an object is placed:
(i) at 22 cm (ii) 14 cm (ii) 40 cm.
in front of mirror without calculations.
Answer : (i) When the object is placed at 22 cm, the image is formed beyond 36 cm, real, inverted image is magnified.
(ii) When the object is at 14 cm then the image formed is virtual, behind the mirror and magnified.
(iii) When the object is placed beyond 40 cm, then the image is formed between 18 cm and 36 cm, it is real inverted and diminished image.

Question. Complete the following ray diagrams:

Question. Define power of lens. What is the S. I. unit of power of a lens? If power of lens is +2D what is the nature and focal length of the lens?
Answer : Power of lens: The degree of convergence or divergence of light rays obtained by a lens is expressed in terms of its power.
Power of a lens is defined as the reciprocal of its focal length.
P = 1/f
S.I. unit of power of a lens is ‘diopter’
P = +2 D.
Lens is convex and the focal length of the lens is +0.50 m.
( P = 1/f’      2D = 1/f’      ∴    f=1/2 = 0.005 )

Question. State the laws of reflection of light. Answer : Laws of reflection of light are:
(i) The angle of incidence is equal to the angle of reflection and
(ii) The incident ray, the normal to the mirror at the point of incidence and the reflected ray, all lie in the same plane.

Question. What are the properties of the image formed by plane mirror?
Answer : Image formed by plane mirror is always virtual and erect. The size of the image is equal to that of the object. The image formed is as far behind the mirror as the object is in front of it and the image is laterally inverted.

Question. With the help of a ray diagram show the type of images formed when object is placed at the following positions in front of concave mirror.
(a) at infinity (b) beyond C
(c) at F (d) at C
(e) between F and C (f) between F and O
 C = centre of curvature O = optical centre of the mirror
F = focus

Question. With the help of a ray diagram show the position, size and the nature of the image formed by a convex lens for various positions of the object.

Question. Name the type of mirror used in the following situations:
(i) Rear view mirror in vehicles
(ii) Solar furnace
(iii) Torch
(iv) Solar cooker
(v) To get the full length image of tall building.
Answer : (i) Rear view mirror in vehicles – convex mirror as it gives virtual image, diminished and cover the wider view.
(ii) Solar furnace – concave mirror to concentrate all parallel beam of light.
(iii) Torch – concave mirror is used.
(iv) Solar cooker – concave mirror is used to concentrate the heat rays at a point.
(v) Convex mirror is used to view a full length tall building.

Question. Draw and explain the ray diagram formed by a convex mirror when
(a) object is at infinity. (b) object is at finite distance from the mirror.
Answer : (a) When the object is at infinity, the image is formed at focus. ‘F’. .

(b) When the object is at finite distance the image is formed behind the mirror, it is virtual image and diminished in size.

Questions On High Order Thinking Skills

Question. Amit visited a fair and saw a mirror in which he got a very funny image. The above part of his body was big in size, middle part was of normal size and the lower part of the body showed very small size. What kind of mirror is this?
Answer : Upper part – Concave mirror; Middle part : Plane mirror; Lower part : Convex mirror.

Question. Nidhi wanted the image of her pencil to be double the size of its original size. Name the mirror used for getting such image.

Question. Give the mirror image of “AMBULANCE”

Question. An incident ray makes an angle of 60° with the mirror. What is the angle of reflection?

Question. Define the following.
(a) What is ray?
(b) What is beam?
(c) What is reflection of light?
(d) What is reflector?
(e) What is focal length?
(f) What is principal focus?
(g) What is refraction?
(h) What is optically rare medium?
(i) What is optically denser medium?
(j) What is power?
(k) What is 1 dioptre?
Answer : (a) It is the path of light.
(b) Group of parallel light rays emitted by the source of light.
(c) Bouncing back of light after striking any surface.
(d) The surface which reflects the light.
(e) The distance between the pole and the principal focus of the spherical mirror.
(f) A point of the principal axis where the rays of light parallel to principal axis meet.
(g) Bending of light ray when it travels from one medium to another.
(h) When the speed of light is more as compared to other medium.
(i) When the speed of light is less as compared to another medium.
(j) The degree of convergence or divergence of light rays achieved by a lens is expressed in terms of its power
(k) It is the power of lens whose focal length is 1 m.

Question. What are the two types of reflection?
Answer : (i) Regular (ii) Irregular

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

Question. Find the focal length of a lens of power –2.0 D. What type of lens is this?
Answer : Given, Power = –2.0
The given lens is concave (power of concave lens is –ve)

P = 1/f   (in metre)
–2.0 = 1/f
f = 1/–2 m
f = – 50 cm
∴ The focal length is – 50 cm.

Question. Which lens can be used as a magnifying glass? For which position of object does a convex lens form:
a. a virtual and erect image?
b. a real and inverted image of same size as that of object? Draw labelled ray diagrams to show the formation of the required image in each of the above two cases.
A convex lens of shorter focal length is used as a magnifying lens (glass).
a. A virtual and erect image in a convex lens is formed when object is placed in between F and O.

b. A real and inverted image of same size as that of an object is formed by a convex lens when the object is placed at 2F^Ch.

Question. (a) Write relation between u, v f for lenses and for mirrors, where u, v f are object distance, image distance and focal length respectively.
(b) The magnification produced by a concave mirror is m =+ 4. Write the information about the image given by this statement.
(c) Draw a ray diagram for the following and show the formation of the images in case of concave mirror when the object is placed:
(i) between the pole and focus point.
(ii) at the centre of curvature.
a. u,v and f reflection is given by lens formula

Question. (a) The refractive index of diamond is 2.42. What is the meaning of this statement?
(b) Redraw the diagram given below in your answer book and complete the path of ray.

(c) What is the difference between virtual images produced by concave, plane and convex mirrors?
(d) What does the negative sign in the value of magnification produced by a mirror indicates about a image?
a. The refractive index of diamond is 2.42 means that speed of light in diamond is (1/2.42) times the speed of light in air.

d. negative magnification produced by a mirror mean image is real.

Question. (a) Write one use of concave mirror as well as convex mirror.
(b) Draw ray diagrams for the following cases when a ray of light:
(i) passing through centre of curvature of a concave mirror is incident on it.
(ii) parallel to principal axis is incident on convex minor.
(iii) is incident at the pole of a convex mirror.
(iv) passing through focus of a concave mirror incident on it.
a. Use of concave mirrors
(ii) Used by ENT doctors to focus the light Use of convex mirror: Used as rear view mirrors in vehicles.

Question. (a) Draw a ray diagram in each of the following cases to show the position and nature of the image formed when object is placed:
(i) between focus and centre of curvature of a concave mirror.
(ii) between focus and pole of a concave mirror.
(iii) between centre of curvature and infinity for a concave mirror.
(b) (i) Give mathematical formulae for determining magnification produced by a spherical mirror.
(ii) What does m =- 1 signify? Identify the mirror that can produce it.

(i) Size of image = Size of object
(ii) Minus sign mean that the image is real and inverted. A concave mirror can produce it when object is kept at 2F^Ch.

Question. An object, 4.0 cm in size, is placed 25.0 cm in front of a concave mirror of focal length 15.0 cm. At what distance from the mirror should a screen be placed in order to obtain a sharp image? Find the nature and the size of the image.
We have
h1 = 4.0 cm
u =- 25 cm
f =- 15.0 cm
v = ?
h2 = ?

The position of screen must be 37.5 cm from the mirror.

Image is enlarged, inverted, real and same side of object.

Question. Draw the ray diagram in each case to show the position, nature of image formed when the object is placed:
a. at the centre of curvature of a concave mirror
b. within focal length of a convex lens
c. between pole and focus of concave mirror
d. in front of concave lens
e. in front of convex mirror

Question. (a) A converging lens forms a real and inverted image of an object at a distance of 100 cm from it.
Where should an object be placed in front of the lens, so that the size of the image is twice the size of the object? Also, calculate the power of a lens.
(b) State laws of refraction.
Answer : (a) v =+ 100 cm
m =+ 2

(ii) Incident ray, normal to the point of incidence and refracted ray lie in the same plane.

Question. (a) Explain the following terms related to spherical lenses:
(i) Optical centre
(ii) Centres of curvature
(iii) Principal axis
(iv) Aperture
(v) Principal focus
(vi) Focal length
(b) A converging lens has focal length of 12 cm.
Calculate at what distance should the object be placed from the lens so that , it forms an image at 48 cm on the other side of the lens.
a. See the key points in the beginning of this chapter.
b. f =+ 12 cm,
v =+ 48 cm
u = ?
Lens formula
1/v – 1/u = 1/f
1/48 – 1/u = 1/12 or 1/u = 1/48 – 1/12
u = -16 cm

Question. Draw a labelled ray diagram to locate the image of an object formed by a convex lens of focal length 20 cm when the object is placed 30 cm away from the lens.
f = 20 cm,
u =- 30 cm
v = ?
Lens formula,  1/v – 1/u = 1/f
1/v – 1/(-30) = 1/20
1/v – 1/20 – 1/30 ⇒ v = 60 cm
f = 20 cm
R = 40 cm,
u = 30 cm

Object is placed in between F and C(2F) image is formed beyond 2F(C) enlarged image.

Question. An object is situated at 8 cm from a convex lens of focal length 10 cm. Find the position and nature of image. Draw ray diagram to illustrate the formation of image (not to scale).
Answer : u =- 8 cm
f =+ 10 cm
Lens formula 1/v-1/u =1/f
1/v-1/-8 =1/10
1/v= -1/10 1/8
1/v= 4-5/40 = -1/40
v =- 40 cm
Virtual, erected and magnified image

Question. (a) Under what condition, a concave mirror produces a virtual and magnified image? Draw a labelled ray diagram to show the formation of image in the above case. Also, state the position of object to produce magnified and real image.
(b) A ray of light moving along principal axis is falling on a concave mirror. Draw the path of reflected ray. Also, state the values of angle of incidence and reflection in this case.
Answer : a. A concave mirror will produce a virtual and magnified image of the object if placed in between F and pole of the mirror.

Real and magnified image will be formed if the object is placed in between 2F^Ch and F in front of a concave mirror.
b. Ray of light moving along the principal axis will retraces it path because incident angle ∠i = reflected angle ∠r = 0.

Question. One half of a convex lens of focal length 10 cm is covered with a black paper. Can such a lens produce an image of a complete object placed at a distance of 30 cm from the lens? Draw a ray diagram to justify your answer.
A 4 cm tall object is placed perpendicular to the principal axis of a convex lens of focal length 20 cm.
The distance of the object from the lens is 15 cm. Find the nature, position and size of the image.
Yes, the lens will produce a complete image but brightness of image will be reduced

Question. (a) Define absolute refractive index.
(b) The path of a light ray from three different media A, B and C for a given angle of incidence is shown below. Study the diagrams and answer the following questions:

(i) Which of the three media A, B or C has maximum optical density?
(ii) Through which of the three media, will the speed of light be maximum?
(iii) Will the light travelling from A to B bend towards or away from the normal?
(iv) Will the refractive index of B relative to C be more than unity or less than unity?
(a) Absolute refractive index of a medium is the ratio of speed of light in air/vacuum to the speed of light in medium.
(b) (i) Angle of refraction in medium C is minimum.
Hence medium C is maximum optical denser.
(ii) In least denser medium i.e. A the speed of light will be maximum.
(iii) Towards normal
(iv) Less than 1

Question. A 2.0 cm tall object is placed perpendicular to the principal axis of a convex lens of 1 focal length 10 cm.
The distance of the object from the lens is 15 cm. Find the position, nature and size of the image forms.
Answer : In convex lens, h1 = 2.0 cm
f = 10 cm
u =- 15 cm
v = ?

Image is real, inverted and four times enlarged.

Question. State the relation between object distance, image distance and focal length of a concave or convex mirror.
A concave mirror produces two times magnified real image of an object at 10 cm from it. Find the position of the image.
For concave or convex mirrors the relation between u, v and f is given by

mirror formula, 1/v + 1/u = 1/f

m =- 2
u =- 10 cm
m = -v/u = -2 or v = 2u =- 20 cm
v =- 20 cm

Question. The focal length of a concave mirror is 30 cm. Find the position of the object in front of the mirror so that the image is three times the size of the object.
Answer : f =- 30 cm, m =- 3
u = ?
a. For real image

object must be 40 cm in front of mirror to get a real inverted image by the concave mirror.
b. For virtual image

(Image 86)

i.e. if the object is placed at a distance of 20 cm in front of mirror then virtual, an erected image is produced by the concave mirror.

Question. A 5 cm tall object is placed perpendicular to the principal axis of a convex lens of focal length 20 cm.
The distance of the object from the lens is 30 cm.
Find the
a. position
b. nature
c. size of the image formed.

Given, h = 5 cm
u =- 30 cm
f =+ 20 cm
v = ?

Question. An object 4 cm high is placed at a distance of 27 cm in front of a convex lens of focal length 18 cm. Find the position, nature and size of the image formed.
We have h1 = 4 cm
u =- 27 cm
f =+ 18 cm
v = ?

Size of image is 8 cm, -ve sign means image is real, inverted and 2 times magnified.

Question. An object 5 cm high is kept 25 cm away from a converging lens (convex lens) of focal length 10 cm.
What is nature, position and the size of the image?
Given: h1 = 5 cm (height of object)
u =- 25 cm
f =+ 10 cm
v = ?
Lens formula

Image is real, smaller in size, inverted at a distance of 16.7 cm from the lens.

Question. An object is placed 15 cm from a convex mirror of radius of curvature 60 cm. Find the position of image and its magnification.
Here, u =- 15 cm
R =+ 60 cm
f = R2  =+ 30 cm
v = ?
m = ?

Question. A point object is placed at a distance of 12 cm from a convex lens on its principal axis. Its image is formed on the other side of the lens at a distance of 18 cm from the lens. Find the focal length of the lens. Is the image magnified? Justify your answer.
Answer : Here, u =- 12 cm
v = 18 cm
f = ?

m =- 1.5 means the image is magnified

Question. (a) Two lenses have power of (i) +2D (ii) -4D.
What is the nature and focal length of each lens?
(b) An object is kept at a distance of 100 cm for a lens of power -4D. Calculate image distance.

93. A 4.5 cm needle is placed 12 cm away from a convex mirror of focal length 15 cm. Give the location of the image and its magnification.
Answer : h1 = 4.5 cm, u =- 12 cm
f =+ 15 cm, v = ?
Mirror formula, 1/v+1/u = 1/f

Size of image is +2.5, virtual, erect and smaller behind the mirror.

Question. A convex mirror used on an automobile has 3 m radius of curvature. If a bus is located at 5 m from this mirror, find the position, nature and size of the image.
We have R =+ 3 m

i.e. image is virtual, erected and smaller in size behind the mirror.

Question. The power of a lens is 2.5 dioptre. What is the focal length and the type of lens?
f = 1/p = 100/2.5 cm
f = 40 cm
Focal length is positive hence it is a convex lens.

Question. A10 cm tall object is placed perpendicular to the principal axis of a convex lens of focal length 30 cm.
The distance of the object from the lens is 20 cm. Find the (i) position, (ii) nature and (iii) size of the image formed.
Answer : We have h1 = 10 cm
u =- 20 cm
f =+ 30 cm
v = ?
Lens formula,

Virtual, erect, enlarged image.

Question. Find the position, nature and size of the image of an object 3 cm high placed at a distance of 9 cm from a concave mirror of focal length 18 cm.
Answer : h1 = 3 cm
u =- 9 cm
f =- 18 cm
v = ?
Mirror formula,
1/v+1/u = 1/f
1/v+1/-9 = 1/-18

Image is formed at a distance of 18 cm behind the mirror which is erected, virtual and magnified image.

Question. (a) A concave mirror produces three times enlarged image of an object placed at 10 cm in front of it.
Calculate the focal length of the mirror.
(b) Show the formation of the image with the help of a ray diagram when object is placed 6 cm away from the pole of a convex mirror.
Answer : (a) u =- 10 cm
Let a real image is formed in concave mirror i.e.,

(b) When u =- 6 cm from pole of a convex mirror.
For convex mirror, the image will be virtual, erected and smaller.

Question. An object 1 cm tall is placed at a distance of 15 cm from a concave mirror of focal length 10 cm. Find the position, size and nature of the image formed.
In concave mirror
h1 = 1 cm
u =- 15 cm
f =- 10 cm
v = ?

Image is real, inverted and 2 times larger.

Question. (a) Describe an activity to find the approximate value of focal length of a concave mirror.
(b) What happens to the size of the image of an object when it is moved gradually away from a concave mirror?
(c) In an experiment to study refraction through a glass slab, it is observed that a ray of light undergoing refraction emerges parallel to the direction of incident ray. Why does it happen so?
Explain with the help of a diagram.