Refraction of Light

   

 

Summary

When a ray of light is incident on a boundary separating the two media having different densities:

A part of the light gets reflected.

Rest of the light changes its direction as it enters the second medium.

The change of direction suffered by a ray of light as it passes obliquely from one medium to

another is known as refraction

Incident Ray

The ray of light striking the surface of separation of the media through which it is travelling

is known as the incident ray.

Point of Incidence

The point at which the incident ray strikes the surface of separation is called the point of

incidence.

Normal

The perpendicular drawn to the surface of separation at the point of incidence is called the

normal.

Refracted Ray

The ray of light that has changed its direction at the surface of separation when travelling

from one medium to another is called the refracted ray.

Angle of Incidence

The angle, which the incident ray makes with the normal at the point of incidence, is called

the angle of incidence.

Angle of Refraction

The angle, which the refracted ray makes with the normal at the point of incidence, is called

the angle of refraction.

Laws of Refraction

The laws of refraction are:

The incident ray, the refracted ray and the normal to the surface at the point of

incidence all lie in one plane.

For any two given pair of media, the ratio of the sine of the angle of incidence to the

sine of the angle of refraction is a constant.

The second law is called Snell's law after the scientist Willebrod Snell who first formulated

it

where nm is the refractive index of the second medium with respect to the first medium.

Refractive Index

Refractive index or absolute refractive index of a medium is equal to the ratio of the speed

of light in vacuum or air to the speed of light in that particular medium.

This ratio is always a constant for a given pair of media.

Lens

A lens is a transparent medium that has at least one curved surface.

Convex Lens

A convex lens is thicker in the middle and thinner at its edges.

Concave Lens

A concave lens is thicker at the edges and thinner in the middle.

Optic Centre

The centre of a lens is called the optical centre or optic centre. It is denoted by the letter O.

Centre of Curvature

The centres of the spherical surfaces forming a lens are known as the centres of curvature

of the lens. The letter C is used to denote centre of curvature.

Principal Axis

The line passing through the centres of curvature C1 and C2 of two spherical surfaces of

the lens is called the principal axis of the lens.

Principal Focus of a Convex Lens

The principal focus F of a convex lens is a point on the principal axis to which rays parallel

to the principal axis converge after refraction.

Principal Focus of a Concave Lens

The principal focus F of a concave lens is a point on the principal axis from which rays

parallel to the principal axis appear to diverge after refraction.

Focal Length

The distance between the optic centre and the focus is the focal length f of the lens.

Rays that are generally considered while constructing ray diagrams:

Any ray of light, travelling parallel to the principal axis, after refraction through the

lens, passes through the principal focus of the lens.

Any ray of light, which first passes through principal focus or which appears to

meet at the focus after refraction, always travels parallel to the principal axis.

Any ray of light which passes through optical centre of lens does not deviate form

its path.

The image is formed at the point of intersection of these refracted rays.

Sign Convention for Lenses

All distances on the principal axis are measured from the optical centre.

The distances measured in the direction of incident rays are positive and all the

distances measured in the direction opposite to that of the incident rays are negative.

All distances measured perpendicular to and above the principal axis are positive.

Thus, height of an object and that of an erect image are positive

And all distances measured perpendicular to and below the principal axis are

negative.

Lens Formula

Where u is the object distance or distance of the object from the lens, v is the image

distance or the distance of the image from the lens and f is the focal length or the distance

of the principal focus from the lens.

Magnification

It is the ratio of the height of the image to the height of the object. It is represented by the

letter m

Magnification produced by a lens is also related to the object distance u, and the image

distance v.

Power of a Lens

Power of a lens is defined as the reciprocal of its focal length in metres.

The SI unit of power is dioptre

1D = 1 m-1

Question (1): The bending of a beam of light when it passes obliquely from one medium to another is known as 

_______. 

1. reflection

2. refraction

3. dispersion

4. deviation

Ans:  2

Question (2): The part of the

lens through which the ray of

light passes without suffering

deviation is called ________.

1. optical centre

2. focus

3. centre of curvature

4. pole

Ans:  1

Question (3): Convex lens

always gives a real image if the

object is situated beyond

_______.

1. optical centre

2. centre of curvature

3. focus

4. radius of curvature

Ans: 3

Question (4): Parallel rays of

light entering a convex lens

always converge at _______.

1. centre of curvature

2. the principal focus

3. optical centre

4. the focal plane

Ans:  2

Question (5): Where should an

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Refraction of Light

Introduction

Refraction of Light

Laws of Refraction

Refraction of Light through a Glass

Slab

Verification of Laws of Refraction -

An Alternate Method

Lenses

Terminology Used in Optics

Formation of Image by a Convex

object be placed so that a real

and inverted image of the same

size is obtained, using a convex

lens?

1. Between O and F

2. At F

3. At 2 F

4. At infinity

Ans:  3

Question (6): SI unit of the

power of a lens is ___________.

1. dioptre

2. cm

3. metre

4. watt

Ans:  1

Question (7): 1 D is the power

of the lens of focal length of

______ cm.

1. 100

2. 10

3. 1/100

4. 1/10

Ans:  1

Question (8): In a simple

microscope lens used is

__________.

1. biconvex

2. biconcave

3. plano convex

4. cylindrical

Ans:  1

Question (9): Reciprocal of focal

length in metres is known as the

______ of a lens.

Lens

Formation of Image by a Concave

Lens

Sign Convention for Lenses

Lens Formula

Magnification

Power of a Lens

Summary

Question and Answers

Multiple Choice Questions

1. focus

2. power

3. power of accommodation

4. far point

Ans:  2

Question (10): A convex lens is

called _________.

1. converging lens

2. diverging lens

3. both converging and diverging lens

4. refracting lens

Ans:  1

Question (11): A positive

magnification greater than unity

indicates

_____________________.

1. real image

2. virtual image

3. neither real not virtual image

4. distorted image

Ans:  2

Question (12): The power of a

convex lens of focal length 50

cm is ______.

1. + 2D

2. - 2D

3. 50 D

4. - 5D

Ans:  1

Question (13): The focal length

of a lens whose power is -1.5 D

is _______.

1. -66.66 cm

2. + 1.5 m

3. + 66.66 cm

4. -1.5 m

Ans:  1

Question (14): Real images

formed by single convex lenses

are always ________________.

1. on the same side of the lens as the object

2. inverted

3. erect

4. smaller than the object

Ans:  2

Question (15): An object is

placed 12 cm from a convex lens

whose focal length is 10 cm. The

image must be.

1. virtual and enlarged

2. virtual and reduced in size

3. real and reduced in size

4. real and enlarged

Ans:  4

Question (16): When a person

uses a convex lens as a simple

magnifying glass, the object

must be placed at a distance.

1. less than one focal length

2. more than one focal length

3. less than twice the focal length

4. more than twice the focal length

Ans:  1

Question (17): The image

produced by a concave lens is

________.

1. always virtual and enlarged

2. always virtual and reduced in size

3. always real

4. sometimes real, sometimes virtual

Ans:  2

Question (18): A virtual image is

formed by _______.

1. a slide projector in a cinema hall

2. the ordinary camera

3. a simple microscope

4. telescope

Ans:  3

Question (19): An object is

placed 25 cm from a convex lens

whose focal length is 10 cm. The

image distance is ________ cm.

1. 50 cm

2. 16.66 cm

3. 6.66 cm

4. 10 cm

Ans:  2

Question (20): The least

distance of distinct vision is

______.

1. 25 cm

2. 25 m

3. 0.25 cm

4. 2.5 m

Ans:  1

Question (21): A convex lens

has a focal length of 20 cm. Its

power in dioptres is

___________.

1. 2

2. 5

3. 0.5

4. 0.2

Ans:  2

Question (22): An object is

placed before a concave lens.

The image formed

_____________-.

1. is always erect

2. may be erect or inverted

3. is always inverted

4. is always real

Ans:  1

Question (23): A ray of light

travels from a medium of

refractive index n1 to a medium

of refractive index n2. If angle of

incidence is i and the angle of

refraction is r.

Then is equal to

1. n1

2. n2

3. n21

4. n12

Ans:  3

Question (24): Two thin lenses

of power +5 D and -2 D are

placed in contact with each

other. Focal length of the

combination is

1. +3 m

2. -3 m

3. 0.33 m

4. -0.33 m

Ans:  3

Question (25): The lens formula

in cartesian frame is

______________.

1.

2.

3.

4.

Ans: 3

Question (1): Define refraction. 

Answer:  The deviation in the path of light when it passes from one medium

to another medium of different density is called refraction.

Question (2): Define refractive index.

Answer:  The ratio of the speed of light in vacuum to the speed of light in a

medium is called the refractive index of the medium.

Question (3): What is the unit of refractive

index?

Answer:  Refractive index is the ratio of velocity of light in two media and

hence it is a mere number without any unit.

Question (4): List out the factors on which the

refractive index of a medium depends.

Answer:  The refractive index of a medium depends on

1) the nature of the medium

2) the colour or wavelength of the incident light

Question (5): Define angle of incidence.

Answer:  The angle which the incident ray makes with the normal at the

point of incidence is called the angle of incidence.

Question (6): What is the angle of incidence if

a ray of light is incident normal to the surface

separating the two media?

Answer:  Angle of incidence is equal to zero if a ray of light is incident

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normal to the surface separating the two media.

Question (7): What is a lens?

Answer:  A lens is a portion of a transparent refracting medium bounded by

two spherical surfaces which are generally spherical or cylindrical or one

curved and one plane surface.

Question (8): What is a concave lens?

Answer:  A lens which is thinner at the middle and thicker at the edges is

called a concave lens.

Question (9): What is the nature of the focus of

a concave lens?

Answer:  The focus of a concave lens is virtual.

Question (10): What type of image is formed

by a concave lens?

Answer:  A concave lens always forms a virtual and erect image.

Question (11): A thin lens has a focal length f

= -12 cm. Is it convex or concave lens?

Answer:  The lens is concave since the focal length is negative.

Question (12): A lens forms an erect image for

all positions of the object in front of it. Is the

lens convex or concave?

Answer:  Concave lens.

Question (13): Where should an object be

placed so that a real and inverted image of

same size is obtained using a convex lens?

Answer:  The object has to be placed at 2F to get a real and inverted image

of same size.

Question (14): Write the relation between u,v

and f of a thin lens.

Answer: 

Refraction of Light

Introduction

Refraction of Light

Laws of Refraction

Refraction of Light

through a Glass Slab

Verification of Laws of

Refraction - An Alternate

Method

Lenses

Terminology Used in

Optics

Formation of Image by a

Convex Lens

Formation of Image by a

Concave Lens

Sign Convention for

Lenses

Lens Formula

Magnification

Power of a Lens

Summary

Question and Answers

Multiple Choice

Questions

Question (15): What is the sign of u, v and f for

a convex lens according to Cartesian sign

convention?

Answer:  According to sign convention u is negative, v is positive for all

positions of the object except when the object is between the optic centre

and first focus and f is positive.

Question (16): An object of height 1m is

placed at a distance of 2f from a convex lens.

What is the height of the image formed?

Answer:  The height will also be equal to 1m since the object placed at 2F

of a convex lens gives an image of the same at 2F on the other side of the

lens.

Question (17): Define power.

Answer:  The power of a lens is defined as the reciprocal of its focal length

in metres.

Question (18): What is least distance of

distinct vision?

Answer:  The minimum distance upto which an eye can see clearly is called

the least distance of distinct vision.

Question (19): What happens when a ray of

light passes through the optical centre of a

lens?

Answer:  The ray of light does not suffer any deviation.

Question (20): State the laws of refraction.

Answer: 

1) The incident ray, the refracted ray and the normal at the point of incidence

all lie in one plane.

2) For any two given pair of media, the ratio of the sine of the angle of

incidence to the sine of the angle of refraction is a constant.

Question (21): Diagrammatically represent the

refraction of light through a rectangular glass

slab.

Answer: 

Question (22): Define convex lens. Why is it

referred to as converging lens?

Answer:  A lens which is thicker in the middle and thinner at the edges is

called convex lens. Convex lens is referred to as a converging lens because

the parallel rays of light after refraction through a convex lens meet at a

point on the principal axis.

Question (23): Draw a diagram to show the

second principal focus of a convex lens.

Answer: 

Question (24): Distinguish between a convex

and a concave lens.

Answer: 

Convex lens Concave lens

Is thicker in the middle and

thinner at the edges

Is thinner at the middle and

thicker at the edges

Focus is real Focus is virtual

It is a converging lens It is a diverging lens

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Question (25): Draw a ray diagram to show the

refraction of light when it passes through the

optic centre of a convex lens.

Answer: 

Question (26): List out the uses of convex

lenses.

Answer:  Convex lenses are used a) as a magnifying glass b) in

photocopying cameras c) as the objective lens of a microscope and a

telescope d) in theatre spot lights

Question (27): With the help of a ray diagram

show how an object gets magnified in a simple

microscope.

Answer: 

When an object is placed between O to F1 we get an enlarged and erect

image of the object.

Question (28): Define the power of a lens.

What is its unit?

Answer:  Power of a lens is defined as the reciprocal of its focal length. The

unit of power is dioptre.

Question (29): With the help of a diagram

explain how light gets refracted when it passes

through a rectangular glass slab.

Answer: 

Place a rectangular glass slab on a white sheet of paper fixed on a

drawing board.

Trace the boundary ABCD of the glass slab.

Remove the glass slab. Draw an incident ray IO on AB.

Draw the normal NN1 at the point of incidence O

Fix two pins P and Q on the incident ray IO.

Place the glass slab within its boundary ABCD.

Looking from the other side of the glass slab fix two pins R and S

such that your eye and the feet of all the pins are in one straight line.

Remove the glass slab and the pins. Mark the pin points P1, P2, P3

and P4.

Join OO1.It is the refracted ray.

Measure are the angle of

incidence, angle of refraction and angle of emergence respectively.

Extend O1E backwards. The emergent ray is parallel to the incident

ray.

The above experiment shows that

When a ray light is passing from air to glass, that is, from a rarer

medium to a denser medium, the refracted ray bends towards the

normal drawn at the point of incidence. In this case But when

the ray of light is passing from glass to air, that is, from a denser to a

rarer medium the refracted ray bends away from the normal. In this case

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Question (30): An object is placed at a

distance of 50 cm from a concave lens of focal

length 20 cm. Find the nature and position of

the image.

Answer:  The distance between the object and the lens (u) = -50 cm

Focal length f = -20 cm

Distance of the image from the optic centre = v

The image is formed 14.3 cm from the lens on the same side as the object

and since v is negative the image formed is virtual and erect.

Question (31): An object is placed 50 cm from

a lens which produces a virtual image at a

distance of 10 cm in front of the lens. Draw a

diagram to show the formation of image and

calculate the focal length of the lens.

Answer:  Distance between the object and the lens = -50cm

Distance between the image and the lens = -10cm

Focal length = ?

The focal length of the lens is -12.5 cm and the negative sign indicates that

the lens is concave.

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Question (32): An object of height 4 cm is

placed at a distance of 10 cm from a convex

lens of focal length 20 cm. Find the position,

nature and size of the image.

Answer:  Distance between the object and the lens u = -10cm

Focal length = f = 20 cm

Distance between the image and the lens v = ?

The image is formed on the same side as the object and since v is negative

the image is erect and virtual and is formed at a distance 20 cm in front of

the lens. We know that

height of the object ho = 4 cm

Height of the image = 8 cm

Question (33): What is the power of a lens

having a focal length of a) 50 cm b) -50cm

Answer: 

When f = 50 cm

When f = -50 cm

<>

Question (34): Draw a ray diagram to show the

position and nature of the image formed by a

convex lens when the object is placed

a) at 2F1

b) between F1 and 2F1

c) beyond 2F1

Answer:  a) at 2F1

b) between F1 and 2F1

c) beyond 2F1

Question (35): State and verify Snell's law.

Answer:  Snell's law states that for any two given pair of media, the ratio of

the sine of the angle of incidence to the sine of the angle of refraction is a

constant.

Place a rectangular glass slab on white sheet of paper fixed on a drawing

board.

Trace the boundary ABCD of the glass slab.

Remove the glass slab and draw a normal N 1 N 2 at O.

Draw a straight line IO inclined at an angle say 30 o with the normal. IO is

the incident ray.

Fix two pins P and Q on the incident ray IO.

Place the glass slab within its boundary ABCD.

Looking from the other side of the glass slab fix two other pins R

and S such that P, Q, R and S appear to lie on the same straight line.

Remove the glass slab and the pins. Mark the pin points P, Q, R

and S.

Join the pins R and S and produce the line on both sides. The ray

O 1 E is the emergent ray.

Join OO 1 . It is the refracted ray.

With O as centre, draw a circle of a convenient radius 'r' in such a

way that it cuts the incident and the refracted rays at F and G

respectively.

From F and G draw perpendiculars to the normal N1N2.

FHO and GKO are right angled triangles.

Measure the length of FH and GK.

Repeat the experiment for different values of angle of incidence.

Record the result in a tabular form

S. No I FH GK

1

2

3

4

5

Find the values of

for different values of I.

Will be a constant verifying