physics chapter-10 light: reflection and refraction

$: Physics Chapter-10 Light: Reflection and refraction$
Introduction:

Light is a form of energy that enables us to see things. Light starts from a source and bounces off objects which are perceived by our eyes and our brain processes this signal, which eventually enables us to see.

Nature of Light

Light behaves as a:

ray, e.g. reflection

wave, e.g. interference and diffraction

particle, e.g. photoelectric effect

Phenomenon of light

image formation by mirrors,

the twinkling of stars,

the beautiful colours of a rainbow,

bending of light by a medium and so on.

Characteristics of light

Speed of light c=λ×�, where

Speed of light is a constant which is

Rectilinear propagation of light: Light travels in a straight line between any two points.

Some key words:

1.Ray of Light: A line drawn in the direction of propagation of light is called a ray of light.

2. Beam of Light: A group of rays of light emitted by a source of light is called a beam of light. A light beam is of three types.

(i) Parallel beam: A group of light rays parallel to each other is known as parallel beam of light.

(ii) Divergent beam: A group of light rays spreading out from a source of light is called divergent beam of light.

(iii) Convergent beam: A group of light rays

Physics

Chapter-10

Light: Reflection and refraction


wave, e.g. interference and diffraction

particle, e.g. photoelectric effect

image formation by mirrors,

the beautiful colours of a rainbow,

bending of light by a medium and so on.

, where λ is its wavelength and � is its frequency.

Speed of light is a constant which is 2.998×108m/s or approximately 3.0×10


A line drawn in the direction of propagation of light is called a ray of light.

A group of rays of light emitted by a source of light is called a beam of light. A light

A group of light rays parallel to each other is known as parallel beam of light.

A group of light rays spreading out from a source of light is called divergent beam of

A group of light rays meeting at a point is called convergent beam of light.


3.0×108m/s.


A line drawn in the direction of propagation of light is called a ray of light.

A group of rays of light emitted by a source of light is called a beam of light. A light

A group of light rays parallel to each other is known as parallel beam of light.

A group of light rays spreading out from a source of light is called divergent beam of

meeting at a point is called convergent beam of light.

3. Reflection of Light: There are some surfaces which have ability to send the light back in the same medium when light strikes it. This phenomenon of sending the light back in the same medium by a surface is called reflection of light.

4. Image: When light rays meet or appear to meet after reflection from a mirror,then it is called an image.

1. Real Image: It is a kind of image which is formed by actual intersection of light rays after reflection.

2. Virtual Image: It is a kind of image which is formed by producing the reflected rays backward after reflection.

Characteristics of images

Images can be real or virtual, erect or inverted, magnified or diminished.

A real image is formed by the actual convergence of light rays.

A virtual image is the apparent convergence of diverging light rays.

If an image formed is upside down then it is called inverted or else it is an erect image.

If the image formed is bigger than the object, then it is called magnified.

If the image formed is smaller than the object, then it is diminished.

5. Plane Mirror: Plane mirror is a piece of glass whose one side is polished by using silver paint, which is covered by a coating of red paint to protect the silver layer.

Any flat and polished surface that has almost no irregularities on its surface that reflect light is called as a plane mirror.

Image formation by a plane mirror

The image formed by a plane mirror is always virtual and erect.

Object and image are equidistant from the mirror

The size of the image is equal to that of the object.

The image is Erect.

laws of reflection of light

(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.

These laws of reflection are applicable to all types of reflecting surfaces including spherical surfaces.

Spherical mirrors:

Mirrors, whose reflecting surface are curved inward or outward spherically are called spherical mirror.

For example – Spoon

A spherical mirror, whose reflecting surface is curved inwards, that is, faces towards the centre of the sphere, is called a concave mirror.

A spherical mirror whose reflecting surface is curved outwards, is called a convex mirror.

Few Basic terms related to Spherical Mirror

Pole: The geometrical central point of the reflecting spherical surface. (aperture), denoted by (P).

Aperture: The width of reflecting spherical surface.

Principal axis: Line joining the pole and centre of curvature of the spherical mirror.

Centre of curvature: The centre of the hollow glass sphere of which the spherical mirror is a part is called as centre of curvature, denoted by C

Radius of curvature: The distance between the pole and the centre of curvature. i.e. PC = R or the radius of the hollow sphere of which the mirror is a part.

Focus point: The point on the principal axis, where all parallel rays meet after reflection is called as Principal Focus or Focus. It is denoted by letter ‘F’.

Focal length: The distance between the pole and focus point i.e. PF = f

Relationship between focal length and Radius of curvature

Radius of curvature= 2 x focal length

R = 2 x f

Or f = R /2

General rules for image formation using ray diagrams:

Ray of light passing through the centre of curvature of any mirror is reflected path

Image Formation by Concave Mirror

By changing the position of the object from the concave mirror, different types of images can be formed. Different types of images are formed when the object is placed:

1. At the infinity 2. Beyond the centre of curvature3. At the centre of curvature 4. Between the centre of curvature and principal focus5. At the principal focus 6. Between the principal focus and pole

When the object is placed:

1. At the infinity

Characteristics of Image:

Image will be formed at the focus F

Highly diminished, (point-sized)

Real and inverted

Rule-3 Ray of light passing through the centre of curvature of any mirror is reflected

Image Formation by Concave Mirror

By changing the position of the object from the concave mirror, different types of images can be formed. Different types of images are formed when the object is placed:

Beyond the centre of curvature

Between the centre of curvature and principal focus

Between the principal focus and pole

formed at the focus F

sized)

Ray of light passing through the centre of curvature of any mirror is reflected back along the same

By changing the position of the object from the concave mirror, different types of images can be formed.

2. Beyond the centre of curvature


Image will be formed between F and C

Diminished,

Real and inverted

3. At centre of curvature C


Image will be formed at C

Same size

Real and inverted

4. Between the centre of curvature C and principal focus F


Image will be formed beyond C

Magnified (enlarged)

Real and inverted

5. At the principal focus

Beyond the centre of curvature

Image will be formed between F and C

the centre of curvature C and principal focus F

Image will be formed beyond C


Image will be formed at infinity

Highly magnified (enlarged)

Real and inverted 6. Between the principal focus and pole


Image will be formed behind the mirror


Virtual and erect

Positions of Object and Image in Concave Mirror

Position of Object Position of Image

At infinity At focus

Between infinity and C

Between F and C

At C At C

Between C and F Beyond C

At F At infinity

Between F and P Behind mirror

Image formation by a Convex Mirror

The image formed in a convex mirror is always virtual and erect, whatever be the position of the object.

1. When an object is placed at infinity


At the focus F, behind the mirror

Highly diminished, point-sized

Virtual and erect

2. When an object is placed at a finite distance from the mirror

infinity

(enlarged)

Between the principal focus and pole

Image will be formed behind the mirror

Positions of Object and Image in Concave Mirror

Position of Image Size of Image

Point sized, highly diminished

Between F and C Diminished

Same size

Enlarged

Highly enlarged

Behind mirror Enlarged

Image formation by a Convex Mirror


When an object is placed at infinity

At the focus F, behind the mirror

sized

2. When an object is placed at a finite distance from the mirror

Nature of I Image

Real and inverted

Real and inverted

Real and inverted

Real and inverted

Real and inverted

Virtual and erect



Between P and F, behind the mirror

Diminished

Virtual and erect

Positions and Nature of Image in Convex Mirror

Position of Object Position of Image Size of Image Nature of Image

At infinity At F, behind mirror Highly diminished Virtual and erect

Between infinity and P Between F and P, behind mirror

Diminished Virtual and erect

Uses of concave mirrors

Concave mirrors are commonly used:

In torches, search-lights and vehicles headlights to get powerful parallel beams of light.

As shaving mirrors to see a larger image of the face.

to see large images of the teeth of patients by dentists.

to concentrate sunlight to produce heat in solar furnaces.

Uses of convex mirrors

Convex mirrors are commonly used as rear-view (wing) mirrors in vehicles because

i)They always give an erect, though diminished, image.

ii) Also, they have a wider field of view as they are curved outwards. Thus, convex mirrors enable the driver to view much larger area than would be possible with a plane mirror.

Sign Convention for Reflection by Spherical Mirrors

Distances are to be measured from the pole (vertex) of the mirror.

Distances measured along the direction of the incident ray are positive. The distance measured opposite the direction of the incident ray is negative.

Distances measured above the principal axis are positive. Distances measured below the principal axis are negative.

Spherical Mirror Formula

A mirror formula can be defined as the formula which gives the relationship between the distance of object 'u', the distance of image 'v', and the focal length of the mirror 'f'.

Object distance is the distance of the object from the pole of the mirror; denoted by the letter u.

Image distance is the distance of the image from the pole of the mirror and it is denoted by the letter v.

And focal length is the distance of the principal focus from the pole of the mirror.

The expression which gives the relation between these three quantities is called the mirror formula which is given as:

Mirror formula is applicable for all spherical mirrors for every position of the object.

Magnification by Spherical Mirrors

Magnification is the increase in the image size produced by spherical mirrors with respect to the object size. It is the ratio of the height of the image to the height of the object and is denoted as ‘m’. The magnification, m produced by a spherical mirror can be expressed as:

m = �� (�’ )

�� ( �)

m = �’

�

Magnification is also equal to the ratio of image distance (v) to the object distance(u).

m = ��

�

It can be expressed as:

Magnification (m) = ��

� =

�’

�

IMP POINTS

Object distance (u) will always be negative(-ve) forboth concaveand convex mirror. Image distance (v) will always be positive (+ve) for convex mirror.

Image distance (v) may be positive (+ve) or negative(-ve) for concave mirror. Focal length (f) for concave mirror will always be negative(-ve). Focal length (f) for convex mirror will always be positive(+ve).

The positive magnitude of magnification indicates virtual and erect image. The negative magnitude of magnification indicates real and inverted image.

Refraction of light

Bending of the light rays as it passes from one medium to another medium is known as refraction of light.

Refraction through a Rectangular Glass Slab

Laws of Refraction

1. Incident ray, refracted ray and normal all lie in the same plane. 2. The ratio of sine of angle of incidence to the sine of angle of refraction is constant. This law is also

known as Snell’s law of refraction.

Refractive Index

When light passes from one medium to another medium, it changes its direction. The extent to which the direction changes is expressed in terms of refractive index. The value of refractive index is dependent on the speed of light in two media. v1 is the speed of light in medium 1 and v2 is the speed of light in medium 2.

Relative refractive index:

When light travels from one medium to another medium, then the refractive index is known as the relative refractive index.The refractive index of medium 2 with respect to medium 1 is represented as n21.

Absolute refractive index

When light travels from vacuum or air to another medium, then the refractive index is known as the

absolute refractive index.

If medium 1 is vacuum or air, then the refractive index of medium 2 with respect to vacuum is known as absolute refractive index of the medium.

Where c is the speed of light in air, v is the speed of light in other medium and nm is the refractive index of the medium.

Optically rarer medium’ and ‘Optically denser medium

In comparing two media, the one with the larger refractive index is optically denser medium than the other. The other medium of lower refractive index is optically rarer. The speed of light is higher in a rarer medium than a denser medium. Thus, a ray of light travelling from a rarer medium to a denser medium slows down and bends towards the normal. When it travels from a denser medium to a rarer medium, it speeds up and bends away from the normal.

An optically denser medium may not possess greater mass density. For example, kerosene having higher refractive index, is optically denser than water, although its mass density is less than water.

Speed of light in air = 3 X 108 m/sec Speed of light in glass = 2 X 108 m/sec Speed of light in water = 2.25 x 108

Spherical Lenses:A transparent material bound by two surfaces, of which one or both surfaces are spherical, forms a lens.

Types of Lenses:

i) Convex lens:

A lens may have two spherical surfaces, bulging outwards. Such a lens is called a double convex lens. It is simply called a convex lens. It is thicker at the middle as compared to the edges. Convex lens converges light rays, hence convex lenses are called

Concave lens

A double concave lens is bounded by two spherical surfaces, curved inwards. It is thicker at the edges than at the middle. Such lenses diverge light rays. Such lenses are called diverging lenses. A double concave lens is simply called a concave lens.

m/sec

m/sec

8 m /sec

A transparent material bound by two surfaces, of which one or both surfaces are

A lens may have two spherical surfaces, bulging outwards. Such a lens is called a double convex lens. It is simply called a convex lens. It is thicker at the middle as compared to the edges. Convex lens converges light rays, hence convex lenses are called converging lenses.

A double concave lens is bounded by two spherical surfaces, curved inwards. It is thicker at the edges than at the middle. Such lenses diverge light rays. Such lenses are called diverging lenses. A double concave

is simply called a concave lens.

A transparent material bound by two surfaces, of which one or both surfaces are

A lens may have two spherical surfaces, bulging outwards. Such a lens is called a double convex lens. It is simply called a convex lens. It is thicker at the middle as compared to the edges. Convex lens converges

A double concave lens is bounded by two spherical surfaces, curved inwards. It is thicker at the edges than at the middle. Such lenses diverge light rays. Such lenses are called diverging lenses. A double concave

Few Basic terms related to Spherical

1.Optical centre

Optical centre is defined as the point on the lens which is on the principal axis and the light ray doesn't deflect when passes through it.

Centres of curvature

Centre of curvature is defined as the centre of the surface of sphere of which the lens is a part. Since, a lens has two surfaces, so the lens has two centres of curvatures.

Principal axis

Principal axis is defined as the straight lines passing

Aperture

Aperture is defined as the diameter of the boundary of the circular lens.

Principal focus of convex lens

Several rays of light parallel to the principal axis are falling on a convex lens. These rays, after refractiofrom the lens, are converging to a point on the principal axis. This point on the principal axis is called the principal focus of the lens.

Principal focus of the Concave lens.

Several rays of light parallel to the principal axis are falling on a concave lfrom the lens, are appearing to diverge from a point on the principal axis. This point on the principal axis is called the principal focus of the concave lens.

Few Basic terms related to Spherical lens:

is defined as the point on the lens which is on the principal axis and the light ray doesn't

Centre of curvature is defined as the centre of the surface of sphere of which the lens is a part. Since, a lens has two surfaces, so the lens has two centres of curvatures.

Principal axis is defined as the straight lines passing through centre of curvature.

Aperture is defined as the diameter of the boundary of the circular lens.

Several rays of light parallel to the principal axis are falling on a convex lens. These rays, after refractiofrom the lens, are converging to a point on the principal axis. This point on the principal axis is called the

oncave lens.

Several rays of light parallel to the principal axis are falling on a concave lens. These rays, after refraction from the lens, are appearing to diverge from a point on the principal axis. This point on the principal axis is called the principal focus of the concave lens.

is defined as the point on the lens which is on the principal axis and the light ray doesn't

Centre of curvature is defined as the centre of the surface of sphere of which the lens is a part. Since, a lens

Several rays of light parallel to the principal axis are falling on a convex lens. These rays, after refraction from the lens, are converging to a point on the principal axis. This point on the principal axis is called the

ens. These rays, after refraction from the lens, are appearing to diverge from a point on the principal axis. This point on the principal axis is

Focal length

Focal length is defined as the distance between the optical centre and principal focus of the lens.

General rules for image formation using ray diagrams:

Image Formation by Lenses

1. When the object is at infinite


Image will be formed

1. At focus

2. real and inverted

3. Diminished in size

2.


Image will be formed between F and 2F

Diminished,

Real and inverted

3. At 2F


Image will be formed at 2F

Same size

Real and inverted

4.


Image will be formed beyond 2F


Real and inverted

5.


Image will be formed at infinity

Highly magnified (enlarged)

Real and inverted

6.


Image will be formed on the same side of the lens as the object.

magnified (enlarged)

virtual and erect

Sign convention is a set of rules to set signs for image distance, object distance, focal length, etc. for mathematical analysis of image formation. According to it:

Object is always placed to the left of lens. All distances are measured from the optical center of the mirror. Distances measured in the direction of the incident ray are positive and the distances measured in

the direction opposite to that of the incident rays are negative. Distances measured along y-axis above the principal axis are positive and that measured along y-

axis below the principal axis are negative. Convex lens has positive focal length and concave lens has negative focal length.

Lens Formula

In optics, the relationship between the distance of an image (v), the distance of an object (u), and the focal length (f) of the lens is given by the formula known as Lens formula. Lens formula is applicable for convex as well as concave lenses. These lenses have negligible thickness. The formula is as follows:

�

� -

�

� =

�

�

Magnification The magnification produced by a lens, is defined as the ratio of the height of the image and the height of the object. It is represented by the letter m. If h is the height of the object and h′ is the height of the image given by a lens, then the magnification produced by the lens is given by,

m = ��

�� =

�′

�

Magnification produced by a lens is also related to the object-distance u, and the image-distance v. This relationship is given by

Magnification (m) = h′/h = v/u

Power of a Lens The degree of convergence or divergence of light rays achieved by a lens is expressed in terms of its power. The power of a lens is defined as the reciprocal of its focal length. It is represented by the letter P. The power P of a lens of focal length f is given by

P = �

�(�)

The SI unit of power of a lens is ‘diopter’. It is denoted by the letter D. If f is expressed in meters, then, power is expressed in diopters. Thus, 1 diopter is the power of a lens whose focal length is 1 meter. 1D = 1�� Note: the power of a convex lens is positiveand that of a concave lens is negative

NCERT INTEXT QUESTION Page No.168 Question 1 Define the principal focus of a concave mirror.

Answer:

The point on the principal axis, where all parallel rays meet after reflection is called as Principal Focus or Focus of concave mirror. It is denoted by letter ‘F’.

Question 2 The radius of curvature of a spherical mirror is 20 cm. What is its focal length? Answer: Focal length = 12 x Radius of curvature = 12 x 20 cm = 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 rear-view mirror in vehicles? Answer: We prefer a convex mirror as a rear-view mirror in vehicles because of two reasons:

A convex mirror always produces an erect image of the objects.

The image formed in a convex mirror is highly diminished or much smaller than the object, due to which a convex mirror gives a wide field of view of the traffic behind. A convex mirror enables the driver to view such larger area of the traffic behind him.

Page No.171

Question 1 Find the focal length of a convex mirror whose radius of curvature is 32 cm. Solution: R = +32 cm and

f=R/2=+32/2=+16cm

Question 2 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? Solution:

Because the image is real, so magnification m must be negative.

Thus, the image is located at a distance of 30 cm from the mirror on the object side of the mirror.

Page Number: 176

Question 1 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 because the ray of light goes from a rarer medium to a denser medium.

Question 2 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 ms-1. Solution: Refractive index of glass, n = 1.50

Question 3 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 highest optical density. Air has lowest refractive index (= 1.0003), so it has lowest optical density.

Question 4 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, n = 1.47 For water, n = 1.33 Because water has the lowest refractive index, therefore light travels fastest in this optically rarer medium than kerosene and turpentine oil.

Question 5 The refractive index of diamond is 2.42. What is the meaning of this statement? Answer: By saying that the refractive index of diamond is 2.42, we mean that the speed of light in diamond is lower by a factor of 2.42 relative to that in vacuum.

Page Number: 184

Question 1 Define 1 dioptre of power of a lens. Answer: One dioptre is the power of a lens whose focal length is 1 metre.

Question 2 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. Solution:

Here ν = +50cm Because the real image is of the same size as the object,

Question -3

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

Solution:

A concave lens has negative focal length, so it is to be written with a minus sign. Thus, Focal length, f=-2 m Now, Power, p=1/f in metres P=1/−2 P=−0.5D Thus, the power of this concave lens is,-0.5 dioptre.

NCERT EXERCISES

Page No. 185

Question 1 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 2 The image formed by a concave mirror is observed to be virtual, erect and larger than the object. Where should be the position of the object ? (a) Between the principal focus and the centre of curvature (b) At the centre of curvature (c) Beyond the centre of curvature (d) Between the pole of the mirror and its principal focus.

Answer: (d) Between the pole of the mirror an

Question 3 Where should an object be placed in front of a convex lens to get a real image of the size of the object ? (a) At the principal focus of the lens (b) At twice the focal length(c) At infinity (d) Between the optical centre of the lens and its principal focus.Answer: (b) At twice the focal length.

Question 4 A spherical mirror and a thin spherical lens have each a focal length of 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 5 No matter how far you stand from 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 6 Which of the following lenses would you prefer to use while reading small letters found in a dictionary ? (a) A convex lens of focal length 50 cm.(b) A concave lens of focal length 50 cm.(c) A convex lens of focal length 5 cm.(d) A concave lens of focal length 5 cm.Answer: (c) A convex lens of focal length 5 cm.

Question 7 We wish to obtain an erect image of an object, using a concave mirror of focal length 15 cm. What should be the range of distance of the object from the mirror? What is the nature of the image? Is the image larger or smaller than the object? Draw a ray diagram to show the image formation in this case. Answer: A concave mirror gives an erect image when the object concave mirror, i.e., between 0 and 15 cm from the mirror. The image thus formed will be virtual, erect and larger than the object.

(d) Between the pole of the mirror and its principal focus.

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

of the lens and its principal focus.

A spherical mirror and a thin spherical lens have each a focal length of -15 cm. The mirror and the

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

No matter how far you stand from mirror, your image appears erect. The mirror is likely to be

Which of the following lenses would you prefer to use while reading small letters found in a

(a) A convex lens of focal length 50 cm. 50 cm.

(c) A convex lens of focal length 5 cm. (d) A concave lens of focal length 5 cm.

(c) A convex lens of focal length 5 cm.

We wish to obtain an erect image of an object, using a concave mirror of focal length 15 cm. What he 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

A concave mirror gives an erect image when the object is placed between the focus F and the pole P of the concave mirror, i.e., between 0 and 15 cm from the mirror. The image thus formed will be virtual, erect and

Where should an object be placed in front of a convex lens to get a real image of the size of the object

15 cm. The mirror and the

No matter how far you stand from mirror, your image appears erect. The mirror is likely to be

Which of the following lenses would you prefer to use while reading small letters found in a

We wish to obtain an erect image of an object, using a concave mirror of focal length 15 cm. What he 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

is placed between the focus F and the pole P of the concave mirror, i.e., between 0 and 15 cm from the mirror. The image thus formed will be virtual, erect and

Question 8 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 mirrors are used as reflectors in headlights of cars. When a bulb is located at the focus of the concave mirror, the light rays after reflection from the mirror travel over a large distance as a parallel beam of high intensity.

(b) A convex mirror is used as a side/rear

A convex mirror always forms an erect, virtual and diminished image of ait.

A convex mirror has a wider field of view than a plane mirror of the same size.

(c) Large concave mirrors are used to concentrate sunlight to produce heat in solar furnaces.

Question 9 One-half of a convex lens is covered with a black paper. Will this lens produce a complete image of the object? Verify your answer experimentally. Explain your observations.Answer: A convex lens forms complete image of an object, even if its one half is covered with black paper. It cexplained by considering following two cases.

Case I: When the upper half of the lens is coveredIn 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 for

Case II: When the lower half of the lens Is coveredIn 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 given figure.

Question 10 An object 5 cm in length is held 25 cm away from a converging lens of focal length 10 cm. Draw the ray diagram and find the position, size and the nature of the imageAnswer: Height of the Object, h0 = 5 cm

Distance of the object from converging lens, u =

Focal length of a converging lens, f = 10 cm

Name the type of mirror used in the following situations.

view mirror of a vehicle.

Support your answer with reason.

(a) Concave mirrors are used as reflectors in headlights of cars. When a bulb is located at the focus of the ht rays after reflection from the mirror travel over a large distance as a parallel beam

(b) A convex mirror is used as a side/rear-view mirror of a vehicle because

A convex mirror always forms an erect, virtual and diminished image of an object placed anywhere in front

A convex mirror has a wider field of view than a plane mirror of the same size.


covered with a black paper. Will this lens produce a complete image of the object? Verify your answer experimentally. Explain your observations.

A convex lens forms complete image of an object, even if its one half is covered with black paper. It cexplained by considering 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

e lens to form the image of the given object, as shown in the given figure.

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.

Distance of the object from converging lens, u = -25 cm

Focal length of a converging lens, f = 10 cm

(a) Concave mirrors are used as reflectors in headlights of cars. When a bulb is located at the focus of the ht rays after reflection from the mirror travel over a large distance as a parallel beam

n object placed anywhere in front


covered with a black paper. Will this lens produce a complete image of

A convex lens forms complete image of an object, even if its one half is covered with black paper. It can be

In this case, a ray of light coming from the object will be refracted by the lower half of the lens. These rays m the image of the given object, as shown in the following figure.

In this case, a ray of light coming from the object is refracted by the upper half of the lens. These rays meet e lens to form the image of the given object, as shown in the given figure.

An object 5 cm in length is held 25 cm away from a converging lens of focal length 10 cm. Draw the

Using lens formula,

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

Question 11 A concave lens of focal length 15 cm forms an image 10 cm from the lens. How far is the object placed from the lens? Draw the ray diagram.Solution: Focal length of concave lens (OF1), f =

Image distance, v= – 10 cm

According to the lens formula,

The negative value of u indicates that the object is placed 30 cm in front of the lens. This is shown in the following ray diagram.

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

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.

oncave lens (OF1), f = – 15 cm

The negative value of u indicates that the object is placed 30 cm in front of the lens. This is shown in the

A concave lens of focal length 15 cm forms an image 10 cm from the lens. How far is the object

The negative value of u indicates that the object is placed 30 cm in front of the lens. This is shown in the

Question 12 An object is placed at a distance of 10 cm from a convex mirror of focal length 15 cm. Find the position and nature of the image. Solution: Object distance, u = -10 cm, Focal length, f = +15 cm, Image distance, ν = ?

Thus, image distance, ν = + 6 cm Because ν is +ve, so a virtual image is formed at a distance of 6 cm behind the mirror. Magnification, m=−υ/u=−6/−10=0.6 (i.e. < 1) The positive value of m shows that image erect and its value, which is less than 1, shows that image is smaller than the object. Thus, image is virtual, erect and diminished.

Question 13 The magnification produced by a plane mirror is +1. What does this mean? Answer: Since magnification, m=h‘/h=−ν/u. Given, m = +1, so h’ = h and ν = -u

(i) m = 1 indicates the size of image is same as that of object. (ii) positive sign of m indicates that an erect image is formed.

The opposite signs of ν and u indicate that image is formed on the other side of the mirror from where the object is placed i.e., image is formed behind the mirror and thus image formed is virtual.

Question 14 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. Solution: Since object size, h = +5 cm, object distance, u = -20 cm and radius of curvature, R = +30 cm

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 15 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 focussed image can be obtained? Find the size and the nature of the image. Answer: Here, object size, h = +7.0 cm, object distance, u = -27 cm and focal length, f = -18 cm

Image distance, ν = ? and image size, h’ = ? From the mirror formula, 1f=1ν−1u, we have

The screen should be placed at a distance of 54 cm on the object side of the mirror to obtain a sharp image.

The image is real, inverted and enlarged in size.

Question 16 Find the focal length of a lens of power Answer: Here, P = -2.0 D The type of lens is concave because the focal length is negative.

Question 17 A doctor has prescribed a corrective lens of power +1.5 D. Find the focal prescribed lens diverging or converging?Answer: Here, P = +1.5 D

Because the focal length is positive, the prescribed lens is converging.

−1u, we have


The image is real, inverted and enlarged in size.

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

The type of lens is concave because the focal length is negative.

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?

Because the focal length is positive, the prescribed lens is converging.


length of the lens. Is the

physics chapter-10 light: reflection and refraction

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