What is meant by power of a lens? Write its SI unit. A student uses a lens of focal length 40 cm and another of - 20 cm . Write the nature and power of each lens.

The power of a lens is a measure of the degree of convergence or divergence of light rays falling on it. It is also defined as the reciprocal of its focal length in metres. The S.I. unit of power is dioptre. Focal length of a convex lens is positive. F c o n v e x = 40 cm = 40 100 m Power = 1 focal length = 1 ( 40 100 ) Power = + 2 . 5 D So, lens having focal length 40 cm is convex. Focal length of a concave lens is negative. F c o n c a v e = - 20 cm = - 20 100 m Power = 1 focal length = 1 ( - 20 100 ) Power = - 5 D Converging (convex ) lenses have positive focal lengths, so they also have positive power values. Diverging (concave ) lenses have negative focal lengths, so they also have negative power values.

State the laws of refraction of light. Explain the term 'absolute refractive index of a medium' and write an expression to relate it with the speed of light in vacuum.

The two laws of refraction are: The Incident ray, the refracted ray and the normal at the point of incidence, all lie in the same plane. The ratio of the sine of angle of incidence( i ) to the sine of angle of refraction ( r ) at a point in a medium is constant, i.e., sin i sin r = C o n s t a n t . Absolute refractive index is the ratio of speed or velocity of light in vacuum to the speed of light in the given medium. A b s o l u t e r e f r a c t i v e i n d e x ( η ) = S p e e d o f l i g h t i n v a c u u m o r a i r ( c ) S p e e d o f l i g h t i n m e d i u m ( v ) .

A 10 cm tall object is placed perpendicular to the principal axis of a convex lens of focal length 12 cm . The distance of the object from the lens is 18 cm . Find the nature, position and size of the image formed.

Given, The height of the object, h 1 = + 10 cm Focal length, f = + 12 cm Object distance, u = - 18 cm We have the lens formula, 1 v - 1 u = 1 f ⇒ 1 v - 1 - 18 = 1 12 ⇒ 1 v + 1 18 = 1 12 ⇒ 1 v = 1 12 - 1 18 ⇒ 1 v = 3 - 2 36 = 1 36 ∴ v = 36 cm Magnification, m = v u = 36 - 18 = - 2 Thus, we can say that the position of the image is formed at a distance of 36 cm from the convex lens. As the value of the magnification is more than 1 , the image formed is larger than the object. The minus sign of magnification indicates that the image is formed below the principal axis. Thus, we can say that the image formed is real as it can be captured on the screen, and it is inverted.

A student holding a mirror in his hand, directed the reflecting surface of the mirror towards the Sun. He then directed the reflected light on to a sheet of paper held close to the mirror. Will he be able to determine the approximate value of focal length of this mirror from this activity ? Give reason and draw ray diagram to justify your answer in this case.

Yes, the sheet of paper will start burning only when he kept it at the focus of the mirror. This will give an approximate value of focal length which is the distance between the mirror and the point where the paper starts burning. Here, we are using a concave mirror because it is having converging properties. The required ray diagram is given below: A real image of the sun is formed by the concave mirror.

A student holding a mirror in his hand, directed the reflecting surface of the mirror towards the Sun. He then directed the reflected light on to a sheet of paper held close to the mirror. Which type of mirror does he have?

The student is having a concave mirror in his hand. Concave mirror is having converging properties. Hence, in order to burn the paper, the student can place the paper at the principal focus of the concave mirror where the intensity of light will be maximum.

A student holding a mirror in his hand, directed the reflecting surface of the mirror towards the Sun. He then directed the reflected light on to a sheet of paper held close to the mirror. What should he do to burn the paper?

Mirror that is having the converging properties will be used to burn the paper. This is because to burn the paper we need to converge the intensity of sunlight at one point. This type of mirror is a concave mirror. The student has to place the paper at such a point where the intensity of light will be maximum, that is the principal focus of the mirror.

The absolute refractive index of Ruby is 1 . 7 . Find the speed of light in Ruby. The speed of light in vacuum is 3 × 10 8 m s .

We know that, Reractive Index ( R . I ) = Speed of light in vacuum Speed of light in medium Therefore, Refractive index of ruby = Speed of light in vacuum Speed of light in ruby Let V R be the speed of light in Ruby. Substituting the values in the above equation, ⇒ 1 . 7 = 3 x 10 8 m s V R ∴ V R = 3 x 10 8 m s 1 . 7 = 1 . 76 × 10 8 m s Thus, the speed of light in ruby is 1 . 76 × 10 8 m s .

An object of height 4 . 0 cm is placed at a distance of 30 cm from the optical centre ‘ O ’ of a convex lens of focal length 20 cm . Draw a ray diagram to find the position and size of the image formed. Mark optical centre ‘ O ’ and principal focus ‘ F ’ on the diagram. Also, find the approximate ratio of the size of the image to the size of the object.

Given, f = + 20 cm , u = - 30 cm and h o = 4 cm We have the relation, 1 f = 1 v - 1 u ⇒ 1 20 = 1 v - 1 ( - 30 ) ⇒ 1 v = 1 60 ∴ v = 60 cm We know, h i h o = v u ⇒ h i 4 = 60 - 30 ⇒ h i = - 8 cm Thus, the size of the image is 8 cm . The minus sign shows that the image is formed below the axis. The ratio of the size of the image to the object = - 8 4 = - 2 The required diagram is given below: Here, the object is kept between F 1 and 2 F 1 . The image is formed beyond 2 F 2 . It is real and inverted.

E M B I B E

Science Crash Course (Based on Revised Syllabus-2023)>Chapter 10 - Light - Reflection and Refraction>Exercise>Q 72

MEDIUM

10th Rajasthan Board

IMPORTANT

Earn 100

Draw the path of a ray of light when it enters one of the faces of a glass slab at an angle of nearly $45 °$ . Label on it (i) angle of refraction, (ii) angle of emergence and (iii) lateral displacement.

Important Questions on Light - Reflection and Refraction

EASY

10th Rajasthan Board

IMPORTANT

Science Crash Course (Based on Revised Syllabus-2023)>Chapter 10 - Light - Reflection and Refraction>Exercise>Q 74

When is a medium said to be optically denser? What happens when a light ray passes from a rarer to denser medium?

EASY

10th Rajasthan Board

IMPORTANT

Science Crash Course (Based on Revised Syllabus-2023)>Chapter 10 - Light - Reflection and Refraction>Exercise>Q 76

Write an equation relating the refractive index of a material to the speed of light in vacuum and the speed of light in the material.

EASY

10th Rajasthan Board

IMPORTANT

Science Crash Course (Based on Revised Syllabus-2023)>Chapter 10 - Light - Reflection and Refraction>Exercise>Q 77

Light enters an optical fiber at an angle of $15^{\circ}$ .

Question Image

The refractive index of the fiber is $1.4$ . Calculate the angle of refraction of the light beam shown entering the fibre.

EASY

10th Rajasthan Board

IMPORTANT

Science Crash Course (Based on Revised Syllabus-2023)>Chapter 10 - Light - Reflection and Refraction>Exercise>Q 78

Through which point of a lens, a ray of light passes undeviated?

EASY

10th Rajasthan Board

IMPORTANT

Science Crash Course (Based on Revised Syllabus-2023)>Chapter 10 - Light - Reflection and Refraction>Exercise>Q 79

How a convex lens can be used as a magnifying lens? Explain with a ray diagram.

HARD

10th Rajasthan Board

IMPORTANT

Science Crash Course (Based on Revised Syllabus-2023)>Chapter 10 - Light - Reflection and Refraction>Exercise>Q 81

An object is placed at a distance of

50 cm

in front of a convex lens of focal length

25 cm

. Find the image distance. If the object is

10 cm

high what will be the nature and size of the image?

MEDIUM

10th Rajasthan Board

IMPORTANT

Science Crash Course (Based on Revised Syllabus-2023)>Chapter 10 - Light - Reflection and Refraction>Exercise>Q 82

An object is placed at a distance of

15 cm

in front of a convex lens of focal length

30 cm

. Find the nature and position of the image?

HARD

10th Rajasthan Board

IMPORTANT

Science Crash Course (Based on Revised Syllabus-2023)>Chapter 10 - Light - Reflection and Refraction>Exercise>Q 83

A convex lens forms a real and inverted image of a needle at a distance of $40 cm$ from it. Where is the needle placed in front of the convex lens if the size of the image is equal to that of the object? Also, find the power of the lens.