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Chapter Review:

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• Light seems to travel in straight lines.
• The laws of reflection of light (i) The angle of incidence is equal to the angle of reflection. (ii) The incident ray, the normal to the mirror at the point of incidence and the reflected ray, all lie in the same plane.
• Image formed by a plane mirror is always virtual and erect. The size of the image is equal to that of the object.
• The reflecting surface of such mirrors can be considered to form
  a part of the surface of a sphere. Such mirrors, whose reflecting surfaces are spherical, are called spherical mirrors.
• 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.
• The centre of the reflecting surface of a spherical mirror is a point called the pole.
• The reflecting surface of a spherical mirror forms a part of a sphere.
  This sphere has a centre. This point is called the centre of curvature.
• The radius of the sphere of which the reflecting surface of a spherical mirror forms a part, is called the radius of curvature.
• A straight line passing through the pole and the centre of curvature of a spherical mirror. This line is called the principal axis.
• The distance between the pole and the principal focus of a spherical mirror is called the focal length.
• The diameter of the reflecting surface of spherical mirror is called its aperture.
• The nature, position and size of the image formed by a concave mirror depends on the position of the object.
• Concave mirrors are commonly used in torches, search-lights and
  vehicles headlights to get powerful parallel beams of light.
• Convex mirrors are commonly used as rear-view (wing) mirrors in
  vehicles.
• The distance of the principal focus from the pole is called the focal length.
• Light travels in vacuum with an enormous speed of 3×10⁸ m s^-1. The speed of light is different in different media.
• 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.
• A lens have two spherical surfaces, bulging outwards. Such a lens is 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 lens have two spherical surfaces curved inwards. It is thicker at the edges than at the middle. Such lenses diverge light rays.Such lenses are called concave lens or diverging lenses.     
• The SI unit of power of a lens is ‘dioptre’.
• The power of a convex lens is positive and that of a concave lens is negative.

Text-book Questions 

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Page no. 168 

Q1. Define the principal focus of a concave mirror.

Ans: The incident rays parallel to the principal axis after reflection intersect at a point F between the pole and the center of curvature of the mirror. This point is called the principal focus of a concave mirror.

Q2. The radius of curvature of a spherical mirror is 20 cm. What is its focal length?

Ans: Radius of curvature = 20 cm

Focus distance = radius of curvature/2

          = 20/2

          = 10 cm

Thus, the focal length of the given spherical mirror is 10 cm.

Q3. Name a mirror that can give an erect and enlarged image of an object.

Ans: A concave mirror, when the object is placed between the pole and the principal focus of the concave mirror, it forms an erect and magnified image.

Q4. Why do we prefer a convex mirror as a rear-view mirror in vehicles?

Ans: Convex mirrors are also preferred because they always produce an erect image though it is smaller. Their field of vision is also very large because they are curved outwards. Hence, convex mirrors enable the driver to see a much larger area behind him as compared to plane mirrors.

Page no. 171 

Q1. Find the focal length of a convex mirror whose radius of curvature is 32 cm.

Ans: In a convex mirror,

radius of curvature R = 32 cm

radius of curvature = 2 × focus distance

Focus distance = radius of curvature/2

           = 32/2

           = 16 cm

Thus, the focal length of that convex mirror = 16 cm.

Q2. 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?

Ans:  In a concave mirror,

Object distance (u) = – 10 cm

Magnification (m) = – 3 [Since the image is real]

 

Page no. 178

Q1. 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?

Ans: The ray of light will bend towards the normal, because the ray of light enters from air which is a rarer medium to water which is a denser medium than air, then in such a case the light will bend towards the normal.

Q2. Light enters from air to glass having refractive index 1.50. What is the speed of light in the glass? The speed of light in vacuum is 3 × 108 m s–1.

Answer: 

Q3. Find out, from Table 10.3, the medium having highest optical density. Also find the medium with lowest optical density.

Ans: The medium of maximum optical density is diamond whose refractive index is 2.42.

The medium of minimum optical density is air whose refractive index is 1.0003.

Q4. You are given kerosene, turpentine and water. In which of these does the light travel fastest? Use the information given in Table 10.3.

Ans: From Table 10.3

Refractive index of kerosene = 1.44

Refractive index of turpentine = 1.47

Refractive index of water = 1.33

In this, the speed of light is highest in water and the speed of light is the least in turpentine oil because the higher the refractive index, the less will be the speed of light in that medium and the lower the refractive index of which is the speed of light in that medium. will be more

Q5. The refractive index of diamond is 2.42. What is the meaning of this statement?

Ans: The refractive index of diamond is 2.42. The meaning of this statement is that the optical density of diamond is high, due to which it is a hard material, it has the lowest speed of light.

Page no. 148 

Q1. Define 1 dioptre of power of a lens.

Ans: If the focal length of a lens is 1 meter, then it is called 1 diopter power of the lens.

Q2. 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.

Q3. Find the power of a concave lens of focal length 2 m.

NCERT Solutions Exercise 

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Q1. Which one of the following materials cannot be used to make a lens?

(a) Water (b) Glass (c) Plastic (d) Clay

Ans: (d) Clay

Q2. 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.

Ans: (a) Between the principal focus and the centre of curvature

Q3. 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.

Ans:  (b) At twice the focal length

Q4. A spherical mirror and a thin spherical lens have each a focal length of –15 cm. The mirror and the lens are likely to be

(a) both concave.

(b) both convex.

(c) the mirror is concave and the lens is convex.

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

Ans: (a) both concave.

Q5. No matter how far you stand from a mirror, your image appears erect. The mirror is likely to be

(a) plane.

(b) concave.

(c) convex.

(d) either plane or convex.

Ans: (d) either plane or convex.

Q6. 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.

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

Q7. 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: In a concave mirror, a virtual and erect image is formed only when the object is between the principal focus and the pole. Since the focal length of a concave mirror is 15 cm, that is, the distance between the pole and the focus is 15 cm. Therefore the object should be placed between 0cm to 15cm in front of the mirror, only then a erect image is formed.

Nature of Image : Virtual and Upright.

Image size: larger than the object.

Q8. 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.

Ans:

(a) The head-light of a car is made of a concave mirror, because if the bulb is placed at the principal focus of the mirror, it reflects from the mirror and forms a parallel beam.

(b) A convex mirror is used for the side/rear-view mirror of a vehicle because it always produces a small but erect image. Since the convex mirror is curved outwards, its field of vision is greatly increased, so that the driver can see a large part of the rear of the vehicle.

(c) Sunlight has to be focused in a solar furnace, for which a concave mirror is suitable. This mirror passes light rays coming through infinity parallel to the principal axis through the focus, due to which the temperature around the focus increases from 1800C to 2000C.

Q9. 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: Yes, even if half of a convex lens is covered with black paper, this lens forms a complete and clear image of a given object.

Testing - Now keep a convex lens covered in half with a black paper with the help of a stand and keep a burning candle on one side of the lens and a white curtain on the other side. In observation we find that the full image of the candle is made on the screen which is real and inverted. 

Q10. 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.

Q11. 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.

Q12. 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.

Q13. The magnification produced by a plane mirror is +1. What does this mean?

Q14. 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.

Q15. 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.

Q16. Find the focal length of a lens of power – 2.0 D. What type of lens is this?

Q17. 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?

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Chapetr-10. Light-Reflection and Refraction

 

Q – By which muscles of eye can change the curvature of eye lens?

A – ciliary muscle.

Q - Whose reflective index is greater hot wind or cool wind? Why?

A – cool wind has greater reflective index. As cool wind is heavier than hot wind.

Q – A optician makes a lens using two different lenses converging and diverging lenses. Which kind of this lens is?

A – bifocal lens.

Q – Why does pencil look turned sinking in glass?

A – The pencil looks turned sinking in the glass due to refraction of light.  

Q- Why does the sky appear dark instead of blue to an astronaut?

Q- What is cataract? How can it be removed?

A- The Crystalline lens of people at old age becomes milky  and cloudy. This Condition is called cataract. This causes partial or complete loss od vision. This situation of eye is called cataract. It is possible to restore vision through a cataract surgery.

Q – Which colour deviates the minimum and the maximum, while white light passing through prism? Why?

A –  Give two reasons for occurring myopia. Draw a ray diagram, how is this defect of vision corrected?

Q: What do you understand by atmospheric refraction? Name two phenomenon caused by this.

Give answers of the following questions;

(i) How does rainbow form?

(ii) What is called the light which forms spectrum similar to sun light?

(iii) What is called the splitting of light into its component colours?

(iv) Who works as a prism to form rainbow?

​(v) Why does the sun appear before the actual sunrise and after actual sunset?