Class X Science / Light – Reflection and Refraction

Class X Science
Notes for Light – Reflection and Refraction

• Light is a form of energy that produces in us the sensation of sight.

• Reflection of light is the phenomenon of bouncing back of light in the same medium on striking the surface of any object.

• The two laws of reflection are:

(i) the incident ray, the reflected ray and the normal (at the point of incidence), all lie in the same plane.

(ii) the angle of reflection (r) is always equal to the angle of incidence (i)

∠r = ∠i

• In a plane mirror, the image of a real object is always

(i) virtual,

(ii) erect

(iii) of same size as the object,

(iv) as far behind the mirror as the object is in front of the mirror.

(v) laterally inverted.

• Absolute refractive index(n) of a medium is the ratio of speed of light in vacuum or air(c) to the speed of light in the medium(v) i.e.

• Refraction of light is the phenomenon of change in the path of light in going from one medium to another.

• In going from a rarer to a denser medium, the ray of light bends towards normal and in going from a denser to a rarer medium, the ray of light bends away from normal.

• Snell’s law of refraction,

• No refraction occurs, when

(i) light is incident normally on a boundary,

(ii) refractive indices of the two media in contact are equal.

• New Cartesian Sign Convention for spherical lenses:

(i) All distances are measured from optical centre C of the lens.

(ii) The distances measured in the direction of incidence of light are taken as positive and vice–versa.

(iii) All heights above the principal axis of the lens are taken as positive and vice versa.

• The linear magnification produced by a lens is

• Power of the combination of lenses

P = p1 + p2 + p3 ...

• Linear magnification produced by a spherical mirror is

• For a convex mirror, m is +ve and less than one, as the image formed is virtual, erect and shorter than the object.

• For a concave mirror, m is +ve when image formed is virtual and m is –ve, when image formed is real.

• According to New Cartesian Sign Convention, for spherical mirror.

(i) All distances are measured from the pole of the spherical mirror.

(ii) The distances measured in the direction of incidence of light are taken as positive and vice–versa.

(iii) The heights above the principal axis of the mirror are taken as positive and vice–versa.

• In spherical mirror, focal length

Class X Science
NCERT Solution for Light - Reflection and Refraction

Q1. Define the principal focus of a concave mirror.

Ans. It is a point on the principal axis where the rays of light parallel to principal axis meet.

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

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

Ans. Concave mirror.

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

Ans. 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.

Q5. Find the focal length of a convex mirror whose radius of curvature is 32 cm.

Ans. Radius of curvature R = 32 cm

Q6. A concave mirror produces three times magnified (enlarged) real image of un object placed at 10cm in front of it. Where is the image located?

Ans. Object distance u = ï¿½10 cm (concave minor)

Q7. A ray of light travelling in cur enters obliquely into water. Does the light ray bend towards the normal or away form the normal? Why?

Ans. The light bends towards the normal on entry into water because water is optically denser than air.

Q8. Light enters train air to glass having refractive index 1.50. What is the speed of light in the glass? The speed ref light in vacuum is 3 × 108 m/s.

Ans. The speed of light in vacuum is = 3 × 108 m/s

Refractive index of glass ng = 1.50

Q9. Find out from folllowing table the medium having highest optical derisity. Also find the medium with lowest optical density.

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

Q10. You are given kerosene, turpentine and water. In which of these does the light travel fastest? Use the information given in table above.

Ans. 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.

Q11. The refractive index of diamond is 2.42. What is the meaning of this statement?

This means the ratio of the speed of light in the air and the speed of light in diamond is equal to 2.42.

Q12. Define I dioptre of newer of a lefts Ans. 1 dioptre is the power of a lens whose focal length is 1 metre.

1 D = 1 mï¿½1

Q13. 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.

Q14. Find the power of a concave lens of focal length 2 m.

Ans. Focal length of concave lens f = ï¿½2 m

QUESTIONS NCERT FROM TEXTBOOK

Q1. Which one of the following rrratcrï¿½ials cannot he used to make a lens?

(a) Water (b) Glass

Ans. (d) Clay

Q2. The Image formed by a concave mirror is observed to be virtual, erect and larger than the object.

Where should he the positron of the object?

(a) Between the principal focus and the centre of carvature

(b) At the centre of curvature

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

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

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

(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 ï¿½15 cm. The mirror and the lens are likely to be

(a) both concave

(b) both convex

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

Ans. (a) Both concave.

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

(a) plane. (b) concave.

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.

(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 minor of focal length 15 cm. What should he 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.

Ans. Focal length of concave mirror = 15 cm.

Object should be place in front of given concave minor at a distance less than 15 cm.

Image formed is virtual and erect.

Image size is larger than object.

Q8. Name the type of mirror used in the following situations.

(a) Headlight of a car: (b) Side/rear-view mirror of a vehicle

Support your answer with rea son.

Ans. (a) For headlight of a car- Concave mirror is used to get a powerful beam of light after reflection.

(b) Convex minor is used for side/rear view mirror of a vehicle. Convex mirror forms an erect and diminished image of vehicles and gives wider view of rear.

(c) In solar furnace concave mirror is used as a reflector, it concentrates sun light at a point where the temperature increases sharply to 180°C ï¿½ 200°C.

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.

Ans. Yes, one-half of a convex lens when covered with a black paper, the lens produces a complete or full image of an object.

To verify experimentally:

Take a convex lens, cover half part of it as shown in the figure, with a paper. Place it on a stand.

Focus a distant object on a screen, the image obtained on the screen is complete.

Observation and conclusion: Image formed on the screen does not depend on the size of the lens. The brightness of the image decreases as less number of rays pass through the lens.

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.

Ans.

Hence, the image formed at 16.67 cm from the lens on the other side. The size of the image is 3.3 cm, i.e., reduced and inverted.

Q11. A concave lens of focal length 15 cm forms an image 10 cm from the lens object placed from the lens? Draw the ray diagram.

Ans. Concave lens

Q12. An object is placed at a distance of I0 cm from a convex mirror of focal length 15 cm. Find the position and nature of the image.

The image is formed 6 cm behind the mirror, virtual image is formed.

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

Ans. Magnification, m = + 1

+ indicates virtual image.

1 indicates that the object size and image size is same.

Q14. An object 5.0 cm in length is placed at a distance of 20 cm in front of a convex ï¿½error of radius of curvature 30 cm. Find the positron 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 he obtained? Find the size and the nature of the image.

Ans. Concave minor u = ï¿½27 cm, f = ï¿½18 cm, h = 7.0 cm

Q16. Find the focal length of a lens of power ï¿½2.0 D. What type of lens is this?

Ans. P = ï¿½ 2.0 D

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?

Power of the lens is +ve, and it is converging lens i.e., convex lens.

LIGHT

MULTIPLE CHOICE QUESTIONS

1. Complete the sentence. Friction always ____________

1. Focal length of plane mirror is

a. At infinity

b. Zero

c. Negative

d. None of these

2. Image formed by plane mirror is

a. Real and erect

b. Real and inverted

c. Virtual and erect

d. Virtual and inverted

3. A concave mirror gives real, inverted and same size image if the object is placed

a. At F

b. At infinity

c. At C

d. Beyond C

4. Power of the lens is -40, its focal length is

a. 4m

b. -40m

c. -0.25m

d. -25m

5. A concave mirror gives virtual, refract and enlarged image of the object but image of smaller size than the size of the object is

a. At infinity

b. Between F and C

c. Between P and F

d. At E

6. In optics an object which has higher refractive index is called

a. Optically rarer

b. Optically denser

c. Optical density

d. Refractive index

7. The optical phenomena, twinkling of stars, is due to

a. Atmospheric reflection

b. Total reflection

c. Atmospheric refraction

d. Total refraction

8. Convex lens focus a real, point sized image at focus, the object is placed

a. At focus

b. Between F and 2F

c. At infinity

d. At 2F

9. The unit of power of lens is

a. Metre

b. Centimeter

c. Diopter

d. M-1

10. The radius of curvature of a mirror is 20cm the focal length is

a. 20cm

b. 10cm

c. 40cm

d. 5cm

Answers

1. A

2. C

3. C

4. C

5. C

6. B

7. C

8. C

9. C

10. B

LIGHT
PREVIOUS YEAR QUESTIONS

1. List four characteristics of the images formed by plane mirrors. (CBSE 2015)

2. Draw a ray diagram to show the path of the reflected ray corresponding to an incident ray which is directed parallel to the principal axis of a convex mirror. Mark on it the angle of incident and the angle of reflection (CBSE 2014)

3. A spherical mirror produces an image of magnification -1 on a screen placed at a distance of 50 cm from the mirror.

(a) Write the type of mirror.

(b) Find the distance of the image from the object.

(d) Draw the ray diagram to show the image formation in this case. (CBSE 2014)

4. State the laws of refraction of light. If the speed of light in vacuum is 3X108 ms-1, find the speed of light in a medium of absolute refractive index 1.5. (CBSE 2014)

5. Which phenomenon is responsible for making the path of light visible? (CBSE 2012)

6. When we place a glass prism in the path of a narrow beam of white light a spectrum is obtained.

7. What happens when a second identical prism is placed in an inverted position with respect to the first prism? Draw a labeled ray diagram to illustrate it. ( CBSE 2012)

8. The power of the lens is -4.0D. What is the nature of this lens? (CBSE 2008)

9. Which type of mirror is used to give erect and enlarged image of an object? (CBSE 2008)

10. Draw the ray diagram and also state the position, the relative size and the nature of image formed by a concave mirror when the object is placed at the centre of curvature of the mirror. (CBSE 2011)

LIGHT
HIGH ORDER THINKING QUESTIONS

1. What is the magnification of a plane mirror

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

3. Why paper catches fire when a convex lens is used to ficus sunlight?

4. What is silvering of mirror?

5. What is refractive mirror?

6. State the formula, lens formula and power of lens

7. The refractive index of water is 1.33 and kerosene is 1.44. Calculate refractive index of the kerosene with respect to water.

8. What kind of mirrors are used in big shopping stores to watch activities of customers?

9. Give mirror image of word ï¿½AMBULANCEï¿½

10. The magnification produced by a plane mirror is +1. What does it mean?

LIGHT

TEST

MAXIMUM TIME: 40 MINUTES

MAXIMUM MARKS: 25 MARKS

1. Name the type of mirror used in following situations (3)

a. Headlights of the car

b. Side/rear view mirror of a vehicle

c. Solar furnace

2. What are two types of reflexive index. (2)

3. Differentiate between concave and convex lens. (3)

4. Draw the diagram of path of light when it travels through a glass slab. (3)

5. What are the properties of the image formed by plane mirror? (2)

6. Write laws of reflection (2)

7. Draw and explain the ray diagram formed by a convex mirror when (5)

a. Object is at infinity

b. Object is at finite distance from the mirror

8. Name the mirror which can show the size of the object to be double of its original. (1)

9. If the speed of light in water is 2.25X108m/s and the speed in vaccum is 3X108m/s . Calculate the refractive index of water. (2)

10. Find the focal length of a lens of power -2.0 D. what type of lens is this? (2)

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