# LIGHT: REFLECTION AND REFRACTION– 2020 Important Questions

1 MARK QUESTION

Q1.  Is it possible that the laws of reflection change, if we use spherical mirror instead of a plane mirror? State your reasons.

Q2.  Out of convex mirror and concave mirror, whose focus is situated behind the mirror?

Q3.  Give the relationship of magnification with focal length of a spherical mirror.

Q4.  Between which two points of a concave mirror should an object be placed to obtain a magnification of -3?

Q5.  What is the velocity of light in a glass slab of refractive index 1.5?

Q6.   Is it possible that a convergent lens in one medium becomes divergent, when placed in another medium?

Q7.  If the magnification of a body of size 1 m is 2. What is the size of the image?

Q8.   When is the magnitude of the power of a lens equal to its focal length?

Q9. Define the term principal axis of a spherical mirror.

Q10. Define the principal focus of a concave mirror.

Q11. What is the magnification of the images formed by plane mirror and why?

Q12. We prefer a convex as a rear-view mirror in vehicles. Why?

Q13. Explain why a ray of light passing through the centre of curvature of a concave mirror, gets reflected along the same path.

Q14. What are the values of angle of incidence i and angle of reflection r for a normal incidence?

Q15. What is the unit of refractive index?

Q16. State a condition for no refraction of light entering from one medium to another.

Q17. Why is refractive index of atmosphere different at different altitudes?

Q18. How does the size of the image change as the object is brought closer from infinity towards the convex lens?

Q19.The refractive index of diamond is 2.42. What is the meaning of this statement?

Q20.  A ray of light incident on a concave mirror after passing through the centre of curvature is reflected back along the same path. What is the angle of incidence?

Q21.  What is the radius of curvature of a plane mirror?

Q22.   What is the power of sunglasses?

Q23.   Does refractive index change with colour of light?

Q24.  Redraw the ray diagram given below in your answer book and complete the path of ray.

Direction In each of the following questions, a statement of Assertion is given by the corresponding statement of Reason. Of the statements, mark the correct answer as

(a) If both Assertion and Reason are true and Reason is the correct explanation of Assertion.

(b) If both Assertion and Reason are true, but Reason is not the correct explanation of Assertion.

(c) If Assertion is true, but Reason is false.

(d) If Assertion is false, but Reason is true.

(e) If Assertion and Reason both are false.

Q25. Assertion: Convex mirror is preferred for rearview mirror in vehicles.

Reason: The field view of a convex mirror is lesser than that of concave mirror.

Q26. Assertion: An object appears to be black.

Reason: All the light falling on the object is absorbed and only black light is reflected from it.

Q27. Assertion: Lateral displacement is the distance between incident ray and the emergent ray.

Reason: The wider the glass slab, the larger the lateral displacement.

Q28. Assertion: In the dispersion of white light by a prism, the red light bends the least.

Reason: The frequency of red light is the highest.

Q29. Assertion: The object distance is the distance of the object from the lens.

Reason: It is measured from the object to the optical centre of the Lens.

Q30.  The refractive index of a denser medium with respect to rarer medium is 5/4 then the refractive index of rarer medium with respect to denser medium will be

(a)5/4

(b) √5/2

(c) 2/√5

(d)4/5

Q31.  When an object is placed on the principal axis of a convex lens between first focus and optical centre, then position of image will be

(a) on same side of the lens as the object between first focus and optical centre

(b) on same side of the lens as the object and away from the first focus

(c) on opposite side of the lens as the object between second focus and optical centre

(d) on opposite side of the lens as the object and away from the second focus

Q32.  Which of the following mirrors are used to get powerful parallel beams of light in headlights?

(a) Plane mirrors

(b) Convex mirrors

(c) Concave mirrors

(d) Both (a) and (c)

Q33.  An object is placed at a distance of 10 cm in front of a concave mirror. It forms a real image three times larger than the object. The distance of image from the mirror is

(a) -30 cm

(b) 30 cm

(c) 10 cm

(d) -10 cm

3 MARKS QUESTIONS

Q1.   What is meant by refractive index? If the speed of light in a medium is 2/3 rd of the speed of light in vacuum, find the refractive index of that medium.

Q2.  “The magnification produced by a spherical mirror is -3”. List four information you obtain from this statement about the mirror/image.

Q3.  The absolute refractive indices of glass and water are 4/3 and 3/2, respectively. If the speed of light in glass is 2 × 108 ms-1, calculate the speed of light in (i) vacuum (ii) water

Q4.         (i)  An object 1 cm high produces a real image 1.5 cm high when placed at a distance of 15 cm from a concave mirror. Calculate the position of the image and the magnification.

(ii)   Write two uses of concave mirrors.

Q5.  A   convex    lens    made    of   a   material of refractive index n2   is kept in a medium of refractive index n1. A parallel beam of light is incident on the lens. Draw the path of rays of light emerging from the convex lens, if

(i)  n1 < n2

(ii)  n1 = n2

(iii)  n1 > n2

Q6.  At what distance should an object be placed from a convex lens of focal length 18 cm to obtain an image at 36 cm from it? What will be the magnification produced in this case?

Q7.   Find the distance at which an object should be placed in front of a convex lens of focal length 10 cm to obtain an image of double its size.

Q8. Ravi is given lenses with powers +5D, -5D, +10D, -10D and   -20D. Considering a pair of lenses at a time, which two lenses will he select to have a combination of total focal length when two lenses are kept in contact in each case.

(i)  -10 cm

(ii)  20 cm

(iii) -20 cm

Q9. A 3 cm tall object is placed 18 cm in front of a concave mirror of focal length 12 cm. At what distance from the mirror should a screen be placed to see a sharp image of the object on the screen. Also calculate the height of the image formed.

Q10.The image of a candle flame placed at a distance of 30 cm from a mirror is formed on a screen placed in front of the mirror at a distance of 60 cm from its pole. What is the nature of the mirror?  Find its focal length. If the height of the flame is 2.4 cm, find the height of its image. State, whether the image formed, is erect or inverted.

Q11. A student wants to project the image of a candle flame on a screen 80 cm in front of a mirror by keeping the candle flame at a distance of 20 m from its pole.

(i) Which type of mirror should the student use?

(ii) Find the magnification of the image produced.

(iii) Find the distance between the object and its image.

(iv) Draw a ray diagram to show the image formation in this case and mark the distance between the object and its image.

Q12. A 5 cm tall object is placed at a distance of 30 cm from a convex mirror  of focal length 15 cm. Find the position, size and nature of the image formed.

Q13. If the image formed by a mirror for all positions of the object placed in front of it is always erect and diminished, what type of mirror is it? Draw a ray diagram to justify your answer. Where and why do we generally use this type of mirror?

Q14. Rohit wants to have an erect image of an object, using a converging mirror of focal length 40 cm.

(i) Specify the range of distance where the object can be placed in front of mirror. Give reason for your answer.

(ii) Will image be bigger or smaller than the object?

(iii) Draw a ray diagram to show the image formation in this case.

Q15. (i) What is the angle of incidence, when a ray of light falls on a spherical mirror from its centre of curvature?

(ii) Two concave mirrors have the same focal length but the aperture of one is larger than the other. Which mirror forms the sharper image and why?

(iii) A convex mirror is held in water. What change you will observe in its focal length?

Q16. State the laws of refraction of light. If the speed of light in vacuum is 3 × 108 m/s, find the speed of light in a medium of absolute refractive index 1·5.

Q17. (i)   Define the term magnification.  Write the formula for magnification of mirror explaining the symbols ‘used in the formula.

(ii) The magnification produced by a convex lens is -2. What is meant by this statement and also write the information regarding image obtained from it.

Q18. If the image formed by a lens for all positions of an object placed in front of it is always erect and diminished, what is the nature of this lens? Draw a ray diagram to justify your answer. If the numerical value of the power of this lens of 10 D, what is its focal length in the Cartesian system?

Q19. (i)  Draw a ray diagram to show the refraction of light through a glass slab and mark angle of refraction and the lateral shift suffered by the ray of light while passing through the slab.

(ii) If the refractive index of glass for light going from air to glass is 3/2, find the refractive index of air for light going from glass to air.

Q20. What is meant by power of a lens? You have three lenses L1, L2 and L3 of powers + 10D, +5D and- 10D respectively. State the nature and focal length of each lens. Explain which of the three lenses will form a virtual and magnified image of an object placed at 15 cm from the lens. Draw the ray diagram in support of your answer.

Q20. Calculate the distance at which an object should be placed in front of a convex lens of focal length 100 cm to obtain an erect image of double its size.

Q21. (i) A ray of light falls normally on a face of a glass slab. What are the values of angle of incidence and angle of refraction of this ray?

(ii) Light enters from air to a medium ‘X’. Its speed in medium ‘X’ becomes 1·5   × 108 m/s. Speed of light in air   3 × 108 m/s.  Find the refractive index of medium ‘X’.

Q22. (i) Water has a refractive index 1·33 and alcohol has refractive index 1·36. Which of the two medium is optically denser?  Give reason for your answer. Draw a ray diagram to show the path of a ray of light passing obliquely from water to alcohol.

(ii) The absolute refractive index of diamond is 2.42 and the absolute refractive index of glass is 1.50. Find the refractive index of diamond with respect to glass.

Q23.  (i) Is it possible that a convergent lens in one medium becomes divergent when placed in another medium?

(ii) Under what conditions there won’t be any refraction of light when it enters from one medium to another?

Q24.  Find the distance at which an object should be placed in front of a convex lens of focal length 10 cm to obtain an image of double its size.

Q25.  (i). What is the value of magnification when the size of image formed by a concave mirror is equal to the size of the object?

(ii) An object 2 cm high produces a real image 2.5 cm high, when placed at a distance of 15 cm from a concave mirror. Find

(a) the position of the image,

(b) focal length of the concave mirror.

Q26.   How far should an object be placed from a convex lens of focal length 20 cm to obtain its image at a distance of 30 cm from the lens? What will be the height of the image, if the object is 6 cm tall?

Q27.   An object of size 7.0 cm is placed at a distance of 27 cm in front of concave mirror of focal length 18 cm. At what distance from the mirror should a screen be placed, so that a sharp focused image can be obtained? Find the size and the nature of the image.

Q28.   A 5.0 cm tall object is placed perpendicular to the principal axis of a convex lens of focal length 20 cm. The distance of the object from the lens is 30 cm. By calculation determine

(i)  the position and

(ii) the size of the image formed.

Q29.   An object placed 45 cm from a lens forms an image on a screen placed 90 cm on the other side of the lens. Identify the type of the lens and find its focal length.

Q30.   A ray of light enters a diamond from air. If the refractive index of diamond is 2.42, what % (percent) is speed of light in diamond with that of speed in air?

5 MARK QUESTIONS

Q1.         (i)  Define the following terms in context of spherical mirrors.

(a)  Pole

(b) Centre of curvature

(c) Principal axis

(d) Principal focus

(ii)   Draw ray diagrams to show the principal focus of a

(a)  concave mirror and

(b) convex mirror

(iii)   Consider the following diagram in which M is a mirror and P is an object and Q is its magnified image formed by the mirror.

State the type of the mirror M and one characteristic property of the image Q.

Q2.   It is desired to obtain an erect image of an object, using concave mirror of focal length of 12 cm.

(i)  What should be the range of distance of an object placed in front of the mirror?

(ii)  Will the image be smaller or larger than the object?  Draw ray diagram to show the formation of image in this case.

(iii) Where will the image of this object be, if it is placed   24 cm in front of the mirror? Draw ray diagram for this situation also to justify your answer.

Show the positions of pole, principal focus and the centre of curvature in the above ray diagrams.

Q3.  A student wants to project the image of a candle flame on the walls of school laboratory by using a lens.

(i) Which type of lens should he use and why?

(ii)  At what distance in terms of focal length F of the lens should he place the candle flame, so as to get

(a)  a magnified and

(b) a diminished image respectively, on the wall?

(iii) Draw ray diagrams to show the formation of the image in each case.

Q4.   One half of a convex lens of focal length 10 cm is covered with a black paper.   Can such a lens produce an image of a complete object placed at a distance of 30 cm from the lens? Draw a ray diagram to, justify your answer.

Q5. A 4 cm tall object is placed perpendicular to principal axis of a   convex lens of focal length 20 cm.  The distance of the object from the lens is 15 cm.   Find the nature, position and the size of the image.

Q6.  What is meant by power of a lens?  Define its SI unit. You have two lenses A and B of focal lengths +10 cm and -10 cm, respectively. State the nature and power of each lens. Which of the two lenses will form a virtual and magnified image of an object placed 8 cm from the lens?   Draw a ray diagram to justify your answer.

Q7. Suppose you have three concave mirrors A, B and C of focal lengths 10 cm, 15 cm and 20 cm. For each concave mirror, you perform the experiment of image formation for three values of object distances of 10 cm, 20 cm and 30 cm. By giving reason, answer the following:

(i) For the three object distances, identify the mirror/mirrors which will form an image of magnification -1.

(ii) Out of the three minors identify the minor which would be preferred to be used for shaving purposes/ makeup.

(iii) For the mirror   B draw ray diagram for image formation for object distances 10 cm and 20 cm.

Q8. A student wants to project the image of a candle flame on the walls of the school laboratory by using a mirror.

(i) Which type of mirror should he use and why?

(ii) At what distance, in terms of focal length ‘f’ of the mirror, should he place the candle flame to get the magnified image on the wall?

(iii) Draw a ray diagram to show the formation of the image in this case.

(iv) Can he use this mirror to project a diminished image of the candle flame on the same wall? State ‘how’ if your answer is ‘yes’ and ‘why not’ if your answer is ‘no.’

Q9. Draw a ray diagram in each of the following cases to show the position and nature of image formed when the object is placed:

(i) Between pole and focus of a concave mirror.

(ii) Between focus and centre of curvature of a concave mirror.

(iii) At the centre of curvature of a concave mirror.

(iv) Between infinity and pole of a convex mirror.

(v) At infinity from a convex mirror.

Q10. (i) Define focal length of a spherical lens.

(ii) A divergent lens has a focal length of 30 cm. At what distance should an object of height 5 cm from the optical centre of the lens be placed so that its image is formed 15 cm away from the lens ? Find the size of the image also.

(iii) Draw a ray diagram to show the formation of image in the above situation.

Q11. (i) Define power of a lens. Write its SI units.

(ii) You are provided with two convex lenses of focal length 15 cm and 25 cm, respectively. Which   of the two is of larger power? Give reason for your answer.

(iii) A 20 cm tall object is placed perpendicular to the principal axis of a convex lens of focal length 10 cm. The distance of the object from the lens is 15 cm. Find Ute nature, position and size of the image. Also find its magnification.

Q12. (i) Draw a ray diagram to show the formation of image by a convex lens when an object is placed in front of the lens between its optical centre and principal focus.

(ii) In the above ray diagram mark the object-distance (u) and the image-distance (v) with their proper signs (+ve  or – ve as per the new  Cartesian sign convention)  and state how these distances are related to the focal length  (f) of the convex lens in this case.

(iii) Find the power of a convex lens which forms a real, and inverted image of magnification -1of an object placed at a distance of 20 cm from its optical centre.

Q13. (i) Two convex lenses A and B have powers P1 and P2 respectively and P2   is greater than P1 Draw a ray diagram for each lens to show which one will be more converging.  Give reason for your answer.

(ii) A 2.0 cm tall object is placed perpendicular to the principal axis of a convex lens of focal length 10 cm. The distance of the object from the lens is 15 cm. Find the nature, position and size of the image. Also find its magnification.

Q14.  (i) Define the following term related to spherical mirror.

(b) Pole

(c) Principal focus

(ii) Mention the differences between real image and virtual image.

Q15.  (i) Draw ray diagrams and show the nature of images formed in the following cases, when

(a) object is placed between F and 2F in front of concave lens,

(b) object is placed between F and 2F in front of convex lens.

(ii) An object 50 cm is placed on the principal axis of a convex lens. Its 20 cm tall image is formed on the screen placed at a distance of 10 cm from the lens. Calculate the focal length of the lens.

Q16.       The image of an object placed at 60 cm in front of a lens is obtained on a screen at a distance of 120 cm from it.  Find the focal length of the lens.  What would be the height of the image, if the object is 5 cm high?

Q17.  ( i) Two lenses  have power of

(a)+ 2D

(b) – 4D.

What is the nature and focal length of each lens?

(ii) An object is kept at a distance of 100 cm from each of the above lenses. Calculate the

(a) image distance and

(b) magnification in each of two cases.

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