# LIGHT: REFLECTION AND REFRACTION– 3 Mark 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?

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