For Grade 10th Students.

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CHAPTER 10
Light – Reflection and Refraction
KEY POINTS & ASSIGNMENT
KEY POINTS
➢ Light seems to travel in straight lines.
➢ Mirrors and lenses form images of objects. Images can be either real or virtual, depending
on the position of the object.
➢ The reflecting surfaces, of all types, obey the laws of reflection. The refracting surfaces
obey the laws of refraction.
➢ New Cartesian Sign Conventions are followed for spherical mirrors and lenses.
➢ Mirror formula, 1/f = 1/u + 1/v, gives the relationship between the object-distance (u),
image-distance (v), and focal length (f) of a spherical mirror.
➢ The focal length of a spherical mirror is equal to half its radius of curvature.
➢ The magnification produced by a spherical mirror is the ratio of the height of the image to
the height of the object.
➢ A light ray travelling obliquely from a denser medium to a rarer medium bends away from
the normal. A light ray bends towards the normal when it travels obliquely from a rarer to a
denser medium.
➢ Light travels in vacuum with an enormous speed of 3×108
m s-1
. The speed of light is
different in different media.
➢ The refractive index of a transparent medium is the ratio of the speed of light in vacuum to
that in the medium.
➢ In case of a rectangular glass slab, the refraction takes place at both air-glass interface and
glass-air interface. The emergent ray is parallel to the direction of incident ray.
➢ Lens formula, 1/f = 1/v -1/u, gives the relationship between the object-distance (u), image-
distance (v), and the focal length (f) of a spherical lens.
➢ Power of a lens is the reciprocal of its focal length. The SI unit of power of a lens is dioptre.
ASSIGNMENT
Q 1. A concave lens has focal length of 15 cm. At what distance should the object from the lens be
Created by:- Er. Murari Parashar Grade 10 (Physics) Mob. 9460991468

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placed so that it forms an image at 10 cm from the lens? Also, find the magnification produced by
the lens.
Q2. 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.
Q3. 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?
Q4. 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
.
Q5. Find out, from Table 10.3, the medium having highest optical density. Also find the medium
with lowest optical density.
Q6. You are given kerosene, turpentine and water. In which of these does the light travel fastest?
Use the information given in Table 10.3.
Q7. The refractive index of diamond is 2.42. What is the meaning of this statement?
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.
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.
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
Created by:- Er. Murari Parashar Grade 10 (Physics) Mob. 9460991468

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the prescribed lens diverging or converging?
Q18. Define 1 dioptre of power of a lens.
Q19. 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.
Q20. Find the power of a concave lens of focal length 2 m.
Q21. Define the principal focus of a concave mirror.
Q22. The radius of curvature of a spherical mirror is 20 cm. What is its focal length?
Q23. Name a mirror that can give an erect and enlarged image of an object.
Q24. Why do we prefer a convex mirror as a rear-view mirror in vehicles?
Q25. A convex mirror used for rear-view on an automobile has a radius of curvature of 3.00 m. If a
bus is located at 5.00 m from this mirror, find the position, nature and size of the image.
Q26. An object, 4.0 cm in size, is placed at 25.0 cm in front of a concave mirror of focal length 15.0
cm. At what distance from the mirror should a screen be placed in order to obtain a sharp image?
Find the nature and the size of the image.
Q27. Find the focal length of a convex mirror whose radius of curvature is 32 cm.
Q28. 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?
Created by:- Er. Murari Parashar Grade 10 (Physics) Mob. 9460991468