April 10, 2026
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Chapter 10: Light – Reflection and Refraction
Complete NCERT Solutions (Intext + Exercise) | CBSE Class 10 Science
Class 10- Science Solutions
Class 10- English Solutions
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Class 10 (CBSE) - SYLLABUS
Class 10 (CBSE) - PYQ
Intext Questions (Page 168)
12 marks
Define the principal focus of a concave mirror.
Answer: The principal focus of a concave mirror is the point on its principal axis where parallel rays of light coming from infinity converge after reflection.
22 marks
The radius of curvature of a spherical mirror is 20 cm. What is its focal length?
Answer: Focal length (f) = R/2 = 20/2 = 10 cm.
32 marks
Name a mirror that can give an erect and enlarged image of an object.
Answer: Concave mirror (when object is placed between pole and focus).
42 marks
Why do we prefer a convex mirror as a rear-view mirror in vehicles?
Answer: Convex mirrors give an erect, diminished, and wide field of view, allowing drivers to see a larger area behind them.
Intext Questions (Page 171)
52 marks
Find the focal length of a convex mirror whose radius of curvature is 32 cm.
Answer: f = R/2 = 32/2 = 16 cm.
63 marks
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?
Answer: Magnification m = -v/u = -3 (real image, inverted). u = -10 cm. So -v/(-10) = -3 => v/10 = -3 => v = -30 cm. Image is 30 cm in front of the mirror.
Intext Questions (Page 176)
72 marks
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.
Answer: n = c/v => v = c/n = (3 × 10⁸)/1.50 = 2 × 10⁸ m/s.
82 marks
The refractive index of diamond is 2.42. What is the meaning of this statement?
Answer: It means the speed of light in diamond is 2.42 times slower than in vacuum (v = c/2.42).
92 marks
Define 1 dioptre of power of a lens.
Answer: 1 dioptre is the power of a lens whose focal length is 1 metre. P = 1/f (f in metres).
103 marks
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.
Answer: Image real and same size means object at 2F. v = +50 cm (real image on other side). For same size, u = -50 cm. Then f = v/2 = 25 cm = 0.25 m. Power P = 1/f = 1/0.25 = +4 D.
112 marks
Find the power of a concave lens of focal length 2 m.
Answer: For concave lens, f = -2 m. P = 1/f = -0.5 D.
NCERT Exercise Questions (Page 185)
Ex 11 mark
Which one of the following materials cannot be used to make a lens? (a) Water (b) Glass (c) Plastic (d) Clay.
Answer: (d) Clay – opaque, does not allow light transmission.
Ex 21 mark
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.
Answer: (d) Between the pole and principal focus.
Ex 31 mark
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 (b) At twice the focal length (c) At infinity (d) Between the optical centre and principal focus.
Answer: (b) At twice the focal length (2F).
Ex 41 mark
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 and the lens is concave.
Answer: (a) both concave. For mirror, concave has negative f; for lens, concave also has negative f.
Ex 52 marks
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.
Answer: (d) either plane or convex.
Ex 63 marks
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.
Answer: (c) A convex lens of focal length 5 cm – gives high magnification for close reading.
Ex 73 marks
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: For erect image with concave mirror, object must be between pole and focus (0 to 15 cm). Image is virtual, erect, and larger than object.
Ex 83 marks
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.
Answer: (a) Concave – to produce a powerful parallel beam. (b) Convex – wide field of view, erect image. (c) Concave – to concentrate sunlight at focus.
Ex 93 marks
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, a complete image is formed but less bright. The uncovered part still refracts light to form the full image.
Ex 102 marks
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 nature of the image formed.
Answer: u = -25 cm, f = +10 cm. 1/v = 1/f - 1/u = 1/10 + 1/25 = (5+2)/50 = 7/50 => v = 50/7 ≈ 7.14 cm (real image on other side). Magnification m = v/u = -7.14/25 = -0.2856, image size = 5 × 0.2856 = 1.43 cm, inverted, diminished.
Ex 113 marks
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.
Answer: For concave lens, f = -15 cm, v = -10 cm (virtual image). 1/u = 1/v - 1/f = -1/10 - (-1/15) = -1/10 + 1/15 = (-3+2)/30 = -1/30 => u = -30 cm. Object is 30 cm from lens on same side.
Ex 123 marks
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.
Answer: f = +15 cm (convex), u = -10 cm. 1/v = 1/f - 1/u = 1/15 - (-1/10) = 1/15 + 1/10 = (2+3)/30 = 5/30 = 1/6 => v = +6 cm. Image is virtual, erect, behind the mirror.
Ex 133 marks
The magnification produced by a plane mirror is +1. What does this mean?
Answer: It means image is same size as object, erect (+ sign), and virtual.
Ex 143 marks
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, nature and size of the image.
Answer: R = 30 cm, f = R/2 = +15 cm (convex). u = -20 cm. 1/v = 1/f - 1/u = 1/15 + 1/20 = (4+3)/60 = 7/60 => v = 60/7 ≈ 8.57 cm. m = -v/u = -8.57/ -20 = +0.4285. Image size = 5 × 0.4285 = 2.14 cm, virtual, erect, diminished.
Ex 153 marks
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 to get a sharp focused image? Find the size and nature of the image.
Answer: f = -18 cm, u = -27 cm. 1/v = 1/f - 1/u = -1/18 + 1/27 = (-3+2)/54 = -1/54 => v = -54 cm (real image on same side). m = -v/u = -(-54)/(-27) = 54/(-27) = -2. Image size = 7 × 2 = 14 cm, inverted, real. Screen at 54 cm from mirror.
Ex 163 marks
Find the focal length of a lens of power -2.0 D. What type of lens is this?
Answer: P = -2.0 D, f = 1/P = -0.5 m = -50 cm. It is a concave lens.
Ex 172 marks
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?
Answer: f = 1/P = 1/1.5 = 0.6667 m = 66.67 cm. Positive power means converging (convex lens).
Additional Important Questions
A12 marks
State the laws of reflection of light.
Answer: (i) Angle of incidence equals angle of reflection. (ii) Incident ray, reflected ray, and normal all lie in the same plane.
A22 marks
What is the difference between a real image and a virtual image?
Answer: Real image can be obtained on screen; inverted; formed by actual convergence of rays. Virtual image cannot be obtained on screen; erect; formed by apparent divergence of rays.
A32 marks
What is Snell's law of refraction?
Answer: The ratio of sine of angle of incidence to sine of angle of refraction is constant for a given pair of media. n = sin i / sin r.
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