Question 1

Q1: Which one of the following materials cannot be used to make a lens? (a) Water (b) Glass (c) Plastic (d) Clay

Accepted Answer

Answer: (d) Clay Reason: A lens is a transparent optical medium that refracts light. Materials like water, glass, and plastic are transparent and can be shaped to form lenses. Clay, however, is an opaque material, which means it does not allow light to pass through it. Therefore, clay cannot be used...

Question 2

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 nat...

Accepted Answer

Given: Height of the object, $h = +5 	ext{ cm}$ Object distance, $u = -25 	ext{ cm}$ (object is on the left) Focal length of converging (convex) lens, $f = +10 	ext{ cm}$ To Find: Image distance, $v$ Height of the image, $h'$ Nature of the image Formula: Lens formula: $$\frac{1}{v} - \frac{1}{u}...

Question 3

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.

Accepted Answer

Given: Focal length of concave lens, $f = -15 	ext{ cm}$ Image distance, $v = -10 	ext{ cm}$ (A concave lens always forms a virtual image on the same side as the object). To Find: Object distance, $u$ Formula: Lens formula: $$\frac{1}{v} - \frac{1}{u} = \frac{1}{f}$$ Calculation: Substituting the...

Question 4

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.

Accepted Answer

Given: Object distance, $u = -10 	ext{ cm}$ Focal length of convex mirror, $f = +15 	ext{ cm}$ To Find: Image distance, $v$ Nature of the image Formula: Mirror formula: $$\frac{1}{v} + \frac{1}{u} = \frac{1}{f}$$ Calculation: Substituting the values in the mirror formula: $$\frac{1}{v} + \frac{1}{...

Question 5

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

Accepted Answer

The magnification ($m$) produced by a plane mirror being $+1$ has two implications: 1.  The positive sign (+): This indicates that the image formed is virtual and erect. An erect image is one that is upright, in the same orientation as the object. 2.  The numerical value (1): This indicates that the...

Question 6

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,...

Accepted Answer

Given: Height of the object, $h = +5.0 	ext{ cm}$ Object distance, $u = -20 	ext{ cm}$ Radius of curvature of convex mirror, $R = +30 	ext{ cm}$ To Find: Image distance, $v$ Nature of the image Height of the image, $h'$ Formula: Focal length, $f = \frac{R}{2}$ Mirror formula: $$\frac{1}{v} + \fra...

Question 7

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 pl...

Accepted Answer

Given: Height of the object, $h = +7.0 	ext{ cm}$ Object distance, $u = -27 	ext{ cm}$ Focal length of concave mirror, $f = -18 	ext{ cm}$ To Find: Image distance (screen position), $v$ Height of the image, $h'$ Nature of the image Formula: Mirror formula: $$\frac{1}{v} + \frac{1}{u} = \frac{1}{f...

Question 8

Q16: Find the focal length of a lens of power –2.0 D. What type of lens is this?

Accepted Answer

Given: Power of the lens, $P = -2.0 	ext{ D}$ (Dioptre) To Find: Focal length, $f$ Type of lens Formula: Power of a lens, $$P = \frac{1}{f 	ext{ (in metres)}}$$ Calculation: $$f = \frac{1}{P}$$ $$f = \frac{1}{-2.0} = -0.5 	ext{ m}$$ To convert to centimeters: $f = -0.5 	imes 100 = -50 	ext{ cm}...

Question 9

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?

Accepted Answer

Given: Power of the lens, $P = +1.5 	ext{ D}$ (Dioptre) To Find: Focal length, $f$ Type of lens (diverging or converging) Formula: Power of a lens, $$P = \frac{1}{f 	ext{ (in metres)}}$$ Calculation: $$f = \frac{1}{P}$$ $$f = \frac{1}{+1.5} = +\frac{1}{1.5} = +\frac{10}{15} = +\frac{2}{3} \approx...

Question 10

Q2: 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) Betw...

Accepted Answer

Answer: (d) Between the pole of the mirror and its principal focus. Reason: A concave mirror forms a virtual, erect, and magnified image only when the object is placed between its pole (P) and principal focus (F). For all other positions of the object in front of the mirror, it forms a real and inve...

Question 11

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) A...

Accepted Answer

Answer: (b) At twice the focal length Reason: For a convex lens, when an object is placed at the center of curvature, which is at a distance of twice the focal length ($2F_1$) from the optical center, it forms a real, inverted image of the same size at twice the focal length on the other side of the...

Question 12

Q4: 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 c...

Accepted Answer

Answer: (a) both concave. Reason: According to the New Cartesian Sign Convention, the focal length of a concave mirror is negative, and the focal length of a concave lens is also negative. A convex mirror and a convex lens both have positive focal lengths. Since both the mirror and the lens have a f...

Question 13

Q5: No matter how far you stand from a mirror, your image appears erect. The mirror is likely to be (a) only plane. (b) only concave. (c) only convex. (d)...

Accepted Answer

Answer: (d) either plane or convex. Reason:  - A plane mirror always forms a virtual, erect image of the same size, regardless of the object's distance. - A convex mirror always forms a virtual, erect, and diminished image, regardless of the object's distance. - A concave mirror only forms an erect...

Question 14

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)...

Accepted Answer

Answer: (c) A convex lens of focal length 5 cm. Reason: To read small letters, a magnifying glass is used, which is a convex lens. A convex lens produces a magnified, virtual, and erect image when the object (the letters) is placed within its focal length. The magnification of a lens is inversely re...

Question 15

Q7: 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 t...

Accepted Answer

To obtain an erect image using a concave mirror, the object must be placed between the pole (P) and the principal focus (F) of the mirror. Range of object distance: The focal length ($f$) is given as $15 	ext{ cm}$. Therefore, the object distance ($u$) must be less than $15 	ext{ cm}$. The range i...

Question 16

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...

Accepted Answer

(a) Headlights of a car: Mirror type: Concave mirror. Reason: The light bulb is placed at the principal focus of the concave mirror. The mirror reflects the light rays from the bulb into a powerful, parallel beam, which illuminates the road over a long distance. (b) Side/rear-view mirror of a vehicl...

Question 17

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. Exp...

Accepted Answer

Answer: Yes, the lens will still produce a complete image of the object. Explanation: An image is formed when rays of light from a point on the object converge at a corresponding point after passing through the lens. Rays from every point on the object travel in all directions and strike the entire...

NCERT Solutions