RowQ
The Vault
RowQ
The Vault
CBSE Class 10 Physics · 11 questions · 26 marks
Light bounces off mirrors and bends through lenses, and CBSE Class 10 asks you to predict exactly how. This chapter builds the mirror and lens formulas, sign conventions, and ray-diagram logic that let you locate any image without guesswork. Master it once and numericals on mirrors, lenses, and power become quick, mechanical wins.
Which of the following correctly describes the nature of the image formed by a plane mirror?
Answer
Virtual, erect, and of the same size as the object — a plane mirror always forms a virtual, erect, laterally inverted image equal in size to the object, at a distance behind the mirror equal to the object's distance in front of it.
An object is placed exactly at the centre of curvature of a concave mirror. Where does the image form and what is its nature?
Answer
At the centre of curvature; real, inverted, same size — when an object is placed at C, its image also forms at C, and is real, inverted, and equal in size to the object.
A ray of light passes from air (a rarer medium) into glass (a denser medium). How does it bend, and why?
Answer
It bends towards the normal because light slows down on entering the denser glass — going from a rarer to a denser medium reduces the speed of light, so the ray bends towards the normal.
A convex lens of focal length 20 cm is being used as a magnifying glass. For the image to be virtual, erect, and magnified, where must the object be placed?
Answer
Between the optical centre and focus (less than 20 cm from the lens) — placing the object within the focal length of a convex lens produces a virtual, erect, magnified image on the same side as the object, which is exactly how a magnifying glass works.
Assertion (A): Concave mirrors are commonly used as shaving or makeup mirrors. Reason (R): A concave mirror forms a virtual, erect, and magnified image only when the object is placed between the pole and the focus.
Answer
Both A and R are true and R is the correct explanation of A — holding the mirror close to the face keeps the face within the focal length, so the concave mirror produces the enlarged, upright virtual image needed for shaving or applying makeup.
State the two laws of reflection of light.
Answer
1. The angle of incidence is always equal to the angle of reflection (angle i = angle r). 2. The incident ray, the reflected ray, and the normal to the reflecting surface at the point of incidence all lie in the same plane.
A concave lens has a focal length of 10 cm. At what distance from the lens should an object be placed so that its image forms 5 cm from the lens? Also find the magnification produced.
Answer
For a concave lens, f = -10 cm. The image of a concave lens is always virtual and on the same side as the object, so v = -5 cm. Using 1/v - 1/u = 1/f: -1/5 - 1/u = -1/10, so 1/u = -1/5 + 1/10 = -1/10, giving u = -10 cm. So the object must be placed 10 cm from the lens. Magnification m = v/u = (-5)/(-10) = 0.5, meaning the image is virtual, erect, and half the size of the object.
Define the refractive index of a medium and give its formula in terms of the speed of light.
Answer
The refractive index of a medium measures how much it slows down light compared to vacuum or air. It is defined as n = c/v, where c is the speed of light in vacuum (or air) and v is the speed of light in that medium. Since it is a ratio of two speeds, refractive index has no units.
With the help of a described ray construction, explain the position, nature, and size of the image formed by a concave mirror when the object is placed beyond the centre of curvature (C). Also state one practical use of a concave mirror based on this property.
Answer
When an object is placed beyond the centre of curvature (C) of a concave mirror, its image can be located using two rays from the top of the object: a ray parallel to the principal axis, which reflects and passes through the focus F; and a ray passing through the centre of curvature C, which strikes the mirror along the normal and reflects straight back along the same path. These two reflected rays meet between F and C, on the same side as the object, and this is where the image forms. Nature and position: the image is real, inverted, and diminished in size, and it lies between the focus F and the centre of curvature C. Use: this property of forming a small, real image of a distant object is used in reflecting telescopes, where a large concave mirror gathers light from a very distant object (effectively beyond C) and forms a small, sharp, real image near its focus for further magnification by an eyepiece.
(a) Define the power of a lens and state its SI unit. (b) Two thin lenses of powers +5.0 D and -2.5 D are placed in close contact. Calculate the power and focal length of the combination, and state whether it behaves as a converging or diverging lens.
Answer
(a) The power of a lens is the reciprocal of its focal length measured in metres: P = 1/f. It measures how strongly a lens converges or diverges the light passing through it. Its SI unit is the dioptre (D); a lens has a power of 1 D if its focal length is 1 m. Power is taken as positive for a converging (convex) lens and negative for a diverging (concave) lens. (b) For lenses held in contact, powers add algebraically: P = P1 + P2 = 5.0 D + (-2.5 D) = 2.5 D. Focal length of the combination, f = 1/P = 1/2.5 = 0.4 m = 40 cm. Since the net power is positive, the combination behaves as a converging (convex) lens of focal length 40 cm.
Read the following and answer the questions that follow: Rahul's father's car has a mirror fitted on the outside, near the driver's seat, so he can watch the traffic behind. Rahul notices this mirror bulges outward and that vehicles seen in it look smaller than they really are, yet the mirror shows a much wider stretch of road than a flat mirror of the same size would. (a) Identify the type of mirror used and give one reason it is preferred over a plane mirror here. (b) What is the nature of the image formed by this mirror (real/virtual, erect/inverted, magnified/diminished)? (c) Such mirrors are often marked with a safety warning about distance. State the warning and explain why it is needed. (d) Name one other common use of this type of mirror.
Answer
(a) A convex mirror is used. It is preferred because its outward-curving (diverging) surface gives a much wider field of view of the traffic behind, letting the driver see a larger area of road than a plane mirror of the same size could show. (b) The image formed is virtual, erect, and diminished (smaller than the object), and it forms behind the mirror, between the pole and the focus, for all positions of the object. (c) Such mirrors are often marked "Objects in the mirror are closer than they appear." This warning is necessary because the diminished image formed by a convex mirror makes following vehicles look farther away and smaller than their true distance, which could otherwise mislead a driver about how close a vehicle really is. (d) Convex mirrors are also used as security mirrors in shops, at blind road turns, and in ATMs, since a single mirror gives a wide-angle view of a large area.
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