Light: Mirrors and Lenses — Complete Answer Key
Every in-text question and every “Keep the Curiosity Alive” exercise from the chapter, answered directly with clear reasoning, ray-diagram logic, and the chapter’s own photographs.
In-text Questions (Probe, Ponder & Thought Bubbles)
Can we make mirrors which can give enlarged or diminished images?
Yes. Curved (spherical) mirrors can do this. A concave mirror gives an enlarged image of an object placed close to it, while a convex mirror always gives a diminished (smaller) image of the object, no matter how far or close it is placed. Only plane mirrors are restricted to always showing a same-size image.
On side-view mirrors of vehicles, there is a warning that says “Objects in mirror are closer than they appear”. Why is this warning written there?
Side-view mirrors are convex mirrors. A convex mirror always forms a diminished (smaller) image of objects behind it. Because the vehicle behind looks smaller than it actually is, the driver’s brain instinctively judges it to be farther away than it really is. The warning reminds drivers that the vehicle is actually closer than the small image suggests, so they don’t misjudge distance and cause an accident.
Why is there a curved line on some reading glasses?
Some reading glasses are bifocal lenses — a single spectacle lens combines two different lens powers in one piece of glass, separated by a visible curved line. The upper part of the lens is ground for distance vision, while the lower part (below the curved line) has extra convex lens power for near/reading vision. This lets the wearer look through the upper part for distant objects and the lower part for reading, without needing two separate pairs of glasses.
How can we distinguish between concave and convex mirrors?
One simple way is to view the mirror from its side, with your eye level with the mirror’s edge (as in Activity 10.2). If the reflecting surface curves inward (away from you, like the inside of a bowl), it is a concave mirror. If it bulges outward (towards you), it is a convex mirror.
A second way (mentioned later in the chapter) is to simply look at the image an object forms in the mirror: an enlarged, erect image close up that inverts farther away indicates a concave mirror, while an image that is always erect and diminished indicates a convex mirror.
What did the shiny metallic spoon show about the images formed by its inner and outer curved surfaces?
The inner curved surface of the spoon (which curves inward, like a concave mirror) produced an inverted image of the face. The outer curved surface of the spoon (which bulges outward, like a convex mirror) produced an image that was erect but smaller in size than the face. This matches exactly how concave and convex mirrors behave.
Where do we find concave and convex mirrors being used in our surroundings?
Concave mirrors are used as: reflectors in torches and vehicle headlights (to concentrate light into a strong beam), dental mirrors (to give an enlarged view of teeth), and the main mirror of reflecting telescopes.
Convex mirrors are used as: side-view mirrors of vehicles (to give a wide-angle, diminished view of traffic behind), road safety mirrors at sharp bends or intersections (to let drivers from both sides see each other), and surveillance mirrors in big stores (to monitor a large area at once).
Are the laws of reflection applicable to spherical mirrors also?
Yes. The two laws of reflection (angle of incidence equals angle of reflection; incident ray, normal, and reflected ray all lie in the same plane) hold true at every single point on a spherical mirror’s surface, exactly as they do for a plane mirror. However, because a spherical mirror’s surface is curved, the normal direction is different at each point on it. This is why, even though each individual ray obeys the laws of reflection, a parallel beam of rays ends up converging (concave mirror) or diverging (convex mirror) after reflection — the curvature changes the direction of the normal from point to point.
Since the concave mirror converges the light beam, wouldn’t light get concentrated in a small area?
Yes, exactly. Activity 10.7 demonstrates this: when sunlight (a set of nearly parallel rays) falls on a concave mirror, the reflected rays converge and meet at a small bright spot. Since a large amount of light energy gets concentrated into this tiny area, it produces enough heat there to ignite a piece of paper placed at that spot. This same principle is used in solar concentrators, solar cookers, and solar furnaces.
Do lenses also converge or diverge the light beam?
Yes. Just like mirrors, lenses also bend (refract) light in a way that depends on their shape. A convex lens converges a parallel beam of light to a point — it is also called a converging lens. A concave lens diverges a parallel beam of light, spreading it out — it is also called a diverging lens. This is confirmed in Activity 10.10, where multiple parallel beams passed through each lens.
Since a convex lens converges a light beam, can it also burn a paper?
Yes. Just like a concave mirror, a convex lens converges parallel sunrays to a small, bright, hot point (a focus). If a piece of paper is held at that exact point for a few minutes, the concentrated heat can ignite it, exactly as described in Activity 10.11. (As the chapter’s safety note warns, one must never look at the Sun directly or through the lens, since this can permanently damage the eyes.)
Where all are lenses used?
Lenses are used very widely: in eyeglasses (spectacles) to help people see clearly, in cameras (including smartphone cameras) to focus light onto the sensor, in telescopes to view distant objects, and in microscopes to magnify tiny objects. Even the human eye itself contains a convex lens, which can change its shape to focus on objects at different distances — letting us read a book up close or see something far away.
How did the water drop on the glass/plastic strip behave like a lens?
The surface of the water drop was curved outward (bulging), just like a convex lens. When the text on the paper underneath was viewed through this curved drop, the letters appeared larger than the letters around the drop. This happened because the curved water surface bends (refracts) light rays similarly to a convex lens, magnifying whatever is viewed through it — exactly how a magnifying glass works.
Keep the Curiosity Alive — Exercise Solutions
A light ray is incident on a mirror and gets reflected by it. The angle made by the incident ray with the normal to the mirror is 40°. What is the angle made by the reflected ray with the mirror?
Correct option: (ii) 50°
Three situations of a light ray falling on a mirror are shown — (i) ray along the normal, (ii) tilted mirror with the ray still along the normal to the tilted surface, (iii) tilted mirror with the ray at 20° from the normal. Draw the reflected ray and state the angle of reflection in each case.
Angles of reflection: (a) 0°, (b) 0°, (c) 20° — in every case r = i, drawn symmetric to the normal.
The cap of a sketch pen is placed in front of three types of mirrors. Match each image with the correct mirror.
| Image | Observation | Mirror |
|---|---|---|
| (i) | Same size as the object, erect | Plane mirror |
| (ii) | Larger than the object, erect (object close to mirror) | Concave mirror |
| (iii) | Smaller than the object, erect | Convex mirror |
(i) → Plane mirror, (ii) → Concave mirror, (iii) → Convex mirror
The cap of a sketch pen is placed behind a convex lens, a concave lens, and a flat transparent glass piece — all at the same distance. Match each image with the correct type of lens or glass.
| Image | Observation | Lens/Glass |
|---|---|---|
| (i) | Larger than the object (magnified) | Convex lens |
| (ii) | Same size as the object | Flat transparent glass piece |
| (iii) | Smaller than the object (diminished) | Concave lens |
(i) → Convex lens, (ii) → Flat transparent glass piece, (iii) → Concave lens
When the light is incident along the normal on the mirror, which of the following statements is true?
The angle of incidence is always measured between the incident ray and the normal. If the ray falls exactly along the normal itself, the angle between the ray and the normal is $0°$.
Correct option: (ii) Angle of incidence is 0°
Three mirrors — plane, concave, and convex — are placed as shown. Based on the images of the graph sheet formed in each, identify the mirrors.
Use this rule to label each mirror in the photo: undistorted grid = plane; lines converging/pinched = concave; lines spreading/diminished = convex.
In a museum, a woman walks towards a large concave mirror. What does she observe?
Correct option: (iii) her inverted image keeps increasing in size and eventually it becomes erect and magnified.
Hold a magnifying glass over text and identify the distance where the text looks bigger. Now move it away from the text. What do you notice? Which type of lens is a magnifying glass?
A magnifying glass is a convex (converging) lens.
Match the entries in Column I with those in Column II.
| Column I | Matches with | Column II |
|---|---|---|
| (i) Concave mirror | → (a) | Spherical mirror with a reflecting surface that curves inwards |
| (ii) Convex mirror | → (b) | Forms an image which is always erect and diminished in size |
| (iii) Convex lens | → (c) | Object placed behind it may appear inverted at some distance |
| (iv) Concave lens | → (d) | Object placed behind it always appears diminished in size |
Assertion: Convex mirrors are preferred for observing the traffic behind us. Reason: Convex mirrors provide a significantly larger view area than plane mirrors. Choose the correct option.
Correct option: (i) Both Assertion and Reason are correct and Reason is the correct explanation for Assertion.
In the figure, O stands for object, M for mirror, and I for image. Which statement is true?
Correct option: (i) Figure (a) indicates a plane mirror and Figure (b) indicates a concave mirror.
Place a pencil behind a transparent glass tumbler. Now fill the tumbler halfway with water. How does the pencil appear when viewed through the water? Explain why its shape appears changed.
The pencil appears bent/displaced at the water surface due to the bending (refraction) of light as it passes from water to air through the curved glass-water boundary.
Discover, Design & Debate
Visit a hospital, ENT specialist, or dentist’s clinic and identify the kind of mirrors used for examining ear, nose, throat, and teeth.
Doctors typically use small concave mirrors for these examinations. A concave mirror, when held close to the area being examined (ear, nose, throat, or teeth), produces an enlarged, erect image, which lets the doctor see small details clearly. Some ENT head-mirrors also use a concave mirror with a hole in the centre to reflect light into the cavity being examined while the doctor looks through the hole.
Think of a design for a solar cooker for your school or home, and prepare a proposal including the budget.
Use online tools or animations to do virtual experiments with spherical mirrors and lenses, and observe how the image changes.
Many free physics-simulation websites let you place a virtual object at different distances in front of a concave/convex mirror or lens and instantly see how the image’s size, position, and orientation (erect/inverted) change. While exploring, try noting: how the image changes as the object moves from very far away to very close to a concave mirror or convex lens (it should match the behaviour summarised in this chapter — enlarging, then inverting, then eventually forming a magnified erect virtual image very close up), and confirm that a convex mirror or concave lens always keeps the image erect and diminished, regardless of object distance.