Answers to all "Keep the curiosity alive" questions of Chapter 10, Light: Mirrors and Lenses (NCERT Class 8 Science, Curiosity, 2026-27): laws of reflection, rays along the normal, images in plane, concave and convex mirrors, convex and concave lenses, a concave mirror in a museum, the magnifying glass, rear-view mirrors and a pencil in water. All 12 questions are answered, with the key answer highlighted.
Laws of reflection: the angle of incidence equals the angle of reflection (both measured from the normal), and the incident ray, the reflected ray and the normal lie in one plane. A plane mirror gives an erect image of the same size; a convex mirror always gives a smaller, erect image; a concave mirror gives an enlarged erect image when the object is close, and an inverted image when it is far. A convex lens can magnify; a concave lens always makes things look smaller.
The incident ray makes 40° with the normal to the mirror. What angle does the reflected ray make with the mirror? (i) 40° (ii) 50° (iii) 45° (iv) 60°
Solution
The angle of reflection equals the angle of incidence, which is 40° from the normal. The normal is at 90° to the mirror, so the angle with the mirror is 90°−40°=50°.
Draw the reflected ray when (a) the ray falls along the normal, (b) the mirror is tilted but the ray still falls along the normal, (c) the mirror is tilted and the ray falls at 20° from the normal. What is the angle of reflection in each case?
Solution(a) and (b): the ray falls along the normal and goes straight back (angle of reflection 0°). (c) The ray at 20° to the normal leaves at 20° on the other side.
(a) The ray goes straight back along the normal. The angle of reflection is 0°.
(b) Again it returns along the normal: 0°. Tilting the mirror does not matter, because the ray is still along that mirror's normal.
(c) The reflected ray leaves on the other side of the normal at 20°.
When light falls along the normal on a mirror, which is true? (i) The angle of incidence is 90° (ii) The angle of incidence is 0° (iii) The angle of reflection is 90° (iv) No reflection takes place
Solution
The angle is measured from the normal. A ray along the normal makes 0° with it, and it reflects straight back, also at 0°.
A woman walks towards a large concave mirror in a museum. What does she see? (i) Her erect image keeps getting smaller. (ii) Her inverted image keeps getting smaller. (iii) Her inverted image keeps getting bigger and finally becomes erect and magnified. (iv) Her erect image keeps getting bigger.
Solution
When she is far from a concave mirror, her image is inverted. As she comes nearer, it grows larger. Once she is very close (within its focus), the image becomes erect and magnified, as in Fig. 10.26.
Hold a magnifying glass over some text and find where the letters look bigger. Then move it away. What do you notice? Which type of lens is a magnifying glass?
Solution
When the lens is close to the text, the letters look bigger and erect. As you move it further away, they get even bigger and blurry. Further still, the image becomes inverted and smaller.
A magnifying glass is a convex lens: it is thicker in the middle than at the edges.
Near the text it magnifies (erect); further away the image blurs and then turns upside down. It is a convex lens.
Match (i) concave mirror (ii) convex mirror (iii) convex lens (iv) concave lens with: (a) a spherical mirror whose reflecting surface curves inwards (b) always forms an erect, diminished image (c) an object behind it may look inverted at some distance (d) an object behind it always looks diminished.
Assertion: Convex mirrors are preferred for seeing the traffic behind us. Reason: Convex mirrors give a much larger field of view than plane mirrors.
Solution
Both are correct, and the reason explains the assertion. A convex mirror always gives an erect image and shows a wider area of the road behind (in a smaller image). That is why it is used as a rear-view mirror.
(i) Both are correct and the Reason is the correct explanation.
In Fig. 10.27, O is the object, M the mirror and I the image. In (a) the image is about the same size as the object; in (b) it is larger. Which statement is true?
Solution
(a) The image is erect and about the same size, which is what a plane mirror gives.
(b) The image is erect and magnified, which only a concave mirror can give (with the object close to it). A convex mirror would make it smaller.
(i) (a) is a plane mirror and (b) is a concave mirror.
A pencil is placed behind a glass tumbler, which is then half-filled with water. How does the pencil look through the water? Why?
Solution
The part of the pencil seen through the water looks thicker (magnified) and shifted, so the pencil seems broken at the water surface.
Light from the pencil bends (refracts) as it passes from water to glass to air. The rounded tumbler filled with water acts like a convex lens, so it magnifies the part of the pencil behind it.
It looks thicker and broken at the water level, because the curved glass of water acts like a convex lens and bends (refracts) the light.
