NCERT Solutions Curiosity Chapter 10 .4 What Is a Lens? — Activity 10.10: Let us investigate
Book page 16410 Updated on2026-09-05
Q1.
Now let the multiple parallel beams of light fall upon the thin glass plate, convex lens, and concave lens one by one as shown in Fig. 10.18. Does the parallel beam of light pass through as it is in all three cases?
Answer
No — only the flat plate leaves the beam alone.
Thin transparent glass plate — the beams pass through as they are and are still parallel on the far side.
Convex lens — the beams are brought together; they converge and meet at a point.
Concave lens — the beams are spread apart; they diverge.
Light bends where it enters the glass and again where it leaves. A convex lens is thicker in the middle, so it bends the outer rays inwards and they meet at F. A concave lens is thinner in the middle, so it bends the rays outwards and they only appear to come from F. The point F is called the focus of the lens.
Why the flat plate changes nothing: its two faces are parallel, so a ray is bent one way going in and bent back by exactly the same amount coming out. It emerges travelling in its original direction — displaced very slightly sideways, but not turned. In a lens the faces are curved and therefore not parallel, so the two bends do not cancel, and how much they fail to cancel depends on how far the ray is from the centre. That is what turns a parallel beam into a converging or a diverging one.
Q2.
Record and analyse your observations.
Answer
What the light passed through
What the emerging beams did
Name
Thin transparent glass plate
Stayed parallel — passed through as it is
—
Convex lens
Came together at a point
Converging lens
Concave lens
Spread out
Diverging lens
Analysis. A convex lens is also called a converging lens and a concave lens a diverging lens. Comparing with Activity 10.6, the convex lens does to transmitted light what the concave mirror does to reflected light, and the concave lens does what the convex mirror does.
Tip: keep the paper sheets on both books at the same level and the room fairly dark. The converging point of the convex lens is easy to miss if the beams are faint.