NCERT Solutions Curiosity Chapter 10 .4 What Is a Lens? — Activity 10.9: Let us experiment
Book page 16310 Updated on2026-09-05
Q1.
Look at the object through the lens from the other side of the lens (Fig. 10.17a) and note your observations in your notebook.
Answer
With the object a small distance behind the convex lens, it appears erect and enlarged — clearly bigger than it looks with the naked eye, and the right way up.
Why it is enlarged at this distance: the convex lens bends the rays leaving the object inwards, but over such a short distance they are still spreading apart when they reach your eye. The eye traces them back along straight lines to a point farther away and wider apart than the object itself — so the object looks bigger, and the right way up.
Tip: record the actual distance in centimetres alongside every observation. Without the distance, the observation cannot be compared with anyone else's.
Q2.
Now slowly move the object farther from the lens and keep observing how the image changes (Fig. 10.17b). How does the distance of the object from the convex lens affect how it looks?
Answer
The distance decides everything:
Small distance — erect and enlarged.
At one particular distance — the view blurs and no clear image can be seen.
Beyond that distance — the object appears inverted, at first still enlarged.
Farther still — it stays inverted and now steadily diminishes in size.
Why the flip happens: the convex lens always converges the light. When the object is close, the rays are spreading so strongly that the converging is not enough to bring them together before your eye — the view stays erect. Past a certain object distance the rays do meet and cross over on their way to you, and once light has crossed, up and down are interchanged. That is the point at which the picture turns over.
Q3.
Now repeat the steps using a concave lens (Fig. 10.17c).
Answer
With a concave lens the answer is the same at every distance: the object appears erect and diminished. As you move it farther from the lens it just keeps getting smaller. It never flips over and it is never enlarged.
Why nothing else can happen: a concave lens is thinner in the middle, so it always makes the rays passing through it spread apart more than they were already spreading. Rays that are pushed apart can never meet and cross, and without a crossing there can be no inverted view and no enlargement. This is the lens counterpart of the convex mirror.
Q4.
Analyse your observations recorded in your notebook and compare the images seen through both lenses. What conclusions do you draw?
Answer
Convex lens
Concave lens
Shape
Thicker at the middle
Thicker at the edges
Object at a small distance
Erect, enlarged
Erect, diminished
Object at a large distance
Inverted, then diminishing
Erect, diminishing
Effect on a parallel beam
Converges it
Diverges it
Behaves like which mirror?
Concave mirror
Convex mirror
Conclusion. The image formed by a convex lens can be enlarged, diminished or of the same size, and erect or inverted, depending on the distance of the object from the lens. The image formed by a concave lens is always erect and diminished.
The single idea behind the whole table: converging optics (concave mirror, convex lens) can make rays cross, so they can produce inverted and enlarged images. Diverging optics (convex mirror, concave lens) never let rays cross, so they are locked into erect, diminished images.