Activity 10.9 — Image Characteristics Through Lenses — "Light: Mirrors and Lenses"

1. 14 marksCuriosity Grade 8, Chapter 10, page 163

   Describe Activity 10.9's set-up and the questions it asks you to observe.

   Hint. The same pattern as Activity 10.3, now with lenses instead of mirrors.

   The set-up. Fix a convex lens upright in a holder. Place a small object behind it, and look at the object through the lens from the other side, recording your observations. Slowly move the object farther from the lens, watching how the image changes. Then repeat the whole procedure with a concave lens.

   How does the distance of the object from the convex lens affect how it looks? ... Analyse your observations recorded in your notebook and compare the images seen through both lenses.

   ✦ This activity is built on exactly the same logic as Activity 10.3 with the mirrors — vary one thing (the object's distance) and hold everything else fixed, then compare two devices against each other. Since the mirror activity had already shown that distance matters for one kind of image-forming device, the chapter tests whether the same is true for lenses using the identical method, which makes the two activities easy to compare once both are complete.

2. 24 marksCuriosity Grade 8, Chapter 10, page 163

   State the full result of Activity 10.9 for the convex lens.

   Hint. As with the concave mirror, both orientation and size change as the object moves.

   When an object is placed behind a convex lens at a small distance from it and seen through the lens, the object appears erect and enlarged in size. As the distance between the object and the convex lens increases, the object appears inverted. It is initially enlarged in size and then diminishes in size.

   Object distance	Orientation	Size
   Small	Erect	Enlarged
   Larger	Inverted	Starts enlarged, then diminishes

   ✦ This is line-for-line the same pattern Activity 10.3 found for the concave mirror. Since both a concave mirror and a convex lens converge light (as later activities confirm), it should not be surprising that they share this signature: erect and magnified up close, inverted farther away, with the size first growing and then shrinking as the object recedes. Recognising that the two results are the same pattern, arrived at from two different devices, is worth more than memorising them as two unconnected facts.

3. 33 marksCuriosity Grade 8, Chapter 10, page 163

   State the full result of Activity 10.9 for the concave lens, and contrast it with the convex lens.

   Hint. One of the two lenses never inverts the image, exactly like one of the two mirrors.

   An object placed behind a concave lens and seen through the lens, always appears erect and diminished in size. Its size changes, as its distance from the lens increases.

   	Convex lens	Concave lens
   Orientation	Erect close, inverted farther	Always erect
   Size	Enlarged close, then diminishing	Always diminished; size still changes with distance

   ✦ Note carefully what the chapter does and does not say about the concave lens's size. It states plainly that the image is always diminished, and separately that its size changes, as its distance from the lens increases — so the diminished image is not a single fixed size at every distance, only consistently smaller than the object, however far away that object is placed. This mirrors precisely what Activity 10.3 found for the convex mirror: always erect and diminished, yet still varying somewhat in exact size.

4. 43 marksCuriosity Grade 8, Chapter 10, pages 163-164

   Compare Activity 10.9's convex-lens result with Activity 10.8's water drop. Are they consistent with one another?

   Hint. The water drop only demonstrated one of the two behaviours a convex lens can show.

   Yes, they are consistent — the water drop showed only the close-up half of the full convex-lens behaviour.

   In Activity 10.8, the text was directly under the water drop — about as close as an object can be to a lens — and the letters appeared larger. Activity 10.9 reports exactly this for a convex lens at a small distance: the object appears erect and enlarged in size.

   ✦ The water drop activity never moved the text farther away, so it could not have shown the second half of the pattern. Since Activity 10.9 goes on to move the object progressively farther from the lens, it reveals what the water-drop set-up was never designed to show — that the same convex-shaped lens, at a greater distance, gives an inverted image that first grows and then shrinks. The two activities are not in tension; the second simply extends the first into territory the first did not explore, using the identical curved-surface principle throughout.

5. 53 marksCuriosity Grade 8, Chapter 10, pages 163-164

   A student looks at a small object through a concave lens from close up, then slowly moves the object farther away. What should the student expect to observe, and what should NOT change?

   Hint. One property in this activity's results is fixed regardless of distance; identify it before predicting what does change.

   Expected observation: the image stays erect and diminished throughout, while its exact size varies somewhat as the distance increases.

   An object placed behind a concave lens and seen through the lens, always appears erect and diminished in size. Its size changes, as its distance from the lens increases.

   What should NOT change: the orientation — the concave lens never inverts the image, at any distance the student tries, unlike the convex lens which does invert its image once the object is moved far enough away.

   ✦ What SHOULD change, and does so continuously rather than in a sudden jump: the exact size of the diminished image, which the chapter states shifts with distance even while staying smaller than the object throughout. Since this is the opposite kind of prediction from the convex-lens case, where a genuine change of kind (erect to inverted) occurs at some distance, a student testing this activity should specifically watch for the absence of any such flip, and treat that absence itself as a confirmed and expected result rather than as a sign the experiment has failed.

6. 63 marksCuriosity Grade 8, Chapter 10, page 163

   Why does the chapter test the convex and concave lens with the same object at the same set of distances, rather than testing them separately with whatever distances happen to be convenient?

   Hint. The comparison at the end of the activity depends on this.

   Because the whole point of the activity is to compare the two lenses against each other, and a fair comparison needs matched conditions.

   Now repeat the steps using a concave lens ... Analyse your observations recorded in your notebook and compare the images seen through both lenses. What conclusions do you draw?

   If the convex lens were tested at one set of distances and the concave lens at an entirely different, unrelated set, any difference observed between them could be blamed on the different distances rather than on a genuine difference between the lenses themselves. Since the activity explicitly asks you to compare the two afterward, using the same object moved through the same progression of distances for both lenses is what makes that final comparison meaningful.

   ✦ This is the same design principle used in Activity 10.3 with the two mirrors, and in Activity 9.1's careful control of stirring — whenever two devices, substances or conditions are being compared, everything except the one variable under test must be held as close to identical as possible, or the comparison proves nothing at all.

Solutions written by the tuition.in editorial team and checked against NCERT Curiosity — Textbook of Science for Grade 8, Chapter 10 (hecu110.pdf), Reprint 2026-27, pages 152-169. Questions are referenced from the NCERT textbook for identification.