Activity 10.8 — What Is a Lens? — "Light: Mirrors and Lenses"

1. 14 marksCuriosity Grade 8, Chapter 10, page 162

   Describe Activity 10.8 step by step, and state its two key observations.

   Hint. A drop of water, sitting on an oiled surface, over some printed text.

   The activity. Rub a few drops of oil (or wax) onto a flat strip of glass or plastic to leave a very thin coating. Place a small drop of water on the oiled spot using a dropper — the oil helps the water hold a round drop shape rather than spreading flat. Place the strip over some printed text so the text sits directly under the drop, then look down through the water drop at the text.

   Two observations, in the chapter's own words:

   The surface of the water drop is curved outside. The letters under the water drop look different — they might appear larger than the letters nearby!

   Observation	What it shows
   The drop's surface is curved outward	It is not a flat sheet of water
   Letters beneath it look larger	The curved surface changes the apparent size of what is seen through it

   ✦ The oil or wax is doing quiet but essential work. Water placed directly on bare, clean glass tends to spread into a thin, nearly flat film rather than holding a rounded shape; since a flat film would show no magnifying effect at all, the very thing the activity is trying to demonstrate depends on that oil coating first being there to keep the drop round.

2. 23 marksCuriosity Grade 8, Chapter 10, page 162

   Why does the chapter call the water drop a lens, and how does a magnifying glass relate to it?

   Hint. The drop and the magnifying glass do the same job by the same means.

   The curved surface of the water drop made the size of the text look different. This curved drop of water is acting like a simple lens. Have you seen a magnifying glass ... It is also a lens that helps in reading small print by making the letters appear bigger.

   Why 'lens' is the right word. A lens is defined by having curved surfaces that change how objects appear when seen through it — not by being made of any particular material. The water drop has a curved outer surface and makes text underneath it look larger, so it qualifies as a lens even though it is only water held in shape by a thin oil film, with no glass involved at all.

   ✦ The magnifying glass is the same idea, made permanent and reliable. Since a drop of water eventually evaporates and has to be reshaped each time, a solid, ground piece of glass or plastic with the same curved shape gives the same magnifying effect without needing to be recreated — which is exactly why the chapter places the two side by side, one as the everyday demonstration and the other as the everyday object built on the very same principle.

3. 33 marksCuriosity Grade 8, Chapter 10, page 162

   Define a lens, and define convex lens and concave lens by their shape.

   Hint. The two definitions turn entirely on where the material is thickest.

   A lens is a piece of transparent material, usually made of glass or plastic, which has curved surfaces. Like mirrors, lenses can also be convex or concave.

   A lens which is thicker at the middle as compared to the edges is called a convex lens.

   A lens which is thicker at the edges as compared to the middle is called a concave lens. (page 163)

   	Convex lens	Concave lens
   Thickest at	Middle	Edges
   Thinnest at	Edges	Middle

   ✦ Both definitions are entirely about the pattern of thickness, not about which way any single surface bends. Since 'thicker at the middle' and 'thicker at the edges' are opposite descriptions, remembering just one of the two pairs (say, that a convex lens bulges at its centre) is enough to derive the other by elimination — a habit worth having when a question gives you a cross-section to identify rather than the word itself.

4. 43 marksCuriosity Grade 8, Chapter 10, page 163

   The chapter says 'unlike mirrors, lenses allow light to pass through them, and we see things through a lens rather than in a lens.' Explain the distinction and why it matters.

   Hint. One device sends light back the way it came; the other lets it continue onward.

   Unlike mirrors, lenses allow light to pass through them, and we see things through a lens rather than in a lens.

   Why the distinction is real and not just wording. A mirror is opaque and coated to reflect — light striking it bounces back toward the same side it came from, which is why you look into a mirror and see an image seemingly located behind its surface. A lens is transparent — light passes all the way through and continues out the other side, which is why you look through a lens at whatever is on the far side of it, exactly as you looked through the water drop at the text lying beneath it in Activity 10.8.

   ✦ Getting the preposition right is not a small point, since the two devices genuinely work by opposite mechanisms. Because a mirror's image sits with the object on the same side as the viewer while a lens's view has the object on the far side, a diagram or description that says you see something 'in' a lens, or 'through' a mirror, has described the wrong physical arrangement — not just used a careless word.

5. 53 marksCuriosity Grade 8, Chapter 10, page 162

   In Activity 10.8, would you expect the same magnifying effect if the water drop were flattened rather than rounded? Explain using what the chapter says about the drop's surface.

   Hint. The curve, not the water itself, is doing the magnifying.

   No — a flattened drop would show little or no magnifying effect.

   The chapter attributes the effect specifically to the drop's shape: the curved surface of the water drop made the size of the text look different. If the water were spread into a flat film instead of held as a rounded drop, its top surface would no longer be curved outward — and a flat sheet of a transparent material behaves like the plain glass window pane the chapter mentions at the start of section 10.4, through which all objects look the same size and shape.

   ✦ This is precisely why the oil coating matters — it is what keeps the drop rounded instead of spreading flat on the glass. Since a flat surface cannot bend the view of what lies beneath it the way a curved one does, removing the curve (by flattening the drop) removes the very feature the activity exists to demonstrate, which is the clearest way to see that it is the shape, not merely the presence of water, that turns the drop into a lens.

6. 63 marksCuriosity Grade 8, Chapter 10, pages 162-163

   A round drop of oil (rather than water) is placed on a glass strip over some text. Would you expect it to behave as a lens too? Explain your reasoning, being careful about what is genuinely established and what is a reasonable extension.

   Hint. The chapter's argument rests on shape, and oil can hold a round shape as well as water can.

   A reasonable expectation: yes, a rounded drop of oil should also act as a simple lens, for the same reason the water drop does.

   The chapter's own reasoning is that a curved transparent surface changes the apparent size of what lies beneath it — nothing in that argument is unique to water as a substance. Oil is also transparent, and a rounded drop of oil has the same kind of curved outer surface as a rounded drop of water.

   ✦ Be precise about what is being claimed here. The chapter itself only performs the activity with water, so a statement about oil is an extension of the same reasoning, not a fact the book states directly — and it should be presented that way rather than as something verified in the text. Since the argument depends only on the shape of the surface and not on which transparent liquid holds that shape, the extension is a sound one to offer, provided it is clearly marked as reasoning from the chapter's principle rather than as a quotation of a result the chapter reports.

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.