In-text — Lenses, Microscopes and the Discovery of the Cell — "The Invisible Living World: Beyond Our Naked Eye"

1. 13 marksCuriosity Grade 8, Chapter 2, Activity 2.1, page 9

   In Activity 2.1 a round-bottom flask is filled with water, corked, and placed on an open book. What do you observe, and why does it happen? Where does the word 'lens' come from?

   Hint. Look at the shape of the water in the flask from the side.

   What you observe. The letters seen through the water-filled flask appear larger.

   Why. The flask full of water is thick in the middle and thinner towards the edges, which is exactly the shape of a magnifying glass. Water in that shape bends light in the same way curved glass does, so the flask acts as a lens and the print beneath is magnified.

   Where the word comes from. Long ago people found that a curved piece of glass made small things look bigger. The piece of glass was shaped like a lentil seed — thick in the middle, thin at the edge — so they called it a lens.

   Why the activity is placed here. It shows that the tool which opened the invisible world is not exotic: the same shape that magnifies a letter, made better and more powerful over time, became the microscope. Following it up by looking at an ant through a real magnifying glass makes the point — details of its body appear that the naked eye cannot resolve.

   ✦ Answer: the letters look larger, because the water-filled flask is thick in the middle and thin at the edges and so acts as a magnifying lens; the word lens comes from the lentil seed, whose shape the first curved glasses resembled.

2. 24 marksCuriosity Grade 8, Chapter 2, Ever heard of…, page 10

   Who was Robert Hooke, what did he publish, and how did the word 'cell' enter science?

   Hint. He was looking at cork, and it reminded him of something built by bees.

   Who and when. Robert Hooke was a careful observer and a skilled artist. In 1665 he published a book called Micrographia, containing detailed drawings of tiny things nobody had seen before, made using a tool we now call a microscope.

   How powerful his instrument was. It made things look 200 to 300 times bigger than the unaided eye could see.

   The observation. One day he looked at a thin slice of cork and saw it was made of many small, empty spaces. The compartments reminded him of a honeycomb, so he drew what he saw and called each small space a cell.

   Why it mattered. This was the first time the word cell was used in science to describe the basic unit of life. Note what he was actually looking at, though: cork is dead plant material, so the 'empty spaces' were the walls left behind after the living contents had gone. The name stuck even though the thing he named was a hollow.

   ✦ Answer: Robert Hooke, who published Micrographia in 1665; looking at a thin slice of cork through a microscope magnifying 200-300×, he saw compartments resembling a honeycomb and named each one a cell — the first scientific use of the word.

3. 33 marksCuriosity Grade 8, Chapter 2, Ever heard of…, page 10

   Who was Antonie van Leeuwenhoek and why is he called the Father of Microbiology?

   Hint. Hooke named the cell; Leeuwenhoek saw things that were alive.

   Who and when. Antonie van Leeuwenhoek was a Dutch scientist working in the 1660s, around the same time as Hooke.

   What he did differently. He made better lenses, which let him build more useful microscopes than those available before.

   What he saw. He was the first person to clearly see and describe tiny living things, including bacteria and blood cells.

   Why the title. Hooke described the compartments of dead cork; Leeuwenhoek described living microorganisms, opening the study of microbes as a subject. That is why he is known as the Father of Microbiology.

   The pattern worth noticing. Both discoveries followed an improvement in the instrument, not a new argument. Better lenses meant more magnification, which meant a world that had always been there became visible.

   ✦ Answer: a Dutch scientist of the 1660s who made better lenses and better microscopes, and was the first to clearly see and describe bacteria and blood cells — which is why he is called the Father of Microbiology.

4. 43 marksCuriosity Grade 8, Chapter 2, pages 10, 17 and 24

   Collect the magnification figures given in this chapter and explain what each level lets you see.

   Hint. The chapter gives three different numbers in three different places.

   The three figures the chapter gives:

   Instrument	Magnification	What it reveals
   Hooke's microscope (1665)	200-300×	The compartments of cork — the first 'cells'
   An ordinary microscope	100-400×	Microorganisms: protozoa, algae, fungi, bacteria
   An electron microscope	about 10,00,000×	Structures inside the cell itself

   Reading the ladder. Each jump does not just make things bigger, it makes a different kind of thing visible. At a few hundred times you can see that a cell exists and that microbes are everywhere; only at around a million times can you make out the components inside a cell — which is why the chapter says it has shown only a few basic cell structures and leaves the rest to higher classes.

   A practical note from the chapter. Scientists have created a low-cost foldable paper microscope. It does not give the detail of a high-powered laboratory instrument, but it makes the microscopic world accessible to many more people — and it is enough for the onion-peel activity.

   ✦ Answer: 200-300× for Hooke's microscope, 100-400× for an ordinary microscope (enough to see microorganisms), and about 10,00,000× for an electron microscope, which is what it takes to see structures inside a cell.

Solutions written by the tuition.in editorial team and checked against NCERT Curiosity - Textbook of Science for Grade 8 (hecu102.pdf), Chapter 2 'The Invisible Living World: Beyond Our Naked Eye', pages 8-27, Reprint 2026-27. HAND-WRITTEN throughout. Page count verified against the printed contents page (hecu1ps.pdf p.18): 20 pages, 8-27, matching exactly. Covers the 3 'Probe and ponder' prompts (p.8), all 9 activities (2.1 to 2.9), every in-text section (2.1 to 2.5), the 9 numbered 'Keep the curiosity alive' exercises (pp.25-26) and the 4 'Discover, design, and debate' projects (p.27). EXERCISE Q1 VENN DIAGRAM READ OFF A 500-DPI RENDER of p.25: three circles labelled 'Only in Animal Cell' (top), 'Only in Bacterial Cell' (lower left) and 'Only in Plant Cell' (lower right), with an arrow labelling the central three-way overlap 'Common to all three cells'. Each of those four labelled regions carries two ruled lines; the three PAIRWISE overlap regions carry none. Working from the book's own statements (p.13 'the cell wall in the plant cell provides rigidity'; p.24 'bacteria do not have a well-defined nucleus... instead they have a nucleoid'; and the Snapshots line 'Plant, fungal, and bacterial cells have an extra covering, called a cell wall'), the correct placement is: cytoplasm and cell membrane in the three-way overlap (exactly two items, matching the two ruled lines); chloroplast in plant-only; nucleoid in bacterial-only; NUCLEUS in the animal-plant pairwise overlap (plant cells have nuclei, bacteria do not); CELL WALL in the plant-bacterial pairwise overlap (not plant-only, since the book says bacterial and fungal cells have walls too); and NO listed part is unique to the animal cell. The solution states both traps explicitly. FACTUAL FIGURES TAKEN VERBATIM FROM THE BOOK: Hooke's Micrographia 1665, microscope 200-300x, cork, honeycomb, first use of 'cell'; Leeuwenhoek 1660s, Father of Microbiology; ordinary microscope 100-400x; electron microscope about 10,00,000x; ostrich egg yolk 130-170 mm as the largest known cell; Ananda Mohan Chakrabarty 1938-2020, oil-degrading bacterium 1971, patent 1980; biogas mainly carbon dioxide with a high proportion of methane; one of India's oldest biogas plants late 1850s; microalgae produce more than half of Earth's oxygen; Spirulina over 60 per cent protein by body weight and a source of vitamin B12, harvested after 3-6 weeks. The MNRE biogas-programme project answer deliberately does NOT assert current programme details - it names the programmes to search for and tells the student to verify against the current official MNRE page, since names and subsidy rules change.. Questions are referenced from the NCERT textbook for identification.