NCERT Class 8 Science Curiosity Chapter 02 The Invisible Living World Beyond Our Naked Eye PDF Download

Chapter 2: The Invisible Living World: Beyond Our Naked Eye

Probe and ponder

• Have you ever wondered what you might see if the invisible world around you became visible?
• How do you think your observation of this hidden world might change the way you think about size, complexity, or even what counts as 'living'?
• Have you thought how these tiny living beings interact with each other?
• Share your questions

The human eye can only see objects that are above a certain size. For a long time, many tiny things around us remained unknown. Long ago, people discovered that a curved piece of glass could make small things look bigger. The piece of glass was shaped like a lentil seed - thick in the middle and thin at the edge - hence they called it a lens. Over time, lenses were improved to become more powerful. Each new tool, from simple magnifying glasses to microscopes, helped humans see what their eyes could not. The invention of the microscope opened a fascinating hidden world filled with tiny living creatures. We will explore some of these life forms in this chapter.

You have already learnt about the amazing variety of living beings. Just look around - there are so many beautiful plants and animals! They are of all shapes, sizes, and colours. Some living beings are tiny, while others are really big. They differ not only in their structure but also in many other features. All these living beings, whether plants or animals, are called organisms. Have you ever noticed the smallest organism around you that is visible to the naked eye? Think about it - how small a thing can your eyes actually see?

You might have seen some people using reading glasses. How does it help them see better? Or what happens when we use a magnifying glass to observe something?

Activity 2.1: Let us observe

• Take a round-bottom flask made up of glass as shown in Fig. 2.1. Fill it with water.
• Close the mouth of the flask with a cork.
• Now, place the flask on an open book and look at the letters through it.

Do you notice something interesting? The letters appear larger when seen through the flask! This happens because the flask filled with water acts like a magnifying glass. Now, use a real magnifying glass to look at small organisms, like an ant. Were you able to see the details of its body more clearly?

For a long time, people were curious to explore the tiny organisms around them, but they could not see them with their naked eyes. So, how did we finally discover this invisible world? Do you know which scientific discovery helped us see the tiny world for the first time?

[Figure 2.1: Round-bottom flask, See in your textbook]

Teacher's Note

When you do Activity 2.1, remember that the water-filled flask works like a lens because of its curved surface. The curved glass bends light rays in a particular way. This is the same principle behind all magnifying glasses and microscopes - they use curved glass or lenses to bend light so that small objects appear much larger.

Ever heard of ...

In 1665, a scientist named Robert Hooke published a book called Micrographia. He was a careful observer, and a skilled artist. In this book, he showed detailed drawings of tiny things that people had never seen before - things he saw using a tool we now call a microscope.

His microscope made things look 200 to 300 times bigger, than what one could see with the unaided eye. One day, he looked at a thin slice of cork and saw it was made of many small, empty spaces. These compartments reminded him of a honeycomb. He drew what he saw and called each small space a cell. This was the first time the word cell was used in science to describe the basic unit of life.

Around the same time in 1660s, Antonie van Leeuwenhoek, a Dutch scientist, made better lenses that allowed him to build more useful microscopes. He was the first person to clearly see and describe tiny living things like bacteria and blood cells. Because of this, he is known as the Father of Microbiology.

[Figure 2.2: (a) Micrographia book; (b) Microscope of Robert Hooke; (c) Cork cells as published in the Micrographia, See in your textbook]

Teacher's Note

Notice that Robert Hooke named the tiny compartments he saw "cells" because they looked like the small rooms (cells) in a monastery. This is just a name he chose based on appearance, but it stuck and we still use the term "cell" today for all the tiny structures that make up living things. The name does not mean that cells are empty - modern microscopes show us they are actually full of structures and materials.

2.1 What Is a Cell?

All living beings are made up of cells. You might wonder what cells actually look like. Let us take a closer look at the basic structure of a cell using a microscope.

Key Points

• For thousands of years, people could only see objects large enough for the human eye to detect. The invention of the microscope opened up an invisible world of tiny living things that were never seen before.
• A lens is a curved piece of glass, thick in the middle and thin at the edges, that makes objects appear larger when you look through it. Water-filled flasks and magnifying glasses work on the same principle.
• Robert Hooke was the first scientist to see and name cells when he observed cork tissue under a microscope in 1665. Antonie van Leeuwenhoek later became known as the Father of Microbiology by being the first to see bacteria and blood cells.
• All living beings, whether plants or animals, are made up of cells, which are the basic units of life.

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