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NCERT Solutions for Class 9th Science Chapter 2 In-text Questions — How to Study Cells?

Book page 9 Updated on2026-09-08

Q1.
What do we call the ability of the human eye to see two very close objects as separate and distinct?
Answer

It is called the resolution of the eye, and the smallest separation it can still manage is the limit of resolution.

near point of the human eye = 25 cm
limit of resolution of the human eye = 0.1 mm
0.1 mm = 0.1 × 1000 µm = 100 µm

Two dots on paper 0.1 mm apart, held 25 cm from the eye, are just seen as two. Bring them closer than that and they merge into a single point.

Why it happens: resolution is not the same as magnification. Magnification only makes the image bigger; resolution decides whether the two points stay distinguishable when it is enlarged. Blowing up a blurred photograph gives a bigger blur, not more detail. That is why scientists worked on all three features of a microscope — resolution, contrast and magnification — and not on magnification alone.
Check it yourself: a typical plant or animal cell is 10–100 µm across. The eye's limit is 100 µm, so only the very largest cells could ever be at the edge of visibility — and even then with no internal detail.
Q2.
How have cell biologists studied the structure and function of cells that are much smaller than the limit of resolution of the human eye?
Answer

By using lenses — a convex lens, or better, a combination of an objective lens and an eyepiece, which is exactly what a microscope is.

  • The objective lens forms an enlarged image of the specimen; the eyepiece enlarges that image again. The two magnifications multiply.
  • A school light microscope carries several objectives (10X, 40X) so the same slide can be studied at different magnifications under visible light.
  • Beyond the reach of light, scientists use an electron microscope, which uses a beam of electrons instead of light and resolves structure at the nanometre scale (1 nm = one-billionth of a metre). Fig. 2.4 in the book is a scanning electron micrograph of stomata on the lower surface of a Colocasia leaf.
total magnification = magnification of eyepiece × magnification of objective
= 10X × 10X = 100X
so an onion cell of 200 µm appears 200 µm × 100 = 20 000 µm = 20 mm across
Why it happens: a lens bends light so that rays leaving a tiny object reach the eye as if they came from a much larger one, at a much larger visual angle. Once that angle is bigger than the eye's own limit, the detail becomes visible. Historically this is exactly how the cell was discovered — Robert Hooke built his own microscope of about 200–300X in 1665, looked at a thin slice of cork and saw small box-like compartments which he named ‘cells’.
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