IGCSE Biology 0610 - 2.2 Size of Specimens

This section focuses on understanding and calculating the size of biological specimens using millimeters as the unit of measurement. It also covers the calculation of magnification using microscopes.

1. Measuring Size with a Microscope

When examining biological specimens under a microscope, it's crucial to accurately measure their size. The eyepiece graticule and the stage micrometer are used for this purpose.

1.1 Eyepiece Graticule

The eyepiece graticule is a scale etched onto the eyepiece of the microscope. It allows you to estimate the size of a specimen viewed under a particular magnification.

1.2 Stage Micrometer

A stage micrometer is a transparent slide with a precisely etched scale. It is used to calibrate the eyepiece graticule and determine the actual size of the divisions on the graticule.

Example: If the stage micrometer has 100 divisions and the eyepiece graticule shows that 10 divisions on the graticule correspond to 1 mm on the stage micrometer, then 1 division on the eyepiece graticule represents 0.1 mm.

2. Calculating Magnification

Magnification is the process of making a specimen appear larger than its actual size. It is determined by the combination of the eyepiece and the objective lens.

2.1 Total Magnification

The total magnification of a microscope is calculated by multiplying the magnification of the eyepiece by the magnification of the objective lens.

$$ \text{Total Magnification} = \text{Eyepiece Magnification} \times \text{Objective Magnification} $$

Example: If the eyepiece magnification is 10x and the objective lens magnification is 40x, the total magnification is 400x.

2.2 Measuring Specimen Size using Magnification

To find the actual size of a specimen, you need to measure its size under the microscope using the eyepiece graticule and the known magnification.

Formula:

$$ \text{Actual Size} = \text{Size on Graticule} \times \text{Magnification} $$

Example: If a specimen appears to be 5 divisions long on the eyepiece graticule, the eyepiece graticule represents 0.1 mm per division, and the total magnification is 400x, then the actual size of the specimen is:

$$ \text{Actual Size} = 5 \times 0.1 \text{ mm} \times 400 = 200 \text{ mm} $$

3. Table Summary

Concept	Description
Eyepiece Graticule	Scale on the eyepiece used for estimating specimen size.
Stage Micrometer	Precise scale used to calibrate the eyepiece graticule.
Magnification	The process of making a specimen appear larger.
Total Magnification	The product of the eyepiece and objective lens magnifications.
Formula for Actual Size	Actual Size = Size on Graticule × Magnification

Suggested diagram: A diagram showing a microscope with labels for the eyepiece, objective lens, eyepiece graticule, and stage micrometer.