IGCSE Biology / Life processes

The light microscope and magnification

Edexcel International GCSE Biology (4BI1), chapter 1: Life processes, specification point 2.1, 2.2. Free lesson.

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A light microscope lets you view small specimens, and its magnification tells you how much larger the image is than the real specimen. You need to know how to use the microscope safely and calculate magnification, image size or actual size.

The parts of a light microscope

The eyepiece lens is the lens you look through. It is usually ×10. The objective lenses are attached to a rotating turret and are typically ×4, ×10 and ×40. Total magnification is found by multiplying the eyepiece magnification by the objective magnification. The stage supports the slide, and the clips hold it still. The coarse focus moves the stage a large amount. Use it only on low power. The fine focus moves the stage a tiny amount. Use it to sharpen the image on high power. The light source shines light up through the specimen.

How to set up and focus the microscope

Start on the lowest power because it gives you a wider field of view, making it easier to find the specimen. Focus in the right order so the objective does not hit the slide. Draw what you see with sharp, unbroken pencil lines and no shading. Aim for a sharp image in the middle of the field of view, with the specimen filling a good part of it.

How to calculate magnification

Magnification compares the size of the image with the size of the real specimen. Image size is measured on the drawing or image, while actual size is the real size. The formula can be rearranged to find any of the three values. Magnification = image size ÷ actual size Image size = magnification × actual size Actual size = image size ÷ magnification Magnification has no units. If image size is plotted against actual size at a fixed magnification, the graph is a straight line through the origin. Its gradient is the magnification.

Convert units before calculating

Image size and actual size must be in the same units before you divide. One millimetre (mm) is 1000 micrometres (µm), so convert first and calculate second. A cell drawn 60 mm wide at ×400 has an actual size of 60 ÷ 400 = 0.15 mm. 0.15 mm × 1000 = 150 µm.

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