WJEC/Eduqas A Level Biology A Student Guide to Practical Work

Grade boost Remember how to de ne magni cation: magni cation = image size object size Pointer Calculation of magni cation of image size with a ×40 objective and ×10 eyepiece: magni cation = objective × eyepiece = 40 × 10 = 400 Grade boost Remember how to de ne resolution: resolution = smallest distance between two points that can be separately distinguished. For a light microscope it is about 2 µm and for an electron microscope it is about 2 nm, although the microscopes in your school may not resolve quite such small distances. Magnification The purpose of a microscope is to magnify small objects. In everyday speech, the word magnification means how much bigger an image is than the actual object. It’s the same in microscopy, except that you can put numbers on it. The object on the slide is first magnified by the objective lens. If all you had was an objective then the image would be four times, or ten times or forty times the object size, depending on which lens you use. The objective lens also determines the resolution of the image. The higher power the objective, the greater the resolution. This means that you can distinguish two points that are closer together, so a higher power objective allows you to see more detail. Eyepiece lenses generally have ×10 or ×15 etched on the side so you can see the power of your eyepiece. The image from the objective lens is magnified by the eyepiece, so the total magnification of the image you see is the product of the two separate magnifications. The eyepiece magnification does not affect the resolution of the image. It just makes it bigger. We will consider magnification again when we look at biological drawings. Calibrating the microscope Calibrating a microscope lets you measure the actual size of structures on the slide. You need an eyepiece graticule, which is the ruler you can see. It is inside the eyepiece. It looks like this: It looks the same through every objective because it is in the eyepiece. So with objectives of different magnifications, the divisions on the graticule represent different lengths. The higher the power of the objective, the smaller length each division of the graticule represents. So calibrating has to be done for each objective. 0 1 2 3 4 5 6 7 8 9 10 8 Biology: A Student Guide to Practical Work