Magnifying Instruments and Biological Drawings
Senior 1 · Biology
Magnifying Instruments and Biological Drawings
Lesson 1 of 4
20 min
Pick up a dry bean seed. With your eyes alone you can see its colour, its kidney shape and a pale scar on one edge. But near that scar is a tiny hole through which water enters the seed, and on a bean leaf there are fine hairs so small that the leaf just looks smooth. The unaided human eye cannot pick out details smaller than about 0.1 mm, while most living things are built from parts far smaller than that.
To study them, biologists use magnifying instruments. The simplest is the hand lens.
Definition
Magnifying instrument: a device with one or more lenses that makes an object look larger than it is. Examples: the hand lens and the light microscope.
Hand lens (magnifying glass): a single convex lens in a frame, usually magnifying 5 to 20 times.
Convex lens: a lens that is thicker in the middle than at the edges.
Magnification: how many times larger the image looks than the real object. It is written with a multiplication sign: "×10" means the image looks ten times larger than the object.
Figure 1: A hand lens. A convex lens bends light so that the object looks larger than it really is.
Figure 2: Using a hand lens. Keep the lens near your eye and bring the specimen towards it, not the other way round.
Warning
Never look at the Sun through any lens. A convex lens concentrates sunlight and can burn the back of your eye, blinding you permanently; it can also set dry grass or paper on fire. Also avoid the common mistake of holding the lens halfway between eye and specimen and moving it about: you see a small, blurred image. Keep it close to your eye.
| Naked eye | Hand lens | Light microscope | |
|---|---|---|---|
| Usual magnification | ×1 | ×5 to ×20 | ×40 to ×1000 |
| Where it is used | anywhere | in the field and the classroom | in the laboratory |
| Specimen | any size | whole, even alive and moving | thin slices or smears on a glass slide |
| Can show | a whole seed, flower or insect | seed scar, leaf hairs, insect legs | cells and single-celled organisms |
Worked example 1: choosing the instrument
Which instrument would you use to study (a) the shape of a coffee leaf, (b) the tiny hole beside the scar on a bean seed, (c) a cheek cell about 0.06 mm across?
Answer: (a) naked eye, (b) hand lens, (c) light microscope. Choose the simplest instrument that shows the detail you need.
Worked example 2: what does "×10" do?
A bristle on a housefly's leg is 0.5 mm long. How long does it appear through a ×10 hand lens?
Answer: the bristle looks 5 mm long: easy to see and draw. (Lesson 3 uses this idea to calculate sizes.)
Agronomists and farmer promoters carry a hand lens as standard kit. With it they check the undersides of coffee leaves for the orange powder of coffee leaf rust, look for the tiny eggs of the fall armyworm on maize, and count aphids and mites on beans and vegetables before deciding whether to spray. A hand lens costs little, needs no electricity and goes into a pocket, which is why it is used far more often than a microscope outside the laboratory.
How to Use a Hand Lens (UC Master Gardeners of San Luis Obispo County). Watch where the lens is held and which object moves to bring the image into focus.
Tip
Memory hook: "Lens to eye, leaf to lens." The lens stays at your eye; the specimen does the moving.
Practice: the hand lens
Key points
Sources and credits
Written by Ganzaa: the lesson text, Figures 1 and 2, the table, the worked examples and the practice questions.
