Unit 4 Magnifying instruments and biological drawings
Key unit competence: Manipulate magnifying instruments for the observation of biological specimen details.
Key ideas
- Many structures of living things are too small for the naked eye. Magnifying instruments make objects look larger so that details can be seen: the hand lens (in the field, for insects, flowers, leaves; magnifies about 2 to 10 times) and the light microscope (in the laboratory, for cells and tiny organisms on a slide).
- In 1665 Robert Hooke looked at thin slices of cork with a microscope and named the small boxes he saw cells. Modern microscopes show cells, bacteria and, with electron microscopes, viruses.
- To use a hand lens, hold it close to your eye and move the specimen (or your head) until the image is sharp.
- Parts of a light microscope: eyepiece (lens you look through, usually ×10), objective lenses (×4, ×10, ×40: the longest has the highest power), nosepiece (turns to change objective), stage and stage clips (hold the slide), diaphragm (controls how much light reaches the specimen), condenser (focuses light on the slide), mirror or lamp (light source), coarse focus knob (large movements, low power only), fine focus knob (sharpens the image at medium and high power), arm and base.
- Steps: start with the lowest-power objective; put the slide on the stage and clip it; look from the side while lowering the objective; then, looking through the eyepiece, raise it with the coarse knob until the specimen is in focus; adjust the light; turn to a higher power and use only the fine knob.
- Care: carry the microscope with two hands (one on the arm, one under the base); never touch the lenses (clean with lens tissue); always use a cover slip; never force the knobs; never use the coarse knob with the high-power objective (it can crack the slide and lens); keep it covered and dry.
- Magnification is how many times larger the image is than the real object. Total magnification of a microscope = eyepiece magnification × objective magnification, for example 10 × 40 = 400 times (×400).
- Magnification of a drawing = size of drawing ÷ actual size of the specimen, both in the same units. A bean seed 3 cm long drawn 15 cm long is magnified 15 ÷ 3 = ×5. Also: actual size = drawing size ÷ magnification.
- A good biological drawing: a clear title, underlined; large (at least half a page); sharp pencil, smooth single lines, no shading or colouring; label lines drawn with a ruler, not crossing, ending on the part; labels printed to the side; the magnification written below the drawing.
- To prepare an onion epidermis slide: put a drop of iodine solution on a clean slide, peel a thin layer of epidermis from a fleshy onion scale, lay it flat in the drop, lower a cover slip slowly at an angle with a needle to avoid air bubbles, and blot away extra liquid.
Common misconceptions
- "Total magnification = eyepiece + objective." No: multiply them: ×10 eyepiece with ×40 objective gives ×400, not ×50.
- "Magnification makes the specimen bigger." No: only the image looks bigger; the specimen does not change size.
- "A good drawing should be shaded and coloured to look real." No: biological drawings use clean single pencil lines, no shading.
- "Use the coarse knob to focus at high power." No: at high power the lens is very close to the slide; use only the fine knob.
Worked examples
Example 1. A learner uses a ×15 eyepiece and a ×40 objective. What is the total magnification?
Solution
Total magnification = 15 × 40 = ×600.
Example 2. A bee is 2 cm long. A learner's drawing of it is 8 cm long. Find the magnification of the drawing.
Solution
Magnification = size of drawing ÷ actual size = 8 ÷ 2 = ×4. Write "×4" under the drawing.
Practice questions
Name five parts of a light microscope and state the function of each.
Show answer
Eyepiece: magnifies the image you look at. Objective lens: main magnification. Stage: holds the slide. Diaphragm: controls light. Fine focus knob: sharpens the image at high power. (Also coarse knob, condenser, mirror, nosepiece.)
Common mistake: Confusing the eyepiece with the objective lens.
List the steps for viewing a prepared slide with a microscope.
Show answer
Put the lowest-power objective in place; clip the slide on the stage; looking from the side, lower the objective close to the slide; looking through the eyepiece, raise it with the coarse knob until in focus; adjust the light; change to higher power and use only the fine knob.
Common mistake: Starting with the high-power objective.
State four ways of caring for a microscope.
Show answer
Carry it with two hands; clean lenses only with lens tissue; never use the coarse knob at high power; cover it and store it in a dry place after use.
Common mistake: Saying "wash it with water".
Calculate the total magnification for a ×10 eyepiece used with ×4, ×10 and ×40 objectives.
Show answer
×40, ×100 and ×400.
Common mistake: Adding instead of multiplying.
An insect 5 mm long is drawn 6 cm long. Calculate the magnification of the drawing.
Show answer
Change to the same units: 6 cm = 60 mm. Magnification = 60 ÷ 5 = ×12.
Common mistake: Dividing 6 by 5 without changing units.
Give five rules for making a good biological drawing.
Show answer
Underlined title; large drawing; sharp pencil and smooth single lines; no shading; ruled label lines that do not cross, with labels to the side; magnification written below.
Common mistake: Colouring or shading the drawing.
Describe how to prepare a temporary slide of onion epidermis.
Show answer
Put a drop of iodine on a clean slide; peel a thin piece of epidermis from an onion scale leaf; spread it flat in the drop; lower a cover slip slowly at an angle with a needle; blot away extra stain; view under low power first.
Common mistake: Using a thick piece of onion that light cannot pass through.
Give one advantage of a hand lens and one advantage of a light microscope.
Show answer
Hand lens: cheap, light and easy to carry for fieldwork on whole specimens. Light microscope: much higher magnification, so cells and microorganisms can be seen.
Common mistake: Saying the hand lens can show cells clearly.
Matching and drag-and-drop
Matching
Match each item in column A with the correct answer in column B. Write the letter next to each number.
Column A Column B 1. A leaf 4 cm long is drawn 12 cm long. The magnification of the drawing is: a. size of drawing ÷ actual size 2. Magnification of a drawing is calculated as: b. ×3 3. You should always begin to observe a slide with the: c. lowest-power objective 4. The flat platform on which a slide is placed is the: d. stage Show answer
1 → b (×3) · 2 → a (size of drawing ÷ actual size) · 3 → c (lowest-power objective) · 4 → d (stage)
- 12 ÷ 4 = 3.
- Divide the drawing size by the real size, in the same units.
- Low power shows a wide field and is easiest to focus.
- Stage clips hold the slide on the stage.
Drag and drop
Choose the correct word or phrase from the box for each gap. One word in the box is not needed.
×50 · ×400 · ×40 · Robert Hooke- The scientist who looked at cork and named "cells" in 1665 was .
- A microscope has a ×10 eyepiece and a ×4 objective. The total magnification is .
- A microscope has a ×10 eyepiece and a ×40 objective. The total magnification is .
Show answer
1: Robert Hooke · 2: ×40 · 3: ×400
- Hooke saw tiny box-like spaces in cork.
- Multiply: 10 × 4 = 40.
- 10 × 40 = 400.
Matching
Match each item in column A with the correct answer in column B. Write the letter next to each number.
Column A Column B 1. A cover slip is placed on a specimen to: a. eyepiece 2. The part of a microscope you look through is the: b. keep the specimen flat and protect the lens 3. The condenser of a microscope: c. focuses light onto the specimen 4. To avoid air bubbles when lowering a cover slip you should: d. lower it slowly at an angle, using a needle Show answer
1 → b (keep the specimen flat and protect the lens) · 2 → a (eyepiece) · 3 → c (focuses light onto the specimen) · 4 → d (lower it slowly at an angle, using a needle)
- It also stops the liquid drying out.
- The eyepiece is at the top of the body tube.
- It gathers light from the lamp or mirror.
- Air escapes from one side as the cover slip goes down.
Drag and drop
Choose the correct word or phrase from the box for each gap. One word in the box is not needed.
diaphragm · 3 cm long · nosepiece · arm- The part that holds the objective lenses and turns to change them is the .
- A drawing of a seed is 12 cm long and its magnification is ×4. The real seed is .
- The part that controls the amount of light passing through the specimen is the .
Show answer
1: nosepiece · 2: 3 cm long · 3: diaphragm
- The revolving nosepiece holds two to four objectives.
- Actual size = 12 ÷ 4 = 3 cm.
- The diaphragm opens or closes like the iris of the eye.