Microscopy

MICROSCOPE

A microscope is an instrument used to see objects that are too small for the naked eye.

The science of investigating small objects using such an instrument is called microscopy.

Microscopic means invisible to the eye unless aided by a microscope.

Parts of the light

microscope

Objective lenses

HP objective lens is close to slide

Total magnification:Magnification = Objective lens X Eyepiece lens

e.g. What is the total magnification if the objective lens is twenty times (X20) and the eyepiece lens five times (X5)?Magnification = 20 X 5 = X100

As magnification increases, detail increases but:

Onion cell 40x

Onion cell 100x

Onion cell 400x

less of the cell is seen

Caring for the microscope:

1. Do not let any liquids to come in contact with the microscope.

2. Always store the microscope inside a box after use.

3. Return the objective lens onto low power after use.

4. Carry the microscope by the arm.5. Use a soft clean tissue to wipe the lenses

Microscope slides

Coverslips

Fig. 2 Preparing a slide as a wet

mount.

Use of stains: some parts of a plant cell can be clearly seen

when the cell is mounted in water E.g. an Elodea leaf cell:

cell wall several chloroplasts

are seen

other cell structures which are not so obvious can often be shown up more clearly by the addition of dyes called STAINS

METHYLENE BLUE To stain animal cells

IODINE SOLUTION To stain plant cells

A thin inner layer of epidermis is peeled off.

An onion is cut into quarters.

One of the fleshy scale leaves is removed.

Snapping leaf backwards exposes the epidermis.

Epidermis is placed on slide & covered with 2-3 drops of distilled water . Coverslip is lowered.

A drop of stain is put at one end of slide.

1 23

5

4

6

7Stain is drawn over specimen using a small piece of filter paper.

Questions1. A student prepared a microscope slide of onion cells

using water.Diagram A shows how the cells looked when first seen with the microscope.Diagram B shows their appearance after the addition of another liquid.What name is given to a liquid used to make cell structures easier to see?

Stain

2. The drawing shows the appearance of a prepared slide.

What causes the ring shapes? (1)Air bubbles trapped beneath the coverslip.

3. This diagram shows the main parts of a typical light microscope.

a) How is a glass slide held in position on the stage?

By the clips.

b) Why must the specimen on the slide be in the center of the hole in the stage?

To allow light pass through the specimen.

c) Why does the nosepiece rotate?To clip in position the required objective lens.

d) What is the mirror for?To reflect light onto the specimen.

e) How can you control the amount of light coming through the microscope?By moving the diaphragm.

f) Explain how you would use the microscope to look at a specimen under the low power.

1. Clipping slide by clips.2. Placing the low power objective in position.3. Moving the tube downwards while looking

from the side until objective lens is close to the slide.

4. Using the coarse adjustment knob, the tube is racked upwards until image is focused.

g) If you had a specimen in focus under the low power, how would you go on to look at it under high power?By using the fine adjustment knob.

h) Why should you never rack downwards with the coarse focusing knob while you are looking down the microscope?Not to break the slide.

i) If the magnifying power of the eyepiece lens is ten times (X10), and that of the low power objective lens is four times (X4), what is the total magnification of a specimen under low power? 10 x 4 = x40