RP1: Light Microscope

Question 1

What is the aim of this experiment?

Answer 1

• use a light microscope to investigate a selection of plant and animal cells

Question 2

What is the function of the mirror in a light microscope?

Answer 2

it reflects light up through the microscope slide

Question 3

Explain the method for preparing a slide

Answer 3

1) use dropping pipette to put one drop of water onto slide
2) separate one thin layer of onion and peel off thin layer of epidermal tissue from inner surface
3) use forceps to put the thin layer on drop of water on slide
4) make sure layer of onion cells is flat on slide
5) put two drops of iodine solution onto onion tissue
6) carefully lower coverslip onto slide by placing one edge on slide and use mounted needle to lower other edge on slide
7) get piece of filter paper and soak up liquid from around edge of coverslip
8) put slide onto stage

Question 4

Explain the method for using the light microscope

Answer 4

1) place prepared slide on stage and use clips to hold slide in place
2) direct mirror to shine light through hole in stage
3) turn nosepiece to lowest power objective lens
4) look from side and turn coarse adjustment knob until end of objective lens is almost touching slide
5) look through eyepiece and turn coarse adjustment knob until cells come into focus
6) rotate nosepiece to use higher power objective lens
7) slightly rotate fine adjustment knob to bring cells into clear focus and use low power objective (40x) to look
8) when you have found some cells, switch to higher power
9) make a clear, labelled drawing of some of these cells - ensure that you draw and label any component parts of cell
10) use an eyepiece graticule to measure length of one of epidermal cells that you have drawn - remember to include units
11) now measure same cell in your drawing
12) calculate magnification of your drawing using formula:
mag = length of drawing of cell / actual length of cell
13) write mag underneath drawing

Question 5

Why should you not look through the eyepiece at the beginning?

Answer 5

• because it could break the slide

Question 6

Name some risks of the practical

Answer 6

• iodine stain is corrosive + an irritant. goggles and gloves should be worn
• bulb of microscope light could get hot
• glass coverslip/slide could shatter

Question 7

Why is a thin sample of tissue required?

Answer 7

• Allows light to pass through so internal structures can be observed

Question 8

Why is water added to the slide before the sample is mounted?

Answer 8

• Allows the sample to be suspended between the slide and the cover slip and ensures the cover slip stays in place

Question 9

Why is iodine solution added to stain the cells?

Answer 9

• Adds contrasts - reacts with starch in plant cells and turns blue-black
• Allows internal structures to become more visible

Question 10

Why is the cover slip lowered at an angle / using a mounting needle?

Answer 10

• Ensures that no air bubbles are trapped

Question 11

How would you measure the length of a cell viewed under the microscope?

Answer 11

• Measure using eyepiece graticule and calibrate with stage micrometer