Practise Microscopy. 21 exam-style questions plus unlimited generated ones on this subtopic, at up to four difficulty levels, with full mark schemes and a progress tracker. Free, no account needed.
How light and electron microscopes compare, how the electron microscope improved our understanding of cells, and how to use magnification = size of image ÷ size of real object. It includes the microscopy required practical: preparing a slide, using a light microscope and drawing cells with a magnification or scale.
Grade by grade
What you need to be able to do, from the first marks up to the top grade.
3
State what magnification meansHow many times bigger the image is than the real object.
4
Calculate the total magnification of a microscopeMultiply the eyepiece lens magnification by the objective lens magnification, e.g. ×10 and ×40 give ×400.
4
Compare light and electron microscopesElectron microscopes have much higher magnification and resolution; light microscopes can show living cells in colour.
5
Use magnification = image size ÷ real sizeRearrange the equation to find the magnification, the image size or the real size.
5
Prepare a slide and focus a microscopeThin sample, stain, coverslip; start on the lowest power and focus with the coarse, then the fine, focus.
6
Convert units in magnification calculationsPut the image size and the real size in the same unit, usually µm, before dividing.
7
Explain how electron microscopes improved understandingTheir much higher resolution revealed small sub-cellular structures, such as ribosomes, and the detail inside mitochondria and chloroplasts.
8
Give real sizes in standard forme.g. a real size of 0.000012 m is 1.2 × 10−5 m.
Notes
Light and electron microscopes
A light microscope uses light and glass lenses. It magnifies up to about ×2000 and shows detail down to about 0.2 µm. It is cheap and can show living cells in colour.
An electron microscope uses a beam of electrons. It has a much higher magnification (up to about ×2 000 000) and a much higher resolving power (detail smaller than 1 nm).
Resolution is the ability to distinguish between two points that are close together. Higher resolution means more detail can be seen.
Electron microscopes are large and expensive, and specimens must be dead because they are viewed in a vacuum.
How microscopy developed
Light microscopes were first used in the 1600s and showed that living things are made of cells. Better light microscopes later showed structures such as the nucleus and chloroplasts.
Electron microscopes were developed in the 1930s, so cells could be studied in much finer detail.
Electron microscopes revealed structures too small to see with a light microscope, such as ribosomes, and the internal structure of mitochondria and chloroplasts. This helped biologists understand how these structures work. grade 7+
Magnification calculations
magnification = size of image ÷ size of real object
Rearranged: real size = image size ÷ magnification, and image size = magnification × real size.
Both sizes must be in the same unit (1 mm = 1000 µm). Magnification has no unit: write it as, e.g., ×400.
Total magnification of a light microscope = eyepiece lens magnification × objective lens magnification.
Required practical: looking at cells
Place a thin layer of tissue (e.g. onion epidermis) on a slide, so light can pass through it. Add a drop of stain (e.g. iodine solution) to make structures such as the nucleus easier to see.
Lower a coverslip at an angle to avoid trapping air bubbles.
Start with the lowest-power objective lens. Focus with the coarse focus, then the fine focus. Then switch to a higher power and refocus using only the fine focus.
Draw with a sharp pencil: clear continuous lines, no shading, ruled label lines, and a magnification or scale bar.
Cheatsheet
magnification = size of image ÷ size of real object
real size = size of image ÷ magnification
total magnification = eyepiece lens × objective lens
1 mm = 1000 µm; 1 µm = 1000 nm
Resolution: the ability to distinguish between two points that are close together
Light microscope: up to about ×2000; can view living cells in colour; cheap
Electron microscope: much higher magnification and resolution; shows ribosomes and fine detail; specimens dead
A stain (e.g. iodine solution) makes cell structures easier to see
How to answer each type of question
Calculate the real size of a cell
2 to 3 marks5
Write the equation, rearranged: real size = image size ÷ magnification.
Convert the image size into the unit the answer needs (usually µm).
Substitute and calculate, and give the unit.
Example. A student draws a cheek cell. The drawing of the cell is 45 mm wide. The magnification of the drawing is ×750. Calculate the real width of the cheek cell in micrometres (µm).
Add a stain, then a coverslip lowered at an angle.
Describe focusing: lowest power first, coarse then fine focus, then a higher power.
Example. Describe how a student could prepare a slide of onion cells and view the cells with a light microscope.
Show the model answer
Any four from: peel a thin layer of onion epidermis and place it flat on a slide (1) add a drop of iodine solution / a stain (1) lower a coverslip at an angle to avoid air bubbles (1) start with the lowest-power objective lens (1) focus with the coarse focus, then the fine focus (1) then switch to a higher power and refocus with the fine focus (1)
Explain the advantage of electron microscopes
2 to 3 marks6
Use the words 'resolution' (or 'resolving power') and 'magnification'.
Say what the higher resolution lets you see: smaller structures, in more detail.
Link to the structure in the question.
Example. Scientists could not see ribosomes until the electron microscope was developed. Explain why.
Show the model answer
Ribosomes are too small to be seen with a light microscope (1). Electron microscopes have a much higher resolution / resolving power (1) and a higher magnification, so they can show much smaller structures in detail (1).
Calculate the magnification, in standard form
2 to 3 marks7
Put the image size and real size in the same unit.
Divide the image size by the real size.
Write the answer in standard form if asked. Magnification has no unit.
Example. A mitochondrion is 2 µm long. In an electron micrograph, the image of the mitochondrion is 50 mm long. Calculate the magnification of the micrograph. Give your answer in standard form.
'I AM': Image size = Actual size × Magnification. Cover the quantity you want to find.
Convert everything to µm before you divide: it avoids most slips.
Sense check: the real size of a cell should usually come out between about 1 µm and 100 µm.
Magnification is about size; resolution is about detail. Magnifying a blurred image does not show more detail.
Where marks are lost
Forgetting to convert mm to µm before dividing, giving an answer 1000 times too big or too small.
Putting a unit on magnification, or leaving the unit off a size.
Confusing resolution with magnification. Resolution is about telling two close points apart, not about size.
Using the coarse focus on high power, which can crush the slide. On high power use only the fine focus.
Shading a drawing, using sketchy lines, or drawing label lines that do not touch the structure.
Exam technique
Write the equation first: it is often worth a mark and helps you rearrange it correctly.
Show the unit conversion as a separate line, e.g. 45 mm = 45 000 µm.
If the question asks for standard form or a number of significant figures, do it in your final answer.
In 'explain' questions about electron microscopes, use the words 'resolution' and 'magnification'.
Required practical:Microscopy (method, variables and exam tips)
Sample questions
Written for this site in the style of AQA exam questions. They are not taken from real past papers.
Question 1Easy5 marks
Microscopes are used to look at cells.
(a) Which equation is correct? Tick (✓) one box.[1]
magnification = size of image ÷ size of real object
magnification = size of real object ÷ size of image
magnification = size of image × size of real object
magnification = size of image − size of real object
(b) A light microscope has an eyepiece lens with a magnification of ×10 and an objective lens with a magnification of ×40. Calculate the total magnification.[1]
(c) The image of a cell is 20 mm wide. The magnification is ×400. Calculate the real width of the cell in mm.[2]
(d) Give the real width of the cell in micrometres (µm). 1 mm = 1000 µm[1]
Show the answer and mark scheme
(a)Answer: magnification = size of image ÷ size of real object
(b)Answer: ×400
(10 × 40 =) 400 (×)
(c)Answer: 0.05 mm
20 ÷ 400
0.05 (mm)
(d)Answer: 50 µm
50 (µm)
Question 2Medium8 marks
A student used a light microscope to observe onion epidermis cells.
(a) Describe how the student should prepare a slide of onion epidermis cells.[3]
(b) Give one reason why the student added a stain to the cells.[1]
(c) The student made a drawing of some of the cells. Give two rules the student should follow when making a scientific drawing.[2]
(d) The student also looked at human cheek cells. Give two structures the student would see in the onion cells that would not be seen in the cheek cells.[2]
Show the answer and mark scheme
(a)
peel off a thin layer of epidermis (using forceps)
place it flat on a (drop of water on a) microscope slide
add a drop of iodine solution / stain
lower a coverslip at an angle / carefully to avoid trapping air bubbles
(b)
to make the cells / nucleus easier to see / to increase contrast
(c)
use a sharp pencil
use clear, continuous lines (no sketching)
no shading / colouring
draw only what is seen / keep structures in proportion
draw label lines with a ruler / label lines should not cross
include a title / magnification / scale bar
(d)
cell wall
(permanent) vacuole
Question 3Hard8 marks
A student wanted to estimate the mean length of onion epidermis cells using a light microscope.
(a) Describe a method the student could use. You should include how the student would prepare the slide and how they would estimate the mean length of the cells.[6]
(b) At a magnification of ×100, the diameter of the field of view was 1.8 mm. The student counted 9 cells end to end across the diameter. Calculate the mean length of one cell in µm.[2]
Show the answer and mark scheme
(a)
peel a thin layer of epidermis with forceps and lay it flat on a microscope slide in a drop of water
add iodine solution to stain the cells and lower a coverslip at an angle to avoid air bubbles
focus on the lowest power objective with the coarse focus, then use the fine focus / move to a higher power
measure the diameter of the field of view by placing a clear ruler (with mm divisions) on the stage at the same magnification
count the number of cells that fit end to end across the diameter of the field of view
mean length of one cell = diameter of field of view ÷ number of cells
repeat in several different areas of the slide / with several slides and calculate a mean
convert the answer to µm (1 mm = 1000 µm)
Marked with levels of response: the full level descriptors are in the app.