### A Brief History of the Microscope

- Microscopy techniques have developed over time, increasing our understanding of cell subcellular structure
- The first
**light microscopes**were developed in the 17th Century - Scientists such as Anton van Leeuwenhoek and Robert Hooke are responsible for using microscopes to develop our first understanding of cells
- Light microscopes use
**light and lenses**to form a magnified image of a specimen - Over the centuries, the design of the light microscope has evolved,
**increasing magnification and resolution**to enhance the detail of what can be visualised - With a light microscope it is possible to see images of
**cells and large subcellular structures**(like nuclei and vacuoles), although**stains**are often required to highlight certain parts of cells - The first
**electron microscopes**were developed in the first half of the 20th Century- Electron microscopes use
**beams of electrons**, rather than light, to visualise specimens - The
**wavelength**of an electron beam is much**smaller**than that of visible light, which gives electron microscopes a much**higher resolution and magnification**

- Electron microscopes use

### Electron Microscopes

- An electron microscope has much higher
**magnification and resolving power**than a light microscope - They can therefore be used to study cells in much finer detail, enabling biologists to see and understand many more
**subcellular structures**such as the**mitochondrion** - They have also helped biologists develop a better understanding of the structure of the
**nucleus and cell membrane**

### Magnification Calculations

- Magnification is calculated using the following equation:

Magnification = Drawing size ÷ Actual size

- A better way to remember the equation is using an
**equation triangle**:

**An equation triangle for calculating magnification**

- Rearranging the equation to find things other than the magnification becomes easy when you remember the triangle –
**whatever you are trying to find, place your finger over it and whatever is left is what you do**, so:- Magnification = image size / actual size
- Actual size = image size / magnification
- Image size = magnification x actual size

- Remember magnification
**does not have any units**and is just written as ‘X 10’ or ‘X 5000’

#### Worked example

An **image** of an animal cell is 30 mm in size and it has been **magnified** by a factor of X 3000. What is the **actual** size of the cell?

To find the **actual** size of the cell:

**Worked example using the equation triangle for magnification**

#### Exam Tip

It is easy to make silly mistakes with magnification calculations. To ensure you do not lose marks in the exam:

**Always look at the units**that have been given in the question – if you are asked to measure something, most often you will be expected to measure it in millimetres NOT in centimetres – double-check the question to see!**Learn the equation triangle**for magnification and always write it down when you are doing a calculation – examiners like to see this!

### Converting Units

- You may be given a question in your Biology exam where the measurements for a magnification calculation have
**different units.**You need to ensure that you**convert them both into the same unit**before proceeding with the calculation (usually to calculate the magnification)

- Remember that 1mm = 1000µm
- 2000 / 1000 = 2, so the actual thickness of the leaf is 2 mm and the drawing thickness is 50 mm
- Magnification = image size / actual size = 50 / 2 = 25
- So the magnification is x 25