Magnification (Edexcel GCSE Biology Combined Science): Revision Notes

Magnification

If not…

Both measurements should have the same units. If they don't, you'll need to convert them first (see below).

Standard form

To convert	Unit	To convert	In standard form:
x 1000 ↓	Millimetre (mm)	÷ 1000 ↑	× 10^-3 m
x 1000 ↓	Micrometre (μm)	÷ 1000 ↑	× 10^-6 m
x 1000 ↓	Nanometre (nm)	÷ 1000 ↑	× 10^-9 m
	Picometre (pm)		× 10^-12 m

So 1 pm = 0.000000000001 m. (That's tiny!)

Common Calculations

1. Magnification of a Light Microscope:

• Magnification of eyepiece lens × Magnification of objective lens.

2. Size of an Object:

• Size of image / Magnification = Size of object (Ensure units are consistent).

Calculations Using Standard Form

Standard form is used to represent very large or very small numbers in a concise way. It is written as a number between 1 and 10 multiplied by a power of 10.

• Example: 3,000,000 = 3 × 10⁶

• Example: 0.000002 = 2 × 10⁻⁶ Common Calculations in Biology:

• Cell Size: If the average human cell is 20 µm (micrometres), this can be written as 2 × 10⁻⁵ metres in standard form.

• Virus Size: If a virus is 100 nm in size, this can be written as 1 × 10⁻⁷ metres in standard form.

• Usage: Helpful for working with very large or small numbers in microscopy.

• Format: A number multiplied by a power of 10, where the number is between 1 and 10.

Number, Size, and Scale in Biology

Number and Scale:

• Cells: Most human cells are about 10-30 µm in size.
• Bacteria: Bacteria are typically around 1-5 µm.
• Viruses: Viruses are much smaller, ranging from 20-300 nm.
• Organelles: Inside cells, structures like ribosomes are around 20 nm in size, while DNA molecules can be about 2 nm in width.

Estimations in Biology

Estimations are often used in biology to simplify complex measurements or when precise values are not necessary.

When to Use Estimations:

• Quick Calculations: Estimations are helpful when you need a rough idea of size or scale without exact measurements.

• Simplifying Large or Small Numbers: When dealing with very large or small values (like the number of cells in a body), it's often easier to estimate. Example:

• If an average cell is 20 µm in size, and you are asked how many cells fit across a 1 mm space, you can estimate:

  • 1 mm = 1000 µm.
  • 1000 µm / 20 µm ≈ 50 cells across 1 mm.