Magnification Calculations (AQA AS Biology): Revision Note

Resolution & magnification

Magnification

• Magnification is the number of times larger an image appears compared to the specimen's actual size

• Light microscopes use:

  • An eyepiece lens (commonly ×10)

  • Several objective lenses with varying magnifications

• Total magnification is calculated by:

Eyepiece magnification × Objective magnification

Resolution

• Resolution is the ability to distinguish between two separate points.

• Limited by the wavelength of light in light microscopes:

  • Longer wavelengths = more diffraction and overlap = lower resolution

• Electron microscopes have a much higher resolution because electrons have a shorter wavelength than visible light

• Light microscopes cannot resolve structures smaller than ~200 nm, such as the 10 nm phospholipid bilayer, because these points are too close together and appear as one

Magnification calculations

• Magnification is how many times bigger the image of a specimen observed is in comparison to the actual (real-life) size of the specimen

• The magnification (M) of an object can be calculated if both the size of the image (I), and the actual size of the specimen (A), is known

To find the actual size of the cell:

• The size of cells is typically measured using the micrometre (μm) scale, with cellular structures measured in either micrometers (μm) or nanometers (nm)

• When doing calculations all measurements must be in the same units. It is best to use the smallest unit of measurement shown in the question

• To convert units, multiply or divide depending if the units are increasing or decreasing

• Magnification does not have units

Step 1: Check that units in magnification questions are the same 

Remember that 1mm = 1000µm

2000 / 1000 = 2, so the actual thickness of the leaf is 2 mm and the drawing thickness is 50 mm

Step 2: Calculate Magnification

Magnification = image size / actual size = 50 / 2 = 25

So the magnification is x 25