Magnification & Resolution (OCR A Level Biology)

• The magnification of an object can be calculated using the formula:

magnification = size of image ÷ size of real object

• The magnification formula can be rearranged to allow the calculation of:
  • magnification (m)
  • size of image (i)
  • size of real object, often referred to as actual size (a)

Magnification equation triangle

An equation triangle allows the magnification formula to be rearranged easily

Converting units during magnification calculations

• Different units of measurement are used to measure different objects:
  • The actual size of cells is typically measured using the micrometre (μm) scale
  • Internal cellular structures are sometimes measured in nanometers (nm)
  • The size of images is usually measured in centimetres (cm) or millimetres (mm)
• Units of measurement relate to each other as follows:
  • 1000 nm = 1 µm
  • 1000 µm = 1 mm
  • 1000 mm = 1 m
  • 10 mm = 1 cm
• When carrying out magnification calculations it is essential that all measurements have the same units, so unit conversions are often required
• Units can be converted by multiplying or dividing by the relevant factor
  • Converting larger units to smaller units = multiply
  • Converting smaller units to larger units = divide
• Note that magnification does not have units

Converting units diagram

Units of measurement can be converted by multiplying or dividing by the relevant factor

Magnification

• Magnification can be defined as:

The number of times larger an image is than the actual object

• The ability of a microscope to magnify an object depends on the type of microscope, and on the features of the microscope itself
  • E.g. for a light microscope the magnification can be calculated by multiplying together the magnification of the eyepiece lens and the objective lens

Resolution

• Resolution can be defined as:

The ability to distinguish separate points on an image as two separate objects

• The higher the resolution, the shorter the distance at which the two objects can be clearly distinguished
• The ability of a microscope to resolve two objects as separate points is dependent on the method of image generation:
  • Light microscopes: the resolution is limited by the wavelength of light
    • As light passes close to physical structures it is diffracted, meaning that light waves spread out
    • The closer the structures are to each other, the more the light waves overlap each other as they are diffracted
    • Points that are closer together than half the wavelength of visible light cannot be clearly distinguished from each other
  • Electron microscopes: the resolution is much higher because electrons have a smaller wavelength than visible light
    • The objects past which the electrons travel can therefore be much closer together before the diffracted beams overlap

Resolution diagram

The resolving power of an electron microscope is much greater than that of a light microscope; this is because electrons have a smaller wavelength than visible light

Comparing light and electron microscopes

• Light microscopes:
  • have a maximum resolution of 200 nm
  • have a maximum useful magnification of around ×1500-2000
  • can be used for viewing living or dead specimens
  • are useful for looking at whole cells, small organisms and tissues within organs such as in leaves or skin
• Electron microscopes:
  • have a maximum resolution of 0.5 nm (TEM) or 3-10 nm (SEM) 
  • are capable of generating images with a magnification of more than ×500 000
    • TEMs are capable of higher magnification than SEMs due to their higher resolution
  • can only be used for viewing dead specimens
  • are useful for looking at organelles and viruses, as well as looking at whole cells in more detail

Light and electron microscopes comparison table