Resolution & Magnification (Cambridge (CIE) AS Biology): Revision Note

Resolution & magnification

Magnification

• Magnification is the number of times that a real-life specimen has been enlarged to give a larger view/image

  • E.g. a magnification of x100 means that a specimen has been enlarged 100 times to give the image shown

• A light microscope has two types of lens which allow it to achieve different levels of magnification:

  • An eyepiece lens, which often has a magnification of x10

  • A series of objective lenses, each with a different magnification, e.g. x4, x10, x40 and x100

• To calculate the total magnification of a specimen being viewed, the magnification of the eyepiece lens and the objective lens are multiplied together:

Total magnification = eyepiece lens magnification x objective lens magnification

Resolution

• Resolution is the ability of a microscope to distinguish two points that are close together as separate objects; it determines how much detail can be seen

  • If resolution is too low, two separate objects appear as one point and the image looks blurry

• Resolution limits useful magnification; increasing magnification beyond this does not reveal more detail

• The resolution of a light microscope is limited by the wavelength of light

  • It cannot resolve points closer together than about half the wavelength of visible light (400–700 nm)

  • The shortest wavelength is 400 nm, so the resolution limit of a light microscope is about 200 nm

  • E.g. a phospholipid bilayer is about 7 nm wide, well below 200 nm, so its structure cannot be seen with a light microscope

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

  • They can achieve a resolution of about 0.5 nm, so they can usefully magnify much more