OCR A Level Biology:复习笔记2.1.4 Magnification & Resolution

• 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

An equation triangle for calculating magnification

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

Converting units of measurement

• There are 1000 nanometers (nm) in a micrometre (µm)
• There are 1000 micrometres (µm) in a millimetre (mm)
• There are 1000 millimetres (mm) in a metre (m)

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

Magnification

• Magnification is how many times bigger the image of a specimen observed is in compared to the actual (real-life) size of the specimen
• A light microscope has two types of lens:
  • An eyepiece lens, which often has a magnification of x10
  • A series of (usually 3) objective lenses, each with a different magnification

   

• To calculate the total magnification the magnification of the eyepiece lens and the objective lens are multiplied together:

eyepiece lens magnification x objective lens magnification

= total magnification 

Resolution

• Resolution is the ability to distinguish between two separate points
• If two separate points cannot be resolved, they will be observed as one point
• The resolution of a light microscope is limited by the wavelength of light
• As light passes through the specimen, it will be diffracted
• The longer the wavelength of light, the more it is diffracted and the more that this diffraction will overlap as the points get closer together
• Electron microscopes have a much higher resolution and magnification than a light microscope as electrons have a much smaller wavelength than visible light
  • This means that they can be much closer before the diffracted beams overlap

   

• The concept of resolution is why the phospholipid bilayer structure of the cell membrane cannot be observed under a light microscope
  • The width of the phospholipid bilayer is about 10nm
  • The maximum resolution of a light microscope is 200nm (half the smallest wavelength of visible light, 400nm)
  • Any points that are separated by a distance less than 200nm (such as the 10nm phospholipid bilayer) cannot be resolved by a light microscope and therefore will not be distinguishable as “separate”

   

The resolving power of an electron microscope is much greater than that of the light microscope, as structures much smaller than the wavelength of light will interfere with a beam of electrons

Comparison of the electron microscope & light microscope

• Light microscopes are used for specimens above 200 nm
  • Light microscopes shine light through the specimen, this light is then passed through an objective lens (which can be changed) and an eyepiece lens (x10) which magnify the specimen to give an image that can be seen by the naked eye
  • The specimens can be living (and therefore can be moving), or dead
  • Light microscopes are useful for looking at whole cells, small plant and animal organisms, tissues within organs such as in leaves or skin

   

• Electron microscopes, both scanning and transmission, are used for specimens above 0.5 nm
  • Electron microscopes fire a beam of electrons at the specimen either a broad static beam (transmission) or a small beam that moves across the specimen (scanning)
  • The electrons are picked up by an electromagnetic lens which then shows the image
  • Due to the higher frequency of electron waves (a much shorter wavelength) compared to visible light, the magnification and resolution of an electron microscope is much better than a light microscope
  • Electron microscopes are useful for looking at organelles, viruses and DNA as well as looking at whole cells in more detail
  • Electron microscopy requires the specimen to be dead however this can provide a snapshot in time of what is occurring in a cell eg. DNA can be seen replicating and chromosome position within the stages of mitosis are visible

   

Light v Electron Microscope Table