Magnification and resolution

Microscopes enhance our sense of sight – they allow us to look directly at things that are far too small to view with the naked eye. They do this by making things appear bigger (magnifying them) and at the same time increasing the amount of detail we can see (increasing our ability to distinguish between two objects or ‘resolve’ them). For this reason, they are one of the most widely used tools in science.

Different kinds of microscopes can show us different amounts of detail (they have different resolving power). Electron microscopes have a far greater resolving power than light microscopes, so we can use them to see even more detail than is visible under a light microscope.

Microscopes magnify and show more detail

When we talk about how microscopes work, we often say that they make things look bigger – that is, they magnify them. We describe what we see down the microscope in the same way, for example, we might say that the dead fly we’re looking at has been magnified 200 times. This helps us to make sense of what we’re seeing. It also helps others who are looking at our photographs or drawings to understand what they’re looking at. This is why all micrographs published in scientific journals must indicate the extent of magnification.

However, making things bigger is only part of the story. If microscopes did nothing but make what we can already see bigger, they wouldn’t be much use! Instead, microscopes increase the amount of detail that we can see. Another word for the level of detail we can see is ‘resolution’.

To understand the difference between magnifying something and increasing the detail that’s visible, have a look at this digital photo of harakeke – you can make it bigger (in this case, 64 times bigger) but it doesn’t show us any more detail because the image is pixelated and blurred.

Thinking about resolution

Scientists think of resolution as the ability to tell that two objects that are very close together are distinct objects rather than just one. The naked eye can tell apart (resolve) two objects (such as grains of sand) that are about a tenth of a millimetre apart – any closer than that, and we see the two as a single shape. If we look under a light microscope on the highest magnification, we can distinguish between objects that are less than a micrometre (a thousandth of a millimetre) apart. If we try to magnify further, we won’t be able to see any more detail than this – just like the digital photo above, the microscope will have reached the limit of its resolution.

 

Understanding the limits of resolution

Scientists have worked out why we can’t see an unlimited amount of detail down a microscope. They found that any object that’s less than half the wavelength of the microscope’s illumination source is not visible under that microscope. Light microscopes use visible light (which has a minimum wavelength of 400 nm, or less than one thousandth of a millimetre). This means that we will never be able to see any object smaller than approximately 200 nm (about the width of an average-sized bacterium) using a light microscope (and in practice, many light microscopes can’t get close to this resolution because of lens quality).

Even more detail: using electrons instead of light

Understanding the limits of light microscopy led to the development of the electron microscope. In the same way that light has a wavelength, the movement of high-speed electrons also has a wavelength. The wavelength of electrons is thousands of times shorter than visible light, so scientists predicted that electron microscopes would be able to resolve objects that are thousands of times smaller. They were right – there are now electron microscopes that can detect objects that are approximately one-twentieth of a nanometre (10-9 m) in size. This means that electron microscopes can be used to visualise viruses, molecules and even individual atoms.

The wavelength of electron movement is measured in picometres (billionths of a millimetre), so electron microscopes should in theory be able to visualise even smaller objects than they currently can. The resolution is currently limited because of technical aspects of viewing samples, but it may eventually be possible to view objects at the theoretical resolution limit of electron microscopes.

Nature of Science

Scientists use a series of conventions when labelling microscope images. They include information about the magnification of the image (for example, 600x) as well as a scale bar, which acts as a ruler and indicates the true size of the object. These conventions help others to make sense of the images.

Useful links

Explore this interactive, which demonstrates the link between magnification and resolution.
www.olympusmicro.com/primer/virtual/magnifying/index.html external link

Try zooming in on images of famous paintings. The high resolution of the photographs lets you see extraordinary detail.
www.googleartproject.com external link

How resolution limits affect what can be seen using satellite imaging.
www.fas.org/irp/imint/resolve5.htm external link

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