microscopes

The principles and limitations of optical microscopes, transmission electron microscopes and scanning electron microscopes. Measuring the size of an object viewed with an optical microscope. The difference between magnification and resolution. Use of the formula: m a g n i f i c a t i o n = s i z e o f i m a g e s i z e o f r e a l o b j e c t

state the definition of magnification

• describes how much bigger an image appears compared to the original object

state the definition of resolution

• the ability of an optical instrument to produce an image that shows fine detail clearly

state the two types of light microscope and describe their general pros and cons

• optical light and laser scanning/conofocal

• cheap, easy to use, portable, can study living specimens

• resolution is limited compared to electron microscopes

state the 2 types of electron microscopes and state their general pros and cons.

• electron transmission and scanning electron

• have the highest resolution because of the short wavelengths of the electrons

• hard to transport, require specialist training to use, samples have to be dead (bc. process in a vacuum), complex staining process required which may introduce artefacts into the sample, samples have to be very thin for the electrons to pass through

describe how an electron transmission microscope works and its specific cons.

• beam of electrons fired from the cathode and pass through the sample stained with metal salts. they are focused by magnets onto a photographic plate → forms a 2D black and white image

• image is black and white, image is 2D

describe how a scanning electron microscope works and its specific pros

• secondary electrons bounce off of the surface of the sample and are focused onto a screen → creates a 3D black and white image with high magnification

• image is 3D, can add false colour