The advantages and limitations of electron microscopy.
The advantages and limitations of electron microscopy There are two main branches of microscopy that are pertinent to cell biology. These branches arise from the two types of microscope; the light microscope and the electron microscope. The basic principles of light microscopy have been known since circa 17th century, however improvements in lens manufacture in circa 19th century allowed the use of microscopy to be much more practically available and useful. This is increased ability inspired rapid research into both the design of microscopes and the preparation of specimens. However, the light microscope can only magnify objects bigger than 0.2 micrometres; due to its limited resolving powers. This is because it utilises a beam of light. Relatively, light has a long wavelength, this means that when there are two small points close together there is too much refraction and wave front overlap, the eye then only sees one point. This can also be considered in terms of objects "crossing the path" of the wavelength. The smallest wavelength of visible light is 400nm, the diameter of mitochondria is 1000nm, and therefore mitochondria cross the path of the light wave. However ribosomes have a diameter of 22nm, and do not cross the path of the light wave and are therefore not seen by the light microscope. As biologists came to realise these limitations they understood that the
Ray tracing
Physics GCSE coursework Ray tracing The objective of this experiment is to find the length of an object and its focal lengths. I will first try to hypothesize where the length points will be. The given results of my hypothesized diagrams will determine the lengths of the focal points. I will then be able to find out where the object image will be situated by following my ray trace diagrams. I believe that I will be able to find the length of the object image if I can find the two focal length points ( 1/v and 1/u displayed in my ray trace diagrams and the focal graph). To achieve this I have decided that I could not get an accurate hypothesis if I did not know this information, so I will carry out a small practical experiment. We found that if the lens is thin, the focal length is longer, and if it is thicker, the focal length is smaller. The focal length for our lens is 10cm. I have created some ray tracing diagrams to show my predicted lengths using the focal length that I found. I have found the longest distance that I can get a clear image is 100cm, the shortest being 15cm. I then carried out an experiment to prove my hypothesis. I used light boxes to create a light source, but this is not extremely accurate as the light rays diverge and are not parallel. I used a screen and a lens to try to find an image. My results for my experiment were as follows. I had three
Physics coursework; Finding the focal length of a lens using a graphical method.
Physics coursework; Finding the focal length of a lens using a graphical method. Planning: Firstly the rough focal length of my lens will need to be found to assist me in my real experiment. A simple way to do this would be, to hold the lens up to a flat white wall opposite a window when it is light outside, by moving the lens closer/ further away from the wall until an upside down image of objects outside the window (e.g. trees,) is produced, I can estimate an focal length for the lens which provides me with the minimum distance of (u), this saves time that would be spent trying to find a point from which I can begin measurements. The equipment will be set up as shown below: Apparatus: * Light source connected to a power pack * Wire grid (object) * 1m ruler (correct to the nearest mm) * a small bi-convex lens * a white 2D screen (approx 100*70 mm) ==> When the light is turned on the light will pass through the mesh creating an image which can be focused by the lens on the screen. ==> It is important to remember that light bulbs will get hot, so precautions should be taken to ensure I am safe from burns during the experiment. ==> It should also be considered that any experiment involving electricity carries risk so due care must be taken when handling any electrical equipment. ==> The light source will be covered with a sheet of grease proof paper, this will
Microscopes. Using electrons instead of light means that the illumination has a much shorter wavelength than light.
Microscopes Cells can be seen with a light microscope but many structures within a cell - organelles - can only be seen clearly with an electron microscope. That is partly because an electron microscope has a greater magnifying power. However, increasing only magnification has its limits because at some point magnification reveals nothing more - the details only look bigger and vaguer. Magnification is how much bigger a sample appears to be under the microscope than it is in real life. Overall magnification = Objective lens x Eyepiece lens Using electrons instead of light means that the illumination has a much shorter wavelength than light. This is good because minute detail can be detected. We say that an electron microscope has a bigger resolving power than an light microscope Resolution is the ability to distinguish between two points on an image. The resolution of an image is limited by the wavelength of radiation used to view the sample. This is because when objects in the specimen are much smaller than the wavelength of the radiation being used, they do not interrupt the waves, and so are not detected. The wavelength of light is much larger than the wavelength of electrons, so the resolution of the light microscope is a lot lower. The actual resolution is often half the size of the wavelength of radiation used. Thus, for the light microscope the maximum