Electron microscopy

oh [Music] [Applause] [Music] the eyes of the scientist look at the World of Nature and seek to understand it marello malpi of bolognia was one of the first to use lenses to extend the range of the scientist's vision these first microscopes could survey the world and its creatures in detail undreamed of in 1661 malpi made history by OB observing the movement of blood in the fine veins of a frog's tongue in the early days of the Royal Society in London Robert Hook designed other microscopes and examined the structure of a host of things and creatures he too astonished His World by his detailed enlargement of PX irans the human flea at the same period the mid 17th century a Dutchman called Van luren Hook began to grind minute single lenses of unexampled power seeking the cause of the hotness of pepper he left some grains of the stuff in water 3 weeks later he found growing in the water incredibly many very little animalcules the smallest of these We Now call bacteria these were the beginnings of the light microscope over the centuries its aberations and distortions were corrected magnifications of over a thousand times became possible by 1900 the Optics of the light microscope were substantially what they are today the light microscope is still essential to the scientist yet there is a limit to the detail it will reveal some objects are so small that they cannot be clearly seen others are too small to be seen at all and this is due to the properties of light itself the behavior of light waves can be simulated in water with a ripple tank light from a given Source has a measurable wavelength an object whose width is greater than half the wav W length of light distorts the pattern of the wave movement and so the eye can see it but a smaller object hardly deflects the wave movement this means it cannot be seen to see and magnify such small objects we must find an alternative to light with a shorter wavelength with light glass lenses are used to focus the beam a stream of electrons in a cathode ray tube has a wavelength thousands of times shorter than that of light and it can be focused with an electromagnetic coil as the current in the coil varies the beam is brought in and out of focus thus an electromagnet provides a new kind of lens for focusing a beam of electrons traveling along its axis it makes possible an electron microscope what is needed first an electron gun to produce a stream of electrons an electromagnetic lens to act as a condenser focusing the stream onto the object to be examined the electrons pass through the object as light passes through a slide then another magnetic lens the objective to create an intermediate image finally a magnetic projector lens to throw a further enlarged image onto a fluorescent screen or a photographic plate the electron gun must work in a vacuum so the column must be solid and vacuum tight with the fluorescent screen viewed through a window here is a modern electron microscope the cable at the top is connected to a high voltage electrical supply up to 100,000 volts inside the casing is the electron gun the Electron Beam is emitted by heated v-shaped filament mounted in the cathode electrons are attracted to the highly polished anode they pass through the hole in the anode plate and on down the column below the anode plate is the condenser lens which focuses the electrons onto the specimen all the lenses are simply electromagnetic coils in metal cases below the condenser lens is the specimen chamber with its Window and Door in the chamber is an air lock which can be pumped out the specimen itself is put in the little copper holder mounted on a grid the grid is so small that it is difficult to see the grid squares without a magnifying glass below the specimen stage is the objective lens the image is focused by varying the current in this lens there are two projector lenses at the bottom of the column below them is the viewing chamber the image is projected onto the viewing screen which can be raised to expose a photographic plate the resolution of photographic Emulsion is normally at least six times that of the human eye so that an enlarged photograph reveals more detail than can be seen on the viewing screen the electrons cannot penetrate more than a millimeter in air but in the microscope they must travel from the electron gun to the viewing screen they can cover this distance only in a very high [Music] vacuum the world of the electron microscopist seems totally unfamiliar an abstracted world of shaped and ples which at first mean nothing yet this is the detail of the real world this is the ultra structure of real things of a chemical smoke of soap of chalk a human tissue cell etched steel and at the extreme part of the microscope the atomic structure of a platinum compound parallel lines of atoms 1 millionth of a millim apart if a heavy current is passed through two carbon electrodes in a vacuum carbon will condense on a slide beneath the carbon forms a sheet 20 millionths of a millimet thick which can be floated off it is thin enough to be easily penetrated by a beam of electrons so it can act as a support for specimens many times smaller than the holes in the grid a suspension of latex rubber is being examined the rubber is supported by the carbon layer preparing specimens for the electron microscope is a painstaking job because everything is so minute the next step is to put the holder into the specimen chamber close the air lock and pump it out at the lowest magnification the squares of the specimen grid are visible the operator selects one square already the latex particles can be seen sitting on the layer of carbon at 20,000 times magnification is changed to the higher range we can see that spongy latex rubber is actually built up of very many small spherical particles each about a thousandth of a millim in diameter magnification on the microscope screen 36,000 on a 10t wide cinema screen 1 and a half million by taking a picture with the built-in camera and enlarging the photograph magnifications of 2 million times or more are possible the prints are known as electron micrographs a freshly broken piece of plate glass is one of the sharpest cutting edges known it can be used to shave off sections of plant and animal tissue so thin that an electron beam will pass through them The Cutting instrument is called an Ultra microtome the slices are 10 to 30 millionths of a millimet thick the thickness of a slice is judged by its interference color normally the tissue is embedded in a plastic support thin tissue sections allow the botanist or the anatomist to observe structure and ways of growth for instance in the sheath round the fibers of the sciatic nerve in a 4-day old mouse the cell membrane appears to be folding inwards around the nerve core as the mouse develops this spiral loops and Loops Again by 16 days the protective covering is tightly wrapped about the nerve itself but many specimens are too dense to be penetrated by electrons if the specimen is put on a slide beneath the carbon rods it is possible to make a cast or replica of the surface of the specimen when a current is passed between the rods the carbon vaporizes and then condenses all over the specimen making the cast now the specimen itself is removed so that only the cast is left the carbon cast can easily be penetrated by El electrons yet an electron micrograph gives a poor impression of structure this enlarged model of the cast shows how the problem has been solved the cast is shadowed with a heavy metal such as platinum or gold with an actual specimen this is done in a vacuum the resulting electron micrograph is reversed to make the Shadows black another way of throwing a surface into relief is to direct the Electron Beam onto the specimen at a low angle this creates Long Shadows the technique is used to study the surface of metals as with this micrograph of a fine scratch on a sheet of copper a new Improvement to the electron mic rcope projects the picture onto a television tube a single bacterial cell is being examined the Electron Beam falls directly onto an electronic camera tube below the viewing screen this image intensifier increases the brightness level allowing higher magnification or the use of lower electron intensities the intensifier can be used in the study of wear a standard laboratory machine records wear between a brass rod and a steel plate a smaller version can be made to fit inside the electron microscope the tendency today is to put the laboratory into the microscope as it were so that experiments can be watched as they happen here the pin on the left is set at an angle to the Electron Beam casting a shadow to the right magnification is 10,000 times on the normal viewing screen the picture is hardly visible but with the image intensifier one can see pieces of the pin breaking off and Welding themselves to the plate in many Industries today the electron microscope plays an important part in research and production the plastics manufacturer uses it to help analyze the structure and properties of synthetic materials in this sample of PVC he can check the shape of particles and the distribution of different particle sizes which will affect the properties of the plastic the paint makers expose samples to wear by Light and Water simulating years of use in a short time a weathered sample can be compared with an unused one a carbon replica shows that the unused paint has a generally uniform surface but in the weathered Paint The Binding material has worn away many synthetic clothing fibers generate static electricity when they are worn before use the surface of a thread of nylon is fairly smooth after it has been worn and washed for a few weeks static electricity leaves burn marks on the fiber pitting and distorting its surface nylon equally worn but washed in an antistatic detergent remains almost as good as new the botanist studying the effect of weed killers takes different weeds and makes replicas of their Leaf surfaces the leaves of poppies and many other weeds have a surface which is porous to liquids but the corn marold has an elaborate structure of wax on the leaves which makes them waterproof this may explain why the mold is immune to contact weed killers the scientist studying air pollution take samples of air from tunnels and city streets with the electron microscope he can distinguish the impurities which we inhale short chains of carbon expelled from the exhausts of vehicles and large carbon particles from domestic chimneys years ago it was predicted that a raft of bubbles could simulate the behavior of the atoms in a metal Crystal if the raft is strained faults or dislocations run through the lines of bubbles now these dislocations can be seen in the microscope in a film of aluminium the Electron Beam Heats and stresses the metal causing the dislocations now watch a film of jalumin being cooled and heated jalumin is a solution of copper in aluminium as the metal cools dark patches appear a copper aluminium compound is being precipitated as the temperature rises the precipitate redissolves watch a cracks spreading in aluminium it has never before been possible to see the way Metals behave almost at the atomic level the biologist too can confirm predictions in the past many diseases were attributed to minute living particles that had never been seen the viruses today many viruses can be grown in tissue cells and sprayed onto specimen grids the scientist can take polio for instance and watch the way the virus replicates inside the cell or he can slice bacteria into sections and St different stages of attack as a bacterial virus colonizes the cell in these smallest forms of life the microscope reveals precision and order as with adino virus parasite of the throat and chest only 30 millionths of a millimeter in diameter our knowledge of it has grown until today we observe its precise structure 252 protein units in a regular 20-sided solid at its core hardly discernable is the thread of nucleic acid the complex molecule that determines the growth and function of every form of life yet this microscope which seems to bring us so near to the heart of things is little more than 20 years old as primitive in its own field as a 17th century instrument but unlike malpi or lven hook the physicist understands the theoretical limitations within which he can improve lenses or step up voltages already the electron microscope has shown how intricate is the detail of everything and how much more there is to learn of the ultra structure of our world [Music] h the [Music]