Film: 5286

Science | 1960 | Sound | Colour


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Sir Lawrence Bragg (Nobel prize winner, jointly with his father W.H.Bragg, Physics, 1915, for their work on the analysis of crystal structure by X-rays), gives a lecture at the Royal Institute about X-Ray Diffraction.

Sir Lawrence Bragg stands facing the audience with several apparatus in front of him, there are also a black board and a screen behind him. Sir Lawrence Bragg starts by explaining the principle of X-ray diffraction. Close up on Sir Lawrence Bragg face while he explains diffraction by multiple layers. He picks up potassium chlorate crystals to explain that when the crystals grow, they grow in twinned layers. Close up on the potassium chlorate crystal, the crystal is turned in several angles to show that the crystal spangles can become green, blue or pink, in some angles it turns firts yellow and then red, that is, at every angle one colour is reflected by the crystal. Sir Lawrence Bragg puts the crystal down and goes to a model on the board which he uses to explain how colour is reflected off the potassium chlorate crystal. He moves away and emphasises that X-rays are electro-magnetic waves, just like light is, the difference is that the X-ray wavelength is shorter, also a crystal is the right kind of body to diffract X-rays, in crystals the atoms are arranged in layers, the layers are spaced about 5000 times closer, that is, it is the right conditions since the wavelength of X-rays is about the same as the spacing of the layers.

Sir Lawrence Bragg picks up a piece of rock salt (Sodium Chloride) crystal. Close up of the Sodium Chloride crystal which is cubic in shape. Sir Lawrence Bragg holds a knife over the Sodium Chloride crystal. Close up the knife cleaving a piece of the Sodium Chloride crystal. Close up of the cleaved piece, it broke along the layers of atoms. Sir Lawrence Bragg explains that the layers go in a number of directions in the crystal. Her walks to a Sodium Chloride crystal model, the lights are dimmed in order to cast a shadow of the crystal on the white screen. Sir Lawrence Bragg turns the model slowly around so that we can see that the shadow cast on the screen reveals that some layers are more closely spaced while other layers are widely spaced. The Sodium Chloride model is stood on its point and the shadow shows that the crystal breaks up into a number of layers, therefore, expect to see diffraction effects in a number of different directions, each diffraction will correspond to one set of layers. The lights are turned up and Sir Lawrence Bragg walks over to a centimetre-wave apparatus where he will perform an experiment to show X-ray diffraction (using electro magnetic waves) on a crystal. He points to show the "sender of waves", the position where he will place the body which is going to reflect or diffract the rays and finaly he shows the receiver (the lamp will light up, then will hear a noise and a beam will be displaced on a cathode-ray oscillograph).

Mr Coates (an assistant) sets the plate going, it reflects twice in each revolution (to show how the apparatus works). Close up of the cathode-ray oscillograph displaying the beam. Mr Coats replaces the plate with a mock crystal. Close up on the mock crystal, each plate is made of perspex each of wich reflects a little of the centimetre waves, these plates are about one centimetre apart. The mock crystal starts rotating. Close up on the cathode-ray oscillograph to observe the reflected beam of the first order, of the second order and of the third order. Sir Lawrence Bragg analyses the Sodium Chloride crystal structure. Close up on the Sodium Chloride model, Sir Lawrence Bragg points to the Sodium and Chloride atoms to emphasis that they are alternate along all the sides. The lights are dimmed and Sir Lawrence Bragg places the Sodium Chloride model on its point, we observe the shadow and detect that the alternate layers are heavy and light, therefore, can determine layer structure in the crystal (the layers are alternately strong reflecting and weakly reflecting). Mr Coats removes the screen to reveal a diagram which Sir Lawrence Bragg uses to explain what one expects from a series of layers like those of a Sodium Chloride crystal. Sir Lawrence Bragg walks to the X-ray apparatus and points to the X-ray tube, to the plaque he wears on his jacket (it indicates how much radiation he is exposed to). He shows where the X-rays come out to fall on the Sodium Chloride crystal. Close up of the crystal.

The Geiger counter picks up the reflected rays. Sir Lawrence Bragg shows how he can move the chamber around to turn the crystal, he pulls the shutter down to let the X-rays reflect on the sides of the cubic Sodium Chloride crystal. Close up of the Geiger counter meter measuring first order and second order reflection. He shuts the X-ray shutter and Mr Coats replaces the cubic crystal by another Sodium Chloride crystal, this crystal has been cut so that the front is parallel to the alternately Sodium and Chloride layers. Close up of the cut Sodium Chloride crystal. Another close up on the Geiger counter readings displayed by the cut crystal. Sir Lawrence Bragg stands next to a more modern and more complicated apparatus used to work out complex crystals, it is used by Professor Phillips at Oxford who uses it to work out the structure of protein molecules. This is an automatic machine which starts and runs itself, measuring the reflections one by one and recording them. Sir Lawrence Bragg shows us the X-ray tube, the glass tube (where the X-ray fall on the liquid protein crystal), the Geiger counter that receives the diffracted rays. Another view of the whole machine with Sir Lawrence Bragg standing next to it. Close up of the "electronic brain" (the computer, analogue I think). Close up "mechanism of screws" (puts the crystal in the right position), the computer gives instructions through a little motor which turns the screws round a given amount, putting the crystal in the right position for one reflection after another. Sir Lawrence Bragg also shows where the reflection will be shown and when the reflection is recorded, the amount will be printed out on punched tape and paper.

Close up of the glass tube containing the protein crystal. Close up of the printed amounts. The punched tape is then fed into a computer which proceeds to work out the density everywhere inside the crystal, where the dendity is heavy the researcher is able to tell where the atoms are. Close up of the punched computer tape. Finally, Sir Lawrence Bragg shows the audience the first protein model to be worked out (possibly haemoglobulin) which was first worked out by Dr Kendrew in Cambridge (Nobel Chemistry prize, 1962). Close up of the protein model which Sir Lawrence Bragg compares with the Sodium Chloride model (worked out over 50 years ago, this film is set in the 1960's), he states that the comparison is a good measure of the way in which X-ray analysis has advanced over half a century.

Science of light-waves and reflection. Demonstrations on a board. Splits piece of rock salt crystal. Use of shadows, diagrams, and sound waves machine. X-ray apparatus linked to Geiger counter dial with readings. X-ray spectrometer. Ticker tape machine. Models of molecules.

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