Production of interference fringes using Young’s slits
Fig. 26.12 shows the general scheme of a modern version of Young’s experiment. Two very narrow, close, and parallel slits S 1 and S2 are illuminated by the light from a single slit S parallel to them, and placed in front of a strong monochromatic (= one colour or wavelength) light source. A sodium discharge lamp giving orange light is suitable, or alternatively, a white source may be used together with a colour filter which transmits only a limited range of wavelengths.
The interference fringes can be seen by setting up a translucent screen and viewing from the side opposite to the slits. Tracing-paper makes a suitable screen. Otherwise they can be examined through a magnifying eyepiece. Note that the fringes are formed in space. They are said to be non-localized. Hence, light and dark bands will be formed on a screen placed anywhere in the
fringe region and the spacing of the bands will increase as the screen is moved further from the slits. The same applies when using a magnifying eyepiece which shows a section across the fringes in its image plane.
The number of fringes seen depends on the width of the slits. The narrower the slits the greater will be the number of fringes, owing to the increased angular diffraction. They will, however, be much fainter since less light energy gets through. The average wavelength of light is about 0.000 5 mm. For convenience, this is usually written as 5 x 10-4 mm or 5 x 10-7 m, the use of a negative index signifying division by the particular power of 10. In order to pass sufficient light energy to give easily visible fringes, the slits have to be a good many wavelengths wide. The angular diffraction of the light passing through them is, therefore, quite small. We may think of it as being somewhere between that of the two water cases shown in Fig. 26.9. Consequently, the fringes are
confined to a much smaller space than our water wave experiments might lead us to suppose. It must be borne in mind, therefore, that all our diagrams relating to Young’s experiment have, for clearness, been drawn with exaggerated scales. The slit sources must be coherent. The reader may have wondered why it is necessary to have a single slit in front of the light source. The reason is that lamps do not emit light waves in the same regular manner as the dipper sends out water waves in a ripple tank.
The atoms in a light source give out millions of wave packets all with different phases and in different directions. If the single slit were not there the two slits would receive.1ight from different parts of the source and so the light emerging from them would be in different and constantly varying phase, The function of the single slit is to cause the light to spread out over a cylindrical diffracted wavefront on which the phase remains constant. Thus, if the two slits are equidistant from the single slit they simultaneously receive light in the same phase. If any sudden phase changes do occur in the light from the single slit, then the two slits will both be affected equally. In these circumstances the light from the two slits is said to be coherent. Thomas Young was well aware of this condition. One cannot btain a constant interference pattern from two independent slit sources.
Practical details. Good results will be obtained only if proper care is given to the preparation of the slits. One of the commonest methods is to use a pin to rule two parallel lines about a third of a millimetre apart on a piece of thin glass which has been coated with Aquadag (colloidal graphite) and allowed to dry. The graphite is removed by the pin point, thus leaving two transparent slits. This is not as easy as it sounds: several trials may be necessary in order to obtain good slits. Good results are obtained by the following method. A hole about I em in diameter is made in a thin sheet of metal and, diametrically across it a short length of copper wire about 0.4 mm diameter is fixed with adhesive. To ensure straightness the wire should be cut from a length which has been stretched slightly by clamping one end in a vice and pulling the other end with pliers. The slits are formed by sticking two pieces of razor blade on either side of the wire. This is best done under a low-power microscope when it will be found comparatively easy to push the two pieces of blade into position to form two narrow, equal, and parallel gaps before the adhesive sets. A pair of slits made in this way, about 0.06 mm wide and about 0.4 mm apart will, if used with a strong light source, give up to 18 fringes. Wider slits give fewer but brighter fringes.
The single slit can be made by the same method, omitting the wire. It is, however, a definite advantage to use a variable slit if one is available, since its width can then be adjusted to give maximum brightness combined with good definition of the
fringes. Needless to say, it is best to work in a dark room or at any rate in a dimly lit laboratory. When setting up the apparatus it is essential to see that the light source and single slit both lie on the perpendicular bisector of the line joining the two slits S 1 and S2. If the fringes are poor, it will probably be due to lack of parallelism between the slits. A slight rotation of the slit S one way or the other should bring about the desired results. The distances between the components is not critical. The slit screens may be some 15 em apart. The eyepiece should be placed where its field of view comfortably contains all the observable fringes: 20 to 30 em from the slits may suit a micrometer eyepiece. If, however, a translucent screen is used it should be placed 50 to 100 em from the slits if rough measurements of the fringe spacing are to be made with a half-millimetre scale. More accurate results will naturally be obtained from micrometer measurements. The micrometer eyepiece mentioned in this description has a vertical rosswire on a horizontal slide which is moved by a micrometer screw (not shown in Fig. 26.12). Readings are taken when the crosswire is centred over the extreme fringes visible and from these the mean distance between adjacent fringes is calculated. We shall now discuss the ray geometry of Young’s experiment and show how the wavelength of the light may be measured.