In the present chapter we consider some examples of experimental techniques. They have been chosen because they contain many ingenious features and show that the same principles of good experimentation apply whether the experiment be advanced or elementary. They come from different branches of physics – optics, electricity, mechanics, and atomic physics – and are illustrated in different contexts – either as an instrument, a complete experiment, or an application. This is the hardest chapter in the book, as some of the physics may be unfamiliar to you. But each section stands alone, so you can omit any one of them – or indeed the whole chapter – at first reading, and carry on with the rest of the book. On the other hand, the experimental ideas contain so many instructive features that you will find a little perseverance well rewarded.

Rayleigh refractometer

(a) Description of instrument. The Rayleigh refractometer is an instrument devised to measure the refractive indices of gases and also small changes in the refractive indices of solids and liquids.

Monochromatic light from a vertical slit S (Fig. 7.1) is collimated by an achromatic lens L1 and falls on two vertical slits S1 and S2. The two beams pass through tubes T1 and T2 of equal length t, lying in the same horizontal plane. The beams then recombine to form vertical interference fringes in the focal plane of the lens L2. The fringes are viewed by a small cylindrical lens L3.