To measure refractive index by the real and apparent depth method
(a) Glass. A glass block is placed vertically over a straight line ruled on a sheet of paper (Fig. 23.10 (a)). A pin on a sliding cork adjacent to the block is then moved up or down until there is no parallax between it and the image of the line seen through
the block. Measurements of the real and apparent depth are then taken as indicated in the diagram and the refractive index of glass calculated.
(b) Water or other liquid. The same method as that described above may be used except that the glass block is replaced by a tall beaker containing the liquid and having a pin lying on the bottom. However, it will be found easier to adjust for no parallax if we use the fact that the image in a plane mirror is as far behind the mirror as the object is in front (Fig. 23.10 (b». A slip of mirror rests across the top of the beaker, and a sliding pin above it is
adjusted until there is no parallax between the image of this pin in the mirror and the image of the pin lying on the bottom of the beaker. . real depth y
Fig. 23.10 (b) makes It clear that n = d h apparent ept x – z Several readings may be obtained, each time varying the depth of the liquid. A graph of real depth against apparent depth is plotted and a mean value of n obtained from its gradient.