Determination of the refractive index of glass using the travelling microscope and a block of glass

The distance from the upper surface of the glass block to the mark 'x' on the paper is the real depth. The distance to the image when the mark is observed normally through the glass block from above is the apparent depth.

If $_{a}n_{g}$ is the refractive index of glass relative to air,

$$_{a}n_{g}=\frac{\text{real depth}}{\text{apparent depth}}$$

When the travelling microscope is focused on to the mark if $x$ is the reading on the vertical scale, when focused on to the image of the mark the reading is $y$ and when focused on the upper surface of the block the reading is $z$,

the real depth $= z-x$, apparent depth $= z-y$

$$_{a}n_{g}=\frac{z-x}{z-y}$$

1. Mark a cross (x) by ink on the white sheet of paper placed at the base of the travelling microscope.
2. As shown in Figure 16.1 place the microscope directly above the mark x pointing towards it and focus on the mark. Obtain the reading ($x$) on the vertical scale.
3. Now place the given glass block on the ink mark.
4. As shown in Figure 16.2 raise the microscope along the scale and focus on the image of the ink mark. Obtain the reading $y$ on the vertical scale.
5. Finally spread the given powder on the upper surface of the glass block vertically above the ink mark.
6. As shown in Figure 16.3 raise the microscope further along the vertical scale and focus on the powder spread on the glass surface. Observe and note down the reading $z$ on the vertical scale.
7. Record all the readings in the table.