On Fig. 2.1, construct a normal to XY at point M and continue it on both sides of the line XY.
From M, draw a line to the left of the normal above line XY so that it makes an angle of 40^{\circ} with the normal. This 40^{\circ} angle represents the angle of incidence. Mark the upper left end of the line as point L.
A narrow beam from an illuminated slit is directed along line LM by the student. The emerging ray is indicated by two crosses as shown in Fig. 2.1. Join the marked crosses and continue the line until it meets the lower edge of the block outline. Label the point as P.
Draw a straight line joining points M and P.
The angle of refraction r is the angle formed between line MP and the normal drawn in (a)(i). Measure angle r and record it.
For the transparent block, the refractive index n is defined by $n=\frac{\sin 40^\circ}{\sin r}$. Calculate n and express your answer to 2 significant figures.
Suggest a modification to the experiment that would ensure your value of n is accurate.
The theory says that the angle between the emerging ray and the normal at the point where the ray exits should be the same as the angle of incidence. In (a)(i), the angle of incidence is given as 40^{\circ}. Add a second normal to your diagram. Measure and record the angle $\alpha$ between the emerging ray and the normal. Explain whether your measurements agree with this theory.