• How do you find the angle of deviation?
Extend both incident and emergent rays till they meet at a point ‘O’. Measure the angle between these two rays. This is the angle of deviation. It is denoted by a letter ‘d’. Note it in table (1). Repeat this procedure for various angles of incidence such as 400 ,500 etc. Find the corresponding angles of deviation and angles of emergence and note them in table (1).
• What do you notice from the angles of deviation? You will notice that the angle of deviation decreases first and then increases with increase in the angle of incidence.
• Can you draw a graph between angle of incidence and angle of deviation?
Take angle of incidence along X- axis and the angle of deviation along Y- axis. Using a suitable scale, mark points on a graph paper for every pair of angles. Finally join the points to obtain a graph (smooth curve). Check your graph with graph shown in figure 9.
• From the graph, can you find the minimum of the angles of deviation?
Yes we can. Draw a tangent line to the curve, parallel to X- axis, at the lowest point of the graph. The point where this line cuts the Y- axis gives the angle of minimum deviation. It is denoted by D. Draw a parallel line to y-axis through the point where the tangent touches the graph. This line meets x-axis at a point showing the angle of incidence corresponding to the minimum deviation. If you do the experiment with this angle of incidence you will get an angle of emergence equal to the angle of incidence. Look at your table (1).
•Is there any relation between the angle of incidence, angle of emergence and angle of deviation?
•Can you find refractive index of a prism? If yes, how?
