Closing your one eye ,adjust the position of your head in such a way that the images of the pins A and B lie in the same straight line. Draw a line AE making angle AEN1 with the normal.The angle should neither too small nor too large (say about 40 degree).
On the line PQ mark a point E and draw a normal N1EN2 at it.Mark the boundary of the glass slab with a sharp pencil and label it as PQRS after removing the slab from its position.Place the given glass slab nearly in the middle of the sheet. Fix a sheet of white paper on a drawing board with drawing pins.A drawing board, rectangular glass slab, office pins, sheet of white paper, a protractor and sharply pointed pencil.
