Question

PRACTICAL 1. You are provided with a rectangular block, a drawing board, drawing sheets, four optical pins and other necessary materials. Use the given diagram as a guide to: measure and record the width of the block; draw the outline ABCD of the block on a drawing sheet; mark a point, Q, on AB such that AQ=1.5cm; draw the normal RQT; draw a line UQ making an angle of 10° with the normal RQT; insert pins P1 and P2 on UQ, look through side DC and insert two other pins P3 and P4 so they appear in line with the images of P1 and P2; remove the block and mark the positions of P3 and P4; draw a straight line joining P3 and P4 and extend it to meet DC at E; produce a line UQ to meet DC at the point F; measure and record the angle r, length EF=D, and evaluate tan i, tan r and V=(tan i - tan r); repeat for i=20°,30°,40° and 50°, recording r, d and V each time; tabulate the results; plot a graph with d on the vertical axis and V on the horizontal axis; determine the slope, s, of the graph; state two precautions taken to ensure good results.

Accepted Answer

This experiment determines the refractive index of the rectangular glass block using the pin (real/apparent depth) method. For each angle of incidence i (20°,30°,40°,50°), the corresponding angle of refraction r is measured, and tan i and tan r are recorded along with V=tan i-tan r and the displacement d=EF. Plotting d (vertical axis) against V (horizontal axis) gives a straight line through the origin, and the slope, s, of this graph relates to the refractive index and width of the block (s=width/refractive index, approximately). Precautions: avoid parallax error when taking readings on the metre rule/drawing sheet; ensure the pins are vertically fixed and properly aligned before taking sightings; ensure the block does not move while marking pin positions.

WAEC Physics 2023 Theory — Question 13

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