## Friday

### (EXPO) SOLUTION TO 2015 WAEC PHYSICS PRACTICAL

You are provided with a triangular glass prism, four optical pins and other necessary materials.

Place the triangular glass prism on a drawing paper and draw its outline UMR.  Remove the prism.  Measure and record the value of the angle U.  Draw a normal to the line UM at N.  Also, draw another line TN to the normal such that Φ = 60o.  Fix two pins at P1 and P2.
Replace the prism and fix two other pins at P3and P4such that the pins appear to be in a straight line with the images of the pins at P1and P2when viewed from the side UR.  Remove the prism.

Join points P3and P4 producing the line to meet TN produced at Z.  Draw the normal XY
Measure and record the angle of emergence, e and that of deviation d.
Repeat the experiment with Φ = 55o, 50o, 40o. and 35o.  In each case, measure and record the corresponding values of e and d.
Plot a graph of d on the vertical axis and e on the horizontal axis starting both axes from the origin (0,0).  Join your points with a smooth curve.
From your graph, obtain the minimum deviation dm and the corresponding angle of emergence em. Hence calculate the refractive index n of the prism using the formula.

State two precautions taken to ensure accurate results

(b)(i)  State the conditions necessary for total internal reflection of light to occur.

(ii)The critical angle for a transparent substance is 39o.  Calculate the refractive index of the substance

The expected responses were as follows:

In part (a) the candidates were required to perform the following tasks to score good points:

Set up an experiment as shown in the diagram;
Draw five complete traces showing at least incident ray, emergent ray and angle of deviation;
Record in degree the value of U;
Measure and record five values of e;
Measure and record five values of d;
(Trend: d decreases, later increases, as Φ decreases)

Record data in a composite table showing Φ, e  and d;
Indicate the graph axes,  select reasonable scales, plot the five points;
Draw a smooth curve through the points;
Show and record minimum deviation dm and the corresponding angle of emergence em;
Calculate the refractive index n using the given expression;
State any two of the following precautions in acceptable form of speech:
Evidence of sharp pencil/neat traces;
Evidence of pin spacing;
Ensured pins were vertical;
Avoided parallax error when reading protractor.
The expected responses to part (b) were as follows:

(i) state the conditions Conditions necessary for total internal reflection of light:
-    Light must be traveling from a denser medium to a less dense medium;
-    Angle of incidence in the dense medium must be greater than the critical                                                      angle of the dense medium.

(ii)    Calculation of refractive index, n

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