Physics
WAEC 2013
Using the diagram above as a guide, carry out the following instructions:
- Fix a plain sheet of paper on the Celotex board.
- Place the rectangular glass prism on the paper and trace its outline, ABCD. Remove the prism.
- Draw a normal NMP to meet AB and DC at M and P respectively such that |AM|=DP=2.0cm.
- Trace the ray PQ with two pins, P\(_{1}\) and P\(_{2}\) at P and Q respectively such that angle MPQ = i =50º
- Replace the prism on its outline. Trace the emergent ray with two other pins P\(_{3}\) and P\(_{4}\) such that they lie in a straight line with P\(_{2}\) and the image of P\(_{1}\) viewed through the glass prism.
- Measure and record \(\theta\), the angle between the emergent ray and face AB of the glass prism.
- Evaluate cos \(\theta\) and sin i.
- Repeat the procedure for four other values of i = 10°, 15°,20° and25°.
- Tabulate your readings.
- Plot a graph with cos \(\theta\) on the vertical axis and sin i on the horizontal axis.
- Determine the slope, s, of the graph
- State two precautions taken to ensure accurate results. Attach your traces to your answer booklet
(b)i. State the laws of refraction of light
ii. Explain what is meant by the statement the refractive index of a material is 1.65
Explanation
| i/o |
\(\theta\)/0 |
cos\(\theta\) |
sini |
| 5.0 |
83.0 |
0.137 |
0.087 |
| 10.0 |
74.0 |
0.276 |
0.174 |
| 15.0 |
65.0 |
0.423 |
0.259 |
| 20.0 |
56.0 |
0.559 |
0.342 |
| 25.0 |
48.0 |
0.669 |
0.423 |
Slope = \(\frac{\bigtriangleup \text{cos}\theta}{\bigtriangleup \text{sin i}}\) = \(\frac{0.720-0.250}{0.450-0.150}\) = \(\frac{0.47}{0.30}\) = 1.567
PRECAUTIONS; i
- Ensured pins were erect/upright/metre rule
- Avoided parallax error on the protractor/metre rule
- Neat traces or sharp pencils shown on traces
- Pins reasonably spaced i.e about 4cm
- Noted and corrected/avoided zero error on metre rule.
(b)i. The incident ray, refracted ray was normal at the point of incidence. i.e in the same plane. The ratio of the sine of the angle of incidence to the sine of the angle of refraction is a constant for a given pair of media.
OR nisini = n\(_{2}\) sinr where the symbols are defined.
OR \(\frac{\text{sini}}{\text{sinr}}\) = N
for a given pair of media, where symbols are defined.
ii. The ratio of the speed of light in vacuum (air) to the speed of light in the material is 1.65
OR
The ratio of the angle of incidence in air/vacuum to the sine of the angle of refraction in the material is 1.65.
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