Using the above diagram as a guide, carry out the following experiment: place the equilateral triangular glass prism on the drawing paper. Trace the outline...
Question 1 Report
Using the above diagram as a guide, carry out the following experiment:
- place the equilateral triangular glass prism on the drawing paper. Trace the outline ABC of the prism.
- remove the prism. Draw a line NO such that it makes an angle \(i\) = 25º with the normal at point O on side AB
- fix two pins \(R_{1}\) and \(R_{2}\) vertically on line N0. Replace the prism on its outline;
- place the reflecting surface of the plane mirror in contact with the face AC of the prism;
- looking through the face BC of the prism, fix two other pins at \(R_{3}\) and \(R_{4}\) such that the pins appear to be in a straight line with the images of the pins at R\(_{1}\) and R\(_{2}\);
- remove the prism, the mirror, and the pins. Draw a line to join points R\(_{4}\) and R\(_{3}\)
- produce line \(R_{4}R_{3}\) to meet line NO produced at T;
- measure and record the angle \(\theta\) at T and e at D;
- repeat the procedure for four other values of \(i\) = 30°, 35°, 40°, and 45°. In each case, measure and record the corresponding values of \(\theta\) and e. Tabulate your readings;
- plot a graph of e on the vertical axis and \(\theta\) on the horizontal axis;
- evaluate \(k = s^{-1}\)
- state two precautions taken to ensure accurate results. [Attach your traces to your answer booklet]
(b)i. State four characteristics of the image of an object formed by a plane mirror.
ii. State two Conditions necessary for total internal reflection to occur in a medium.
Answer Details
Practical: deviation through a prism with a plane mirror on face AC
The prism is placed on its traced outline ABC and light is aimed at the point O on face AB at an angle of incidence \(i\). The plane mirror is held against face AC. For each value of \(i\) the emergent direction is fixed by the no-parallax pin method: pins \(R_1\) and \(R_2\) are set on the incident line NO, and pins \(R_3\) and \(R_4\) are set while viewing through face BC. Line \(R_4R_3\) is produced to meet NO produced at T, where the angle \(\theta\) is measured, and the angle \(e\) is measured at D. The construction is shown below.
Table of readings (observations)
| S/N | i / ° | θ / ° | e / ° |
|---|---|---|---|
| 1 | 25 | 62 | 28 |
| 2 | 30 | 54 | 32 |
| 3 | 35 | 45 | 35 |
| 4 | 40 | 40 | 37 |
| 5 | 45 | 30 | 41 |
Graph of e against \(\theta\)
A graph of \(e\) (vertical axis) against \(\theta\) (horizontal axis) is plotted, and a straight line of best fit is drawn through the points.
Slope and evaluation of k
Two widely separated points are read from the line of best fit: \((\theta_1,e_1)=(10,49)\) and \((\theta_2,e_2)=(83,20)\).
\[ s=\frac{\Delta e}{\Delta\theta}=\frac{20-49}{83-10}=\frac{-29}{73}=-0.397 \]The magnitude of the slope is \(|s|=0.397\) and the intercept on the \(e\)-axis is \(c=53\). Therefore
\[ k=s^{-1}=\frac{1}{0.397}=2.52 \]Two precautions
- The plane mirror was pressed firmly against face AC so that it did not shift while the emergent pins were being fixed.
- All pins were fixed vertically and spaced about 4 cm apart, and viewing was done with the eye low so that parallax between the pins and their images was eliminated.
(b)(i) Four characteristics of the image formed by a plane mirror
- It is virtual (cannot be received on a screen).
- It is erect (upright).
- It is the same size as the object.
- It is laterally inverted and is the same distance behind the mirror as the object is in front.
(b)(ii) Conditions for total internal reflection
- Light must be travelling from an optically denser medium to an optically less dense medium.
- The angle of incidence in the denser medium must be greater than the critical angle for the two media.
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