- Determine and record the approximate focal length f\(_{o}\) of the concave mirror provided.
- Arrange the ray box, the mirror, and the screen as shown in the diagram above.
- Adjust the ray box to a distance b = 20.0cm from the mirror.
- Adjust the position of the screen until a sharp image of the cross wire of the ray box is formed on it.
- Measure and record the distance, a, of the screen from the mirror. Evaluate \(\frac{a}{a}\) =1.
- Repeat the procedure for four other values of b = 25.0, 30.0, 35.0, and 40.0cm. Tabulate your readings.
- Plot a graph of l on the vertical axis against a on the horizontal axis.
- Determine the slope,.s, of the graph. Evaluate S\(^{-1}\).
- State two precautions taken to ensure accurate results.
(b)i. An object is placed at a distance of 10cm in front of a concave mirror of focal length of 15cm. Determine the characteristics of the image formed.
ii. Briefly describe how you obtained f\(_{o}\) in (a)i) above.
Explanation
1. f\(_{o}\) = 15cm
5. a = 60.0cm, b = 20.0cm
Hence, L = \(\frac{a}{b} = \frac{60}{20}\) = 3
6. Tables of values/observation
| S/N | b(cm) | a(cm) | L=\(\frac{a}{b}\) |
| 1 | 20.0 | 60.00 | 3.00 |
| 2 | 25.0 | 37.00 | 1.50 |
| 3 | 30.0 | 30.00 | 1.00 |
| 4 | 35.0 | 26.00 | 0.75 |
| 5 | 40.0 | 24.00 | 0.6 |
7.
8. Slope (s) = \(\frac{\bigtriangleup {L }}{\bigtriangleup {a}} = \frac{3-0.25}{60-18.6cm}\)
= \(\frac{2.75}{41.4}\) = 0.066425cm\(^{-1}\)
S\(^{-1}\) = \(\frac{1}{s} = \frac{1}{0.066425(cm)^{-1}}\)
= 15.05 = 15
N.B: S\(^{-1}\) = f\(_{o}\) = 15cm
Precautions taken to ensure accurate results are as follows:
- Parallax error in reading meter rule avoided
- Readings repeated must De shown on the table
- Collinear alignment of apparatus ensured
(b)i. u = 10cm, f = +15cm, V= ?
\(\frac{1}{v} + \frac{1}{u} = \frac{1}{f }\)
\(\frac{1}{v} + \frac{1}{10} = \frac{1}{15}\)
\(\frac{1}{v} = \frac{1}{15} - \frac{1}{10} = \frac{2-3}{30} =\frac{1}{30}\)
= The characteristics of the image formed are:
- it is virtual
- it is enlarged or magnified i.e twice or two times as big as the object (m = 2)