You are provided with a converging lens and holder, a screen, a ray box containing an illuminated object pin, and a meter rule. i. Place the lens in its hol...
Question 1 Report
You are provided with a converging lens and holder, a screen, a ray box containing an illuminated object pin, and a meter rule.
i. Place the lens in its holder such that it is facing a distant object seen through a well-lit laboratory window. Move the screen to and fro until a sharp image of the distant object is formed on it. Measure the distance, \(f_{0}\), between the screen and the lens.
ii. Clamp the meter rule securely to the table. Place the illuminated object pin at the end R of the meter rule.
iii. Place the lens at a position P such that \(X = RP =\) 20cm.
iv. Move the screen to a position Q to receive a sharp image of the object. Measure the distance \(Y = PQ\).
v. Evaluate \(Z = (X+Y)\)
vi. Repeat the procedure for five other values of x = 25cm. 3Ocm, 35cm, 40cm and 45cm. In each case, record X,Y and evaluate Z.
vii. Tabulate the results.
viii. Plot a graph with Z on the vertical axis and X on the horizontal axis. Draw a smooth curve through the points.
ix. Determine from your graph the minimum value of \(Z=Z_{0}\) and its corresponding distance
x. Evaluate \(W = ½ \left(\frac{Z_0}{4} + \frac{X_0}{2}\right)\)
xi. State two precautions taken to ensure accurate results.
(b) i. Draw a ray diagram to show how a Convex lens forms an image of magnification less than one.
ii. Name two pairs of features in the human eye and a lens camera that performs similar functions.
Answer Details
(a) Determination of the focal length of a converging lens
On focusing the distant object sharply on the screen, the focal length obtained was:
\(f_0 = 13.0\text{ cm}\).
The readings obtained with the illuminated object pin were as follows.
| S/N | \(X=RP\) (cm) | \(Y=PQ\) (cm) | \(Z=X+Y\) (cm) |
|---|---|---|---|
| 1 | 20 | 60 | 80 |
| 2 | 25 | 38 | 63 |
| 3 | 30 | 30 | 60 |
| 4 | 35 | 26 | 61 |
| 5 | 40 | 24 | 64 |
| 6 | 45 | 21 | 66 |
Graph of \(Z\) against \(X\):
From the lowest point of the curve,
\(Z_0=60\text{ cm}\), and \(X_0=30\text{ cm}\).
\[W=\frac{1}{2}\left(\frac{Z_0}{4}+\frac{X_0}{2}\right)=\frac{1}{2}\left(\frac{60}{4}+\frac{30}{2}\right)=\frac{1}{2}(15+15)=15\text{ cm}.\]
Precautions
- The object pin, optical centre of the lens and centre of the screen were kept at the same height and on one straight line.
- The screen was moved until the image was sharply focused, and metre-rule readings were taken with the eye vertically above the mark to avoid parallax.
(b)(i) Ray diagram for a diminished image formed by a convex lens
The object is placed beyond \(2F_1\). The image formed is real, inverted and diminished, between \(F_2\) and \(2F_2\).
(b)(ii) Corresponding parts of the human eye and lens camera
| Human eye | Lens camera |
|---|---|
| Retina | Film or image sensor |
| Eye lens | Camera lens |
| Iris | Diaphragm |
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