Question 2A
The principal foci, P and Q, of a converging lens were determined. The image of an illuminated object O, placed at a distance, m, from P is obtained on a screen S placed at a distance n from Q as shown in the diagram above. The procedure is repeated for various values of m and the corresponding values of n were measured and recorded.
In figure 2(a), POi represents the distance mi between P and the positions of the illuminated object Oi.
In figure 2(b), QS represents the distance n, between Q and the screen S, where i = 1, 2, 3, 4, 5 and 6.
- Measure and record the values of m = POi using the given scale.
- Measure and record the corresponding values of n = QSi using the given scale.
- Evaluate n-1 in each case.
- Tabulate your readings.
- Plot a graph with n-1 on the vertical axis and m on the horizontal axis.
- Determine the slope, s, of the graph.
- State two precautions that are necessary to ensure accurate results when performing this experiment.
b (i) State two applications of a converging lens.
(ii) An object is placed 10 cm from a converging lens. When a plane mirror is placed vertically behind the lens, the image formed coincides with the object. Calculate the focal length of the lens.
Observation
Part (a) Performance was below average. Majority of the candidates could not convert the raw values of m and n using the given scale. Most candidate evaluated for n-1 instead of n-1 and failed to calculate to 3 d.p.
Part (b) Candidates’ responded poorly in this part as many candidates lack the knowledge about the theory of the experiment. They lack the knowledge that when object/image is at infinity, the image/object is at the principal focus. Finally, candidates failed to realise that the reciprocal of infinity is zero.
The expected answers are that candidates should:
- Measure correctly six values of m = PO
- Convert six values of m
- Measure correctly six values of n = QS
- Convert six values of n correctly converted
- Evaluate correctly six values of n-1
- Show composite with m, n and n-1
- Plot a graph using reasonable scales
- draw line of best it
- determine the slope of the graph
- state any two of the following precautions.
e.g.
- Ensured lens is vertical
- Avoided parallax error on rule
- Noted/corrected zero error on rule
- Ensured collinearity of apparatus
- Repeated readings stated
- Ensured sharp image
b(i) Application of converging lens
e.g - (Photographic) camera
- Periscope
- Telescopes
- Correcting eye defects/long sightedness/hypermetropia
- Microscopes/hand lens/simple microscope
- Binoculars
- Projector
