Alternative A
Question 2
- You are provided with a converging lens and holder; a screen, a ray box containing
an illuminated object pin and a meter rule.
- 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, fo , between the screen and the lens.
- Clamp the meter rule securely to the table.
- Place the illuminated object pin at the end R of the meter rule. Place the lens at a position P such that X = RP = 20 cm.
- Move the screen to a position Q to receive a sharp image of the object. Measure the distance Y = PQ.
- Evaluate Z = (X + Y).
- Repeat the procedure for five other values of x = 25 cm, 30 cm, 40 cm and 45 cm. In each case, record X, Y and evaluate Z.
- Tabulate the results.
- Plot a graph with Z on the vertical axis and X on the horizontal axis. Draw a smooth curve through the points.
- Determine from your graph the minimum value of Z = Zo and its corresponding distance Xo.
- Evaluate W = ½(Zo/4+Xo/2)
- State two precautions taken to ensure accurate results.
-
- Draw a ray diagram to show how a convex lens forms an image of magnification less than one.
- Name two pairs of features in the human eye and a lens camera that performs similar functions.
Observation
Part (a) This question was poorly attempted. Many candidates failed record findings with the
appropriate decimal places. Many could not identify the minimum point on the graph of Z against
X.
Part (b) Candidates performance was poor.
The expected response:
2(a) OBSERVATIONS [09]
(i) fo correctly recorded to at least 1 d.p. in cm.
(ii) Six values of x or (X) correctly recorded to at least 1 d.p. in cm.
(Deduct ½ mark for each wrong or missing value)
(iii) Six values of Y correctly recorded to at least 1 d.p. in cm
and in trend.
(Trend: As x increases, Y decreases)
(Deduct 1 mark for each wrong or missing value)
(iv) Six values of Z = (X + Y)
(Deduct ½ mark for each wrong or missing value)
(v) Composite table showing at least x or (X), Y and Z.
GRAPH [06]
(i) Both Axes correctly distinguished (½ mark each)
(ii) Reasonable Scales (½ mark each)
(iii) Six points correctly plotted
(Award ½ mark each)
(iv) Curve of best fit
DEDUCTIONS [02]
(i) Determination of Z0 [01]
Correctly shown
Correctly read
(ii) Determination of X0 [01]
Correctly shown
Correctly read
EVALUATION OF W [01]
W = ½(Zo/4+Xo/2)
correct substitution
Correct arithmetic
ACCURACY [01]
Based on W = f0 (candidate’s value) 10%
PRECAUTIONS [02]
Award 1 mark each for any 2 correct precautions stated in acceptable tense.
e.g.
Coaxial arrangement of optical instruments/Ray box, lens and screen on a straight line
Avoided parallax error in reading metre rule
Lens kept upright
Repeated readings shown on table
Surface of lens cleaned
Noted/corrected/Avoided zero error on metre rule
(b)(i)
Position of object beyond 2f
Two correct incident rays
Two correct refracted rays
Image between f and 2f
NOTE: If no arrows on rays, deduct ½ mark
(ii) Features performing the same function
| Human eye | Lens camera |
| Retina | Film/screen |
| Eye lens | Camera lens |
| Iris | Diaphragm |
| Pupil | Aperture |