The diagram above illustrates an experimental set-up for the determination of the focal length of a lens. The principal foci A and B of the lens were determined.

The image of an illuminated object O placed at a distance X from A is obtained on a screen S placed a distance Y from B.

The procedure is repeated for five other values of X and the corresponding values of Y were measured and recorded.

Fig. 2(a) on page 8, represents the distances xi between A and the illuminated object O.

Fig. 2(a) on page 8, represents the corresponding distances yi between B and S, where i = 1,2,3,4,5 and 6.

(i) Measure and record the raw values of x = AOi.

(ii) Measure and record the corresponding raw values of y = BSi.

(iii) Use the given scales to convert xiand yi to the real values Xiand Yirespectively.

(iv) In each case, evaluate Y-1

(v) Plot a graph with X on the vertical axis and Y-1 on the horizontal axis. (vi) Determine the slope, s, f the graph.

(vii) Evaluate K= √(s

(viii) State two precautions necessary if you were to perform this experiment in the laboratory.

(b) (i) State two differences between a real image and a virtual image.

(ii) Define the focal length of a converging lens.

The expected response:



Six values of xcorrectly measured and recorded to at least 1 d.p. and within

tolerance of ± 0.1 cm

(Award ½ mark each)

Six values of ycorrectlymeasured and recorded to at least 1 d.p and within

tolerance of ± 0.1 cm

(Award ½ mark each)

Six values of X correctly evaluated

(Deduct ½ mark for each wrong or missing value)

Six values of Ycorrectly evaluated

(Deduct ½ mark for each wrong or missing value)

Six valves of Y-1 correctly evaluated to 3 dp.

(Deduct ½ mark for each wrong or missing value)

Composite table showing at least x,y,x,y,and y-1

Plot a graph of reasonable scales

Draw line of best fit

Determine the slope

Evaluate k√s

State any two of the following precautions

e.g

- Ensure clean lens

- Ensure the lens stands vertically in the holder

- Ensure sharp image of object

- Ensure collinear arrangement of apparatus

- Avoid parallax error on metre rule

- Note zero error on metre rule

- Repeated reading stated



(b)(i)

Real image	Virtual image
Formed by actual intersection of rays	Formed by apparent intersection of rays
Can be formed on a screen	Cannot be formed on a screen
Inverted	Erect

(ii) Focal length of a converging lens is the distance between the optical centre and the principal focus of the lens.