In this experiment, you will ha two converging lenses, lens P and lens Q. You is asked to plan an experiment to determine which lens has the shorter focal l...

(a) Method to measure the focal length of one lens: [4]

Place the illuminated crosswire (object) at one end of the optical bench. [1] Position the converging lens in a holder at a measured distance u from the crosswire. [1] Place the white screen on the other side of the lens and slide it along the bench until a sharp image of the crosswire forms on the screen. Record the image distance v from the lens to the screen. [1]

Labelled diagram of the arrangement:

The diagram shows the object, lens and screen in a line on the optical bench, with u and v marked. [1]

(b) Measurements to record: [2]

• The distance u from the object (crosswire) to the lens for each position. [1]
• The distance v from the lens to the screen when the sharpest image is obtained. Repeat for at least three different values of u. [1]

(c) The equation to calculate the focal length from the measurements is:

\[ f = \frac{uv}{u + v} \] [1]

(This is a rearrangement of \( 1/f = 1/u + 1/v \).)

(d) To improve accuracy: [1]

Repeat each measurement of v at each value of u and take the average. Use at least five different object distances, calculate f from each, and average all the values of f. This reduces the effect of random errors in judging the position of the sharpest image.

(e) To determine which lens has the shorter focal length: [1]

Calculate the focal length for each lens using the method above. The lens with the smaller value of f has the shorter focal length. A shorter focal length means the lens is more powerful (bends light more).

(f) One safety precaution (any one): [1]

• Do not look directly at the bright lamp, as it may damage the eyes.
• Handle the lenses carefully to avoid breakage (glass edges can cause cuts).
• Do not leave the lamp on for long periods close to combustible materials, to avoid overheating.