Measure the distance $x$ on Fig. 3.1 from the lens to the screen, and record it in centimetres to the nearest $0.1\,\text{cm}$.
Use your answer from (a)(i) to find the true distance between the lens and the screen. This distance is the focal length $f$ of the lens.
Calculate the values of $(u+v)$ and $uv$.
Add the appropriate units to the headings of the last two columns.
On the grid in Fig. 3.3, plot $uv$ on the $y$-axis against $(u+v)$ on the $x$-axis, then draw the straight line of best fit.
Calculate the line's gradient.
Compare your value of $f$ from (a)(ii) with the gradient value from (e). State whether the two values can be regarded as the same, and support your statement with a calculation.
State how the student avoids parallax errors during the experiment.
Describe a technique the student uses to make sure that the image on the screen is as sharply focused as possible.