Briefly describe one way to obtain an image on the screen that is as sharp as possible in this experiment.
Using Fig. 3.2, measure the image height $h_I$ and write it in the first line of Table 3.1.
Calculate, and write in Table 3.1, a value for D. Use your $h_I$ from Table 3.1 together with $D=\frac{1}{h_I}$.
Draw a graph of $u$ / cm on the $y$-axis against $D$ / $(1/\text{cm})$ on the $x$-axis. Begin the axes at the origin $(0,0)$. Add a straight best-fit line.
Use your graph to determine $u_0$, the value of $u$ when D is zero.
Determine the gradient G of the graph. Make it clear on the graph how you obtained the required information.
Measure and record the height $h_O$ of the illuminated triangular object shown in Fig. 3.3. Calculate the focal length f of the lens. Use your $h_O$, your $G$ from (d)(ii) and the equation: $f=\frac{G}{h_O}\times k$, where $k=1.0\text{ cm}^2$.
Describe one difficulty that may arise when measuring the height of the image in this experiment. Suggest one improvement that would remove or reduce this difficulty.