Question 20N.3.SL.TZ0.11

wavefront separation identical and equal to separation before the lens ✓

wavefronts converging, approximately centered on $f$ ✓

By eye.

Dotted construction lines are not required, allow wavefronts to extend beyond or be inside the dotted lines here.

Allow [1 max] if only two wavefronts drawn. 

$\frac{1}{v}=\frac{1}{4.00}-\frac{1}{4.50}$ ✓

$v=36.0 «\mathrm{cm}»$ ✓

$\mathrm{A}:\hspace{0.17em}\frac{1}{-2.0}=\frac{1}{8}+\frac{1}{u}$ ✓

$u=-1.6 «\mathrm{cm}»$ ✓

distance necessary =$«36.0-1.6=»34.4 «\mathrm{cm}»$ ✓

Allow [2 max] for ECF for no negative in MP1. Gives $u=\mathit{2}\mathit{.}\mathit{7}$ and distance of $\mathit{38}\mathit{.}\mathit{7}«\mathrm{cm}»$

Allow ECF from (b) in MP3.EG use of $\mathit{0}\mathit{.}\mathit{4}\mathit{ }m\mathit{/}\mathit{40}\mathit{ }cm$.

«$m=-\frac{i}{o}=\frac{-36}{4.5}$ for A or $\frac{-8}{-1.6}$ for B»

$m_{\mathrm{A}}=«–»8$  OR  $m_{\mathrm{B}}=«+»5$ ✓

total magnification $=«–» 40$ ✓

Allow [2] marks for a bald correct answer

Allow ECF from (b) and (c).

Eg if $u=\mathit{2}\mathit{.}\mathit{7}\mathit{ }cm$ in (c) then $m_{\mathit{B}}\mathit{=}\mathit{3}$ and total $m\mathit{=}\mathit{24}$