A student wants to determine the focal length $f$ of a lens. An arrow-shaped slit cut in a card is used as the object, as shown in Fig. 1.1.
The object is positioned at one end of a metre rule, and the lens is held a distance $u$ from the object, as illustrated in Fig. 1.2. The slit is lit from behind by a lamp.
A screen is then set on the metre rule until a sharp, focused image of the arrow is produced. The distance $v$ from the lens to the screen is then measured.
(a)[1]
Describe how the student makes sure that the image on the screen is sharply focussed.
(b(i))[1]
Use Fig. 1.3 to measure $u$ and $v$.
(b(ii))[1]
Use the equation $f = \frac{uv}{v + u}$ to calculate $f$ from your readings in (b)(i).
(b(iii))[1]
Suggest one method the student uses to make his value of $f$ accurate.
Worked solution & mark scheme
This 4-mark question has a full step-by-step worked solution and mark scheme. One marking point: “Moves screen forwards and backwards / moves screen slowly” …