Intro
Accommodation is the ability of the eye to focus on objects at different distances. This phenomenon occurs because the lens is able to change shape, altering it focal distance. The effect of lens shape on focal distance can be modelled using bi-convex lenses of different thicknesses.
Materials needed
White cardboard, 5cm candle attached to a jar with plasticine, bi-convex lenses of different thicknesses, ruler
Procedure
Set up the equipment as shown: (draw diagram showing scrren at one end and the candle at other with the lens in between on the ruler to vary distances)
(a) move the lens until a sharp image of the candle is produced on the cardboard screen. Measure the distance from the mid-line of the lens to the sharp image (focal distance). Repeat this procedure with lenses of different thicknesses and record your results in table
(b) now leave the screen and the lens in the same position. For each differently shaped lens, move the candle until a sharp image appears on the screen. Record the distance of the candle from the lens in each case.
Results
Lens thicknesses a) Focal length b) Dist. of candle from lens
Thin 27cm > 100cm
Medium 22 cm 98cm
Thick 5cm 6cm
(then there are some general questions to follow up)
