Very few candidates attempted this question and almost all the responding candidates missed it. Performance was poor as most candidates did not know that u = a + f and
v = b+ f
(ii) This part was fairly well attempted. Most candidates were able to calculate f.
(iii) This was fairly well attempted.
Part (b): Few candidates could not properly explain (i),(ii) and (iii). However,
performance was average.
The expected answers are:
(a)(i) u = a + f
v = b + f
= +
= +
=
( )
( ) = )
ab +af +bf + f2 = bf + f2 + af + f2
f2 = ab
(ii) 2 = ab
= 16 x 25
f = 20cm
(iii) Magnification =
=
= 1.25
b(i) Principal focus of a lens/ mirror is the point on the principal axis to which all rays parallel and close to the principal axis either converge or (appear) to diverge after refraction/ reflection by the lens/ mirror
(ii) Optical centre is a point through which rays of light pass without deviation by the lens
(iii) Focal length is the distance between the optical centre/ centre of curvature and the principal focus of a lens/ mirror .Additional information: e.g. f = ; diagrams e.t.c