Ketika benda diletakkan di depan cermin cekung dg jarak 8cm,jarak bayangan yg terbentuk adalah 24cm.Jika jarak benda digeser 2cm menjauhi cermin,jarak bayangan

1/f = 1/s + 1/s'
1/f = 1/8 + 1/24
1/f = 3/24 + 1/24
1/f = 4/24
1/f = 1/6
f = 6 cm

jika jarak benda digeser 2 cm menjauhi cermin , maka :
s = 8 + 2 = 10 cm
jadi :
1/f = 1/s + 1/s'
1/6 = 1/10 + 1/s'
1/6 - 1/10 = 1/s'
5/30 - 3/30 = 1/s'
2/30 = 1/s'
1/15 = 1/s'
s' = 15 cm (b)