sebatang lilin menyala berada didepan cermin cekung seperti gambar. jika jarak fokus 15cm, maka tentukan jarak bayangan dan perbesaran bayangan!

f = 15 cm
s = 20 cm
s'= ?
M = ?
Penyelesaian:

1) s' = ?

1/s' = 1/f - 1/s
1/s' = 1/15 - 1/20
1/s' = 4/60 - 3/60
1/s' = 1/60
s' = 60 cm

2) M = ?
M = |s'/s|
M = |60/20|
M = 3

Optik Geometri.

f = 15 cm dan s = 20 cm
1 / f = 1 / s + 1 / s'
s' = sf / (s - f)
   = (20 cm)(15 cm) / (20 cm - 15 cm) = 60 cm

M = -s' / s
    = -60 cm / (20 cm) = -3 kali ← Bayangan terbalik.