Page 267 - text book form physics kssm 2020

Chapter 6
Light and Optics
Solving Problems Involving Thin Lens Formula

Table 6.10 Sign convention used in thin lens formula

Positive (+) Negative (−)
Converging lens or convex lens Diverging lens or concave lens
Focal length, f
• Real image • Virtual image
Image distance, v
• On the opposite side of the object • On the same side as the object

Example 1

A thin convex lens has a focal length of 12 cm. Determine the characteristics and the position
of the images formed and linear magnifi cation when the object distance is:
(a) 18 cm
(b) 4 cm

Solution:

(a) u = +18 cm (b) u = +4 cm
f = +12 cm f = +12 cm
1 1 1 1 1 1
Th in lens formula, = + Th in lens formula, = +
f u v f u v
1 1 1 1 1 1
= − = −
v f u v f u
1 1 1 1
= − = −
12 18 12 4
v = +36 cm v = –6 cm
v v
Linear magnifi cation, m = Linear magnifi cation, m =
u u
36 6
= =
18 4
= 2 = 1.5

Image is real, inverted and magnifi ed. Image is virtual, upright and magnifi ed.
Image is located 36 cm from the lens and Image is located 6 cm from the lens and
on the opposite side of the object. Image on the same side as the object. Image is
is magnifi ed 2 times. magnifi ed 1.5 times.

261
6.4.2 261
6.4.2