What is the focal length of a compound lens composed of a converging lens of focal length 15 cm and a diverging lens of focal length 12 cm? Does the compound lens behave as a converging or diverging lens?
SAMPLE ANSWER 13F Distances must be converted into SI units, therefore f1 0.15 m; f2 –0.12 m;.
P P1 P2Q 1 f 1
f1
1 f2
Qf f1f2 f1 f2
The power of the compound lens is 1.67 m1. It behaves as a diverging lens. Qf
(0.15)( 0.12) 0.15 0.12 0.018
0.03 3 5 QP5 3
EXERCISE 13.4 COMPOUND LENSES
Q1 What is the power of a: (i) converging lens of focal length 25 cm
Q2 State the type of lens and the focal length of the lens with a power of (i) 10 m1
(ii) diverging lens of focal length 40 cm. (ii) 8 m1.
Q3 Calculate the power of a compound lens made from two converging lenses, one of focal length 20 cm and the other of focal length 25 cm.
Q4 A converging lens of focal length 30 cm and a diverging lens of focal length 20 cm are placed in contact. (i) What is the focal length of the compound lens? (ii) Does the compound lens behave as a converging or a diverging lens?
Q5 The power of a compound lens is 5 m1. One of the lenses is a converging lens of focal length 15 cm. (i) What is the focal length of the other lens?
The Eye
The iris controls the light entering the eye by making the hole in its centre (called the pupil) larger or smaller. In bright light the pupil is small; in dim light it gets larger.The colour of the iris is the colour of your eye. The cornea and the internal (crystalline) lens together form a converging system.The ciliary muscles can change the shape, and therefore focal length, of the internal lens. The jelly-like liquids, the aqueous humour and the vitreous humour,also form part of this focusing system. The retina is a light-sensitive screen at the back of the eye. It consists of many nerve endings which are sensitive to light.The focusing system produces a real, inverted image on the
Vitreous humour Ciliary muscle
Iris
Cornea Pupil
Internal lens Aqueous humour
Fig 13.28: Optical structures of the eye REFRACTION OF LIGHT 227
Retina Fovea
Blind spot Optic nerve
(ii) Is the other lens a converging or a diverging lens?
