Page 212 - Physics_Form_2
P. 212
Physics for Secondary Schools
front of the lens. Determine the focal Exercise 5.4
length and the magnification of the 1. (a) Draw ray diagrams to show the
lens.
nature and position of images
Solution formed by a convex lens with
focal legth of 30 cm for the
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Given following object distances:
h = 0 10 cm, v = 7.5 cm, u = 30 cm (i) 15 cm
(ii) 30 cm
1 1 1
Using the lens formula = + (iii) 45 cm.
f u v
(b) Use the lens formula to
determine the nature and
uv 30 cm
( 7.5 cm)
f 10 cm position of the image formed
v u ( 7.5 cm) 30 cm
by the objects in (a).
f 10 cm (Negative sign indicates 2. A vertical object 10 cm high, is
that f is measured against the incident placed 30 cm in front of a diverging
ray) lens. An image of an object is formed
v 7.5 cm in front of the lens. Determine
( 7.5 cm)
m 0.25
u 30 cm the focal length of the lens by using
a ray diagram.
The positive value of magnification 3. An object 2 cm high is placed 9 cm
indicates that the image is erect or from a convex lens of focal length
upright. Therefore, the focal length, f 6 cm. Determine the magnification,
is 10 cm and the magnification, m is position and nature of the formed
0.25. image.
4. A converging lens forms an upright
Uses of lenses in life image that is four times the size
Lenses have different applications in of the object. Given that the focal
our daily life. Convex or concave lenses length of the lens is 20 cm, determine
are used in medical eyeglasses for the object distance.
correcting long-sightedness and short- 5. An object is placed 36 cm from a
sightedness respectively. Convex lenses converging lens of focal length 24
are also used in binoculars, cameras, cm. If a real image which is 4 cm
telescopes, microscopes and other optical high is formed, calculate the height
instruments. On the other hand, concave of the object.
lenses are used in flashlights to magnify 6. A lens of focal length 15 cm forms
the light produced by a bulb. These lenses an upright image four times the size
are also used in pinholes.
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Student’s Book Form Two
Physics Form 2 Final.indd 206 25/10/2025 10:28
