Page 53 - Physics 10_Float

GEOMETRICAL OPTICS

1 1 1
=
q f p
1 = 1 1
–
q 0.05 m 2.5 m
1
= 19.6 m -1
q

or q = 0.05 m
Since the image distance is positive, so a real image is formed
on the film at the focal point of the lens. A camera without lens!
Example 12.6: A concave lens has focal length of 15 cm. At
what distance should the object from the lens be placed so Wall of box
that it forms an image at 10 cm from the lens? Also find the Object as screen
Image
magnification of the lens. Pinhole
Solution: A concave lens always forms a virtual, erect image Even simpler than a camera
on the same side of the object. Given that, q = –10 cm with one lens is a pinhole
f = –15 cm, p = ? 1 1 1 camera. To make a pinhole
Using the lens formula: f = p + q camera, a tiny pinhole is made
in one side of a box. An inverted,
real image is formed on the
1 1 1
= + opposite side of the box.
p q f
1 1
= – +
(– 10 cm) ( 15 cm)–

1 1
=
10 cm 15 cm
1 = 3 cm – 2 cm
p 30 cm 2
1 = 1
p 30 cm

p = 30 cm

Thus, the object distance is 30 cm, on the left side from the
concave lens. q 1
10 cm
=
Magnification of the lens is = 3
m =
p
(Ignore nagetive sign) 30 cm
The image is reduced to one-third in size than the object.

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