Page 242 - Physics_Form_2

Physics for Secondary Schools

Astigmatism can be corrected by either For the object at infinity, u = ∞, the far
using corrective lenses or performing eye
surgery. point of the defective eye, v =−50 cm.
By the lens formula:
Example 6.6
1 1 1
FOR ONLINE READING ONLY
The far point of a myopic eye person is =+
50 cm in front of the eye. What is the f v u
nature and power of the lens required 1 1 1
to correct the problem?   50 cm     50 cm

Solution f  50 cm  0.5 m
The nature of the lens should be a Therefore, power
concave lens (diverging lens). A
concave lens makes the objects at  1  1  2.0 .D
infinity appear at the far point. f  0.5 m

Comparison between the lens camera and the human eye
The lens camera has many similarities with the human eye. Table 6.1 shows the
differences and similarities between the lens camera and the human eye.

Table 6.1: Similarities and differences between the lens camera and the human eye
Criteria Lens camera Human eye

Lens Uses convex lens Uses convex lens
The image is real, reduced and The image is real, reduced and
Image
inverted inverted
Light control The amount of light entering The amount of light entering the
the camera is controlled by the eye is controlled by the pupil
diaphragm

Image point The image is formed on a The image is formed on the retina
special film

Focusing Focusing is done by altering Focusing is done by altering the
the distance between the lens shape of the lens which alters its
and the film focal length
Image The image is processed The image is converted to electrical
processing chemically in a process called signals that travel through the optic
“Developing” nerve to the brain for processing

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Student’s Book Form Two

Physics Form 2 Final.indd 236 25/10/2025 10:28