Page 238 - Physics_Form_2

Physics for Secondary Schools

The extremes of the range over which 5. Without moving your finger, switch
distinct vision is possible are known as the to the other eye.
far point and the near point of the eye. The 6. Now move to about 4 m from the
far point of a normal eye is the maximum wall and repeat the process.
distance at which the light from the object 7. Move as far as possible from the
can still be focused onto the retina. The wall and repeat steps 3-5. Record
FOR ONLINE READING ONLY
near point is the shortest distance from the all your observations.
eye at which the light from the object can
be focused onto the retina. The eye tends Question
to be blind when the object is closer to the
eye than the near point. The position of Explain the appearance of the line at
the near point depends on the amount by each of the distances.
which the ciliary muscle can increase or
decrease the curvature of the crystalline Defects of the human eye
lens. The range of accommodation For an object to be seen by the human
gradually diminishes with age because eye, light from the object must be focused
the crystalline lens grows throughout on the retina. However, there are cases
a person’s life, and the ciliary muscles when light from the object cannot be
become less capable of distorting a focused on the retina. This arises mainly
larger lens. For this reason, the near point from incorrect distance relationships in
gradually recedes as one grows older. The the eye. A normal eye can form an image
recession of the near point is a condition on the retina for an object at infinity
called presbyopia. when the ciliary muscles are relaxed,
and for a nearby object when the ciliary
muscles are contracted, as illustrated in
Activity 6.3 Figure 6.20. In some cases, the size of the

Aim: To determine far and near eyeball is defective, leading to conditions
points of the eye known as short-sightedness (myopia) and
far-sightedness (hyperopia).
Materials: manila paper, sellotape
Short-sightedness (myopia)
Procedure
In the myopic (short-sightedness or near-
1. Draw a vertical line about 50 cm sightedness) eye, the eyeball is too long
long on a sheet of manila paper. from front to back in comparison with the
2. Stick the paper on the wall. radius of curvature of the cornea (or the
3. Stand about 2 m from the wall and cornea is too sharply curved). This causes
cover one of your eyes with your the rays of light from an object at infinity
hand. to be focused in front of the retina. The
4. Cover the line by bringing the index most distant object for which an image
finger of your other hand close to can be formed on the retina is then nearer
your face. than infinity. Therefore, a person suffering

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Student’s Book Form Two

Physics Form 2 Final.indd 232 25/10/2025 10:28