Page 3 - Lesson note - introduction

Persistence of Vision:The impression or sensation of an image on the retina
remains (or persists) for about (l/16) th of a second even after the removal of the
object. The phenomenon of the continuation of the impression of an image on the
retina for some time even after the light from the object is cut off is called persistence
of vision.
Power of accommodation. The power of accommodation of the eye is the
maximum variation of its power for focusing on near and far (distant) objects. For a
normal eye, the power of accommodation is about 4 diopters.

1. Eye lens becomes thin 1. Eye lens becomes thick
2. Increases the focal length 2. Decreases the focal length
3. Enable us to see distant object clearly 3. Enable us to see nearby object
clearly

Least distance of distinct vision. The minimum distance from the eye, at which the
eye can see the object clearly and distinctly without any strain is called the least
distance of distinct vision. It is denoted by the letter D. For a normal eye, its value is
25 cm.
Near point. The nearest point from the eye, at which an object can be seen clearly
by the eye is called its near point. The near point of a normal eye is at a distance of
25 cm.
Far point. The farthest point from the eye, at which an object can be seen clearly by
the eye is called the far point of the eye. For a normal eye, the far point is at infinity.

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