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Optical instruments

lens is required to form a virtual image at or beyond the near point. The lens converges
the rays of light from a near object to form the image at or beyond the near point. The
eye lens then focuses the image of the glass lens onto the retina. Figure 6.25 shows the
correction of the hyperopic or presbyopic eye.

Object formed
FOR ONLINE READING ONLY
beyond the retina
Near object

Hyperopic eye

Virtual image created by the
converging lens beyond the Object formed
eye's near point on the retina

converging lens

Figure 6.25: Correction of the hyperopic eye

Astigmatism combination with near-sightedness or far-
Astigmatism is the imperfection in sightedness.
the curvature of the eye that causes the Normal eye
formation of blurred images on the retina.
It occurs when either the cornea or the lens
has mismatched curves. That is, instead
of having a curve like a ground ball, the
surface is egg-shaped. This causes the
lens to have different focal points when Cornea
looking at the same object. Consequently,
rays of light from the object are focused
at different points, leading to image
formation at different positions on the
retina. The result is blurred vision. Figure Astigmatic eye
6.26 illustrates astigmatism. Astigmatism
is often present at birth and may occur in Figure 6.26: Astigmatism

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Physics Form 2 Final.indd 235 25/10/2025 10:28