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Optical instruments

Accommodation of the human eye the curvature of the lens to decrease,
For an object to be seen sharply, the image thereby increasing its focal length (Figure
must be formed exactly at the location of 6.20(a)). To focus sharply on a closer
the retina. The eye adjusts to different object, the tension in the ciliary muscle
object distances u by changing the focal increases, causing the ciliary muscle to
length f of its lens. This is because the contract. This causes the lens to bulge,
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lens-to-retina distance, corresponding resulting in the increase of its curvature
to v, does not change. However, this is and decrease of its focal length (Figure
different from the lens camera in which the 6.20(b)). The process by which the eye
focal length is fixed and the lens-to-sensor focuses on objects at different distances
distance is changed. For the normal eye, a by varying the focal length of the lens is
distant object is sharply focused when the
ciliary muscles are relaxed. This causes called accommodation.

Relaxed muscles

Distant
focusing

(a)

Contracted muscles

Near
focusing

(b)

Figure 6.20: Focusing distant and near objects

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