Figure ( a ) shows the basic structure of a human eye. Light refracts into the e
ID: 1562347 • Letter: F
Question
Figure (a) shows the basic structure of a human eye. Light refracts into the eye through the cornea and is then further redirected by a lens whose shape (and thus ability to focus the light) is controlled by muscles. We can treat the cornea and eye lens as a single effective thin lens as shown in figure (b). A "normal" eye can focus parallel light rays from a distant object O to a point on the retina at the back of the eye, where processing of the visual information begins. As an object is brought close to the eye, however, the muscles must change the shape of the lens so that rays form an inverted real image on the retina as in figure (c).
(a) Suppose that for the parallel rays of figure (a) and figure (b), the focal length f of the effective thin lens of the eye is 2.40 cm. For an object at distance p = 30.0 cm, what focal length f' of the effective lens is required for the object to be seen clearly?
cm
(b) Must the eye muscles increase or decrease the radii of curvature of the eye lens to give focal length f'?
increase
decrease
Lens Retina Muscle Cornea Light from distant object o (a) (c) Effective lens RetinaExplanation / Answer
(a) when the rays are coming from a distant object, object distance= do=infinity
f=2.40
1/f= 1/di + 1/do
1/f= 1/di + 1/infinity
di=f= 2.4 cm
now object distance=-30 cm
again using mirror formula
1/di+ 1/do= 1/f'
1/2.4 + 1/-30= 1/f'
f'=2.61 cm.
(b) focal length increases in the problem from2.4 to 2.61 cm. so eye muscles must increase the radius of curvature of eye.
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