Question

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The effective focal length of the cornea and the eye-lens can be considered as the focal length of an eye. The muscles controlling the curvature of the lens permit the eys to focus on the retina light from objects at different distances from the eye. When the eye muscles are relaxed the focal length of a healthy eye of a child is about 2.5 cm. The near point of his eye is at a distance of 25 cm.

(a) Draw a ray diagram for the situation where a point source of light is placed at the near point is clearly seen by the child with healthy eye. Calculate the focal length of the eye at this instant.

(b) Another child has the focal length equal to that of the focal length of the healthy for the situation in (a). However, the position of his retina is located 0.2 cm behind the position of the retina of the healthy child. Calculate the distance from the eye-lens to the near point of this child.

(c) Sketch a ray diagram illustrating how the required correction can be done using a suitable lens. Calculate the focal length of the corrective lens and its power.

Answer 1

Check if position of the image was 0.2 cm behind retina or position of the retina was 0.2 cm behind healthy eye retina. For first case, we would require a corrective lens! If there is no typing error than the above answer is correct.