A person can see distinctly any object situated in between the distance 50 cm and 300 cm. what type of spectacles are to used (i) to extend the far point up to infinity and

Explanation

To extend the far point from 300 cm to infinity spectacles should have a concave lens. Since the far point of the defective eye is at 300 cm , so the focal length of the concave lens of the spectacles will be 300 cm . Because the parallel rays coming from infinity after refraction by the concave lens appear to diverge from a point at a distance 300 cm from the lens.
With these spectacles , the near point of the range of vision will be that object distance for which the image distance is 50 cm .
Here, v= -50 cm and f =-300 cm.
From the equation `1/v-1/u=1/f` we have ,
`-1/(50)-1/u=-1/(300) or, -1/u=1/(50)-1/(300)=1/60`
or, u=-60 cm
So with this pair of spectacles, the range of vision will be from 60 cm up to infinity .