# Question about lenses in the human eye

To see near objects,the ciliary muscles contract and the suspensary ligaments loosen,making the lense thicker. İ don't understand how that helps us to see distant objects .

To see distant objects the ciliary muscles relax stretching the suspensary muscles making the lense thinner.

İ haven't been able to find information about how this helps us to see , why it happens.

A thinner lens has a greater focal length, hence rays get less refracted. Inversely it is for a thicker lens. For a far object the rays are almost parallel. Therefore they do not have to get refracted so much. But for a near object the rays are more divergent. Hence you need a smaller focal length in order to refract the rays as much that you get a sharp picture. See the picture below for the ray paths. Take also a look at Lensmaker‘s equation describing how the focal length depends on the radius of the lens. The muscles deform the lens curvature. This changes the power D=1/f of the lens and thus the visual system can focus far and near objects. $$D_i=\frac{n-n_0}{R_i}$$ The more curved surface will have smaller radius R and higher power or smaller focal length.
Also the power changes a bit due to the thickness d as $$D=D_1+D_2-\frac{d}{n}D_1D_2=\frac{1}{f}$$