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Image formation - Thin lenses

Vision and eyes

1) How does human eye work ?

To see an object, an eye has to receive light emitted by this one.

What happens when the light enters the eye?
The light passes through several transparent media: cornea, aqueous humor, lens and vitreous humor.
The light then strikes the back of the eye which is covered with light sensitive cells constituting the retina.
The retina produces a nerve impulse that is transmitted to the brain via the optic nerve.
Nevertheless, a correct vision of the object and the production of an understandable nerve signal requires that an image of the object forms on the retina.
Transparent media through which the light travels therefore play the role of a converging lens.
In the study of converging lenses it was seen that an image can be formed on a screen if the distance between the screen and the lens is correctly adjusted.
This adjustment of distance is not possible for the eye since it has constant dimensions: the eye doesn't modify its dimensions but its focal lenght in order to form the image on its retina.
There is accommodation of the eye.


2) Vision defects and their correction

There are many defects that can affect vision, but most frequent are hyperopia and myopia that are a lack of accommodation of the eye.

A hyperopic person is no longer able to properly see close objects that appear blurry to him. This person is forced to keep them away so that they appear more clearly.
A hyperopic eye is an eye which does not converge enough therefore a sharp image is formed behind the retina when objects are too close.
This defect can be corrected simply by wearing glasses with converging lenses in order to increase the convergence of the eye.

A nearsighted person is unable to see distant objects because its eyes are too convergent.
This defect can be corrected by using glasses with divergent lenses that make eyes less convergent.