Once through the lens, the ray should pass through the principal focus. Draw a ray which passes from the object through the centre of the lens. Some ray diagrams may also show a third ray.

The human eye is a simple, but extremely robust, optical instrument. Analysis by sophisticated wavefront-sensing technology and customized ray-tracing has now revealed that the eye is actually an ...

Short-sightedness can be corrected by placing a concave lens in front of the eye, as shown in the diagrams below. Someone who is long-sighted can see distant objects clearly, but they cannot focus ...