Bernadette Green, of Villa Maria College, asks :-

How do glasses work?

John Campbell, a physicist at the University of Canterbury, responded.

Glasses work because they change the direction of travel of an incoming ray of light.

We see things because of the light rays which are scattered by them to our eye. Our eye then focuses these rays onto the light sensitive region (the retina) at the back of the eyeball. Most of the focussing takes place because of the curved at the front of the eyeball. For some people, who have eyeballs of the wrong length or curvature of the front surface, these rays focus in front of or behind the retina and they see fuzzy images. They need glasses in order to obtain sharply focused vision.

When a ray of light passes from air into a transparent material (e.g. glass or the eyeball fluid) the light slows down. If the ray meets the interface between the air and the transparent material other than at right angles then the ray changes direction. The new direction depends on the angle at which the ray meets the interface and the ratio of the speeds.

Consider the person who needs glasses which are thicker at the centre than at the edge in order to see clearly. A ray of light from the object which meets the glass surface at right angles is undeviated. A ray which leaves the object and meets the outer edge of the lens meets the surface at an angle so is bent inwards. This then passes into the eye and meets the undeviated ray at the retina. i.e. all rays are now in focus at the retina and the person sees a clear image.