Page 55 - Curriculum Visions Dynamic Book

Nearsighted people cannot Focuses in front Concave lens see things clearly in focus if of retina in eyeglasses they are far away.
Far object
A concave lens is used for people who cannot see things in focus when they are at a distance. In this case the lens of the eye brings light to a focus in front of the back of the eye.The glass lens slightly diverges the light, so that the eye lens can then bring objects to a focus on the back of the eye.
A convex lens is used for people who cannot see things in focus when they are very close.This is the more usual eye problem. In this case the lens of the eye brings light to a focus behind the back of the eye.The glass lens slightly converges the light, so that the eye lens can then bring objects to a focus on the back of the eye.
Near object (less than 25cm)
Focuses behind retina
Farsighted people cannot see things clearly in focus if they are close up.
Convex lens in eyeglasses
6. Lens glass
Glass lens eyeglasses are mainly lead-silicate glass. They are also sometimes called flint glass.
When making lens glass, it is vital to have exact control of the way that light moves through the glass. That controls the magnification and clarity of the lens and its usefulness to the wearer.
(Below) Reflected light rays through lenses and prisms of a pair of binoculars.
Second glass prism
High lead content glass is used
because it bends the light strongly (it has
a high refractive index). To achieve this, exceptionally pure materials have to be used.
Lens glass is normally melted in an electric furnace using platinum metal to line the furnace. This reduces the chances of impurities getting into the melt. The glass is poured out into moulds that resemble lenses and so need relatively little finishing.
Path of light
Objective lens has a bulging shape to gather light.
Path of light
The prisms behave like mirrors and bounce the light from the objective lens.
First glass prism
(Below) How eyeglasses work for both near- and far-sightedness.
55