Betzold 89947 Handbuch Seite 26

7.2 Excursion Optical Lenses
What actually is an optical lens?
The term "optical lens" was originally used in physics. It refers to an object that _______________ light due
to its particular shape.
There are different kinds of lenses: ________________________ or _______________________ .
Converging lenses
Converging lenses have an _________________ bulging shape. They are also called ______________
lenses. Parallel light rays passing through the lens come together and meet at a spot just beyond the lens
known as the _______________________ . A lens is ___________________ if both sides bulge out-
wards. If one side of a lens is flat (planar) it is called _________________________ .
Diverging lenses
Diverging lenses curve _____________. They are also called ______________________________ .
Parallel light rays passing through the lens are diverged (spread). A lens with both sides curving inwards is
__________________________. If one side of a lens is flat because it does not curve inwards, it is called
__________________________ .
7.3 Interesting facts about eyeglasses
Actually, eyeglass lenses are optical ________________ made out of glass or plastic that are inserted in
an eyeglass frame. Their visual effect, meaning the refractive power of a lens, is expressed in an
optical unit called ___________________ (D). There are different kinds of eyeglasses. Farsighted eyes are
treated with __________________ lenses, so called converging lenses. These types of eyeglasses allow
a stronger refraction of light or causes light to focus closer. This is also referred to as a positive power lens
or plus lens. The optometrist specifies the strength of these eyeglasses with a "+" sign placed in front of
the diopter strength (D). With farsightedness, the refractive power is not strong enough. The focal point of
the eye lies __________________ the retina. With positive eyeglasses it is possible to shift the focal point
_________________ so that is lies exactly on the retina.
On the other hand, concave shaped eyeglasses are considered as ____________________ lens and
as the name indicates, they have a spreading (diverging) effect. They are also called minus lenses as
they have a negative refraction power because they spread rays of light outwards. The optometrist
specifies the strength of these eyeglasses with a "-" sign placed in front of the diopter strength. With
___________________________, the refractive power is to strong. The position of the focal point falls in
____________________ of the retina. Negative eyeglasses are able to refract light so the focal point is
shifted to the back so that it lies exactly on the retina.
So called bifocal glasses combine positive and negative lenses together in one eyeglass lens. Usually a
smaller section of a positive lens is worked into a negative eyeglass. If a line between both segments is not
visible the glasses are called varifocals.
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