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© Arnulf Betzold GmbH
2. Introduction on the subject of eyeglasses
Meanwhile almost 2/3 of all people living in Germany wear eyeglasses. Nearly every retired person owns at least one pair of
reading glasses. Some people always need to wear their glasses and some get by with wearing them from time to time.
Together with your pupils figure out which family members or friends wear eyeglasses. How many of the pupils in the class wear
glasses themselves and why?
Your pupils will learn about optical lenses with the following experiments. They can find out how eyeglasses work and most
important, how they work in particular. Prior to the subject of eyeclasses the topic of visual defects (nearsightedness and
farsightedness) should have been covered in glass.
3. Excursion Optical Lenses
The following text is also available as a fill-in-the-blanks text (without the part about meniscus lenses) (see 7.2)
What actually is an optical lens?
The term “optical lens” was originally used in physics. It refers to an object that refracts light due to its particular shape.
There are different kinds of lenses: converging lenses or diverging lenses.
Converging lenses
Converging lenses have an outward bulging shape. They are also called convex lenses. Parallel light rays passing through
the lens come together and meet at a spot just beyond the lens known as the focal point. A lens is biconvex if both sides bulge
outwards. If one side of a lens is flat (planar) it is called plano-convex.
Diverging lenses
Diverging lenses curve inward. They are also called concave lenses. Parallel light rays passing through the lens are diverged
(spread). A lens with both sides curving inwards is biconcave. If one side of a lens is flat because it does not curve inwards it is
called plano-concave.
Meniscus lenses
Meniscus lenses have a concave and a convex side. Depending on which side the curve is more pronounced, it is either a con-
verging or diverging lens. This type of lens is not mentioned in any of the experiments or in the fill-in-the-blanks text.
4. Interesting facts about eyeglasses
The following text is also available as a fill-in-the-blanks text (see 7.3)
Actually, eyeglass lenses are optical lenses 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 diopters (D). There are different kinds of
eyeglasses. Farsighted eyes are treated with convex 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 beyond the retina. With positive
eyeglasses it is possible to shift the focal point inwards so that is lies exactly on the retina.
On the other hand, concave shaped eyeglasses are considered as diverging 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 nearsightedness, the refractive power is to strong. The position of the focal point falls in front 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.
