# Magnifying glass

magnifying glass as Brennglas

a magnifying glass, also Brennglas mentioned, is a convex lens of small focal length, with which the article which can be illustrated is within the focal length f. It produces an upright virtual picture. Their invention becomes the Arab scholar Abu Ali aluminium-Hasan Ibn aluminium-Haitham(Alhazen latinisiert) attributed.

The name Brennglas is due to the characteristic to bundle sunbeams in such a way in one point that the ignition temperature of paper, wood, o.a. in the focus one exceeds. With a magnifying glass one can approach an article more strongly and see him so larger,as it the eyes - accommodation without lens would permit. When read assistance uses, it adjusts for defective vision (like far or myopia).

In order to measure the enlargement achievement of a magnifying glass, one selects the clear Sehweite (250mm) as base factor.

Construction of the virtual picture when regarding an article by oneMagnifying glass.

An article appears at this distance under the angle α:

[itex] \ tan \ alpha= \ frac {G} {250 \ rm \, mm}< /math>
with G: Object size (object size) in mm

the eye is relaxed, if it akkomodiert on large distance. That is the case for a large image intercept [itex] S_2< /math>. It increases, if itselfthe article width [itex] S_1< /math> brings f of the magnifying glass to the focus closer (see illustration).

In the border line [itex] S_1=f< /math> the increased picture under the angle appears [itex] \ alpha_v< /math>:

[itex] \ tan \ alpha_v = \ frac {B} {S_2} = \ frac {G} {f}< /math>

For the enlargement [itex] V< /math> follows:

[itex] V= \ frac {\ tan \ alpha_v} {\ tan \ alpha} = \ frac {\ frac {G} {f}} {\ frac {G} {250 \ rm \, mm}} = \ frac {250 \ rm \, mm} {f}[/itex]

## it

• permits examples a magnifying glass with the focal length to 50 mm to regard an article from the distance of 50 mm instead of from the distance of the clear Sehweite of 250 mm. According to the definition the magnifying glass increases 5-fold (V=250/50).
• Someone is much short-sighted. It can read the newspaper still in the distance of 50 mm. Related to its personal Sehweite the magnifying glass with 50 mm of focal length does not offer a profit for the enlargement. But the magnifying glass helps to ease the eye. Instead of its eyes on 50 mm distanceto focus, he holds the magnifying glass in such a way that eases the eyes the picture in large virtual distance regard.