Magnifying glass

A magnifying glass ( loupe from French ), also called magnifying glass is a convex positive lens - or a system of lenses, which acts as - smaller focal length for hand use. If an object is within the focal length f it makes an upright virtual image of him.

Operation

The magnification effect by means of water is occupied since the 1st century by Seneca the Younger. The invention of the magnifying optical device as the Arab scholar Abu Ali al -Hasan Ibn Al- Haitham ( Alhazen Latinized ) attributed ( 11th century, writing " Treasure of Optics" ).

Through a magnifying glass objects appear larger, the other side of the viewer between motion and focal plane ( ie within the focal length) are located. This effect of a magnifying glass results partly from the fact that you can accommodate on an object with her at a closer distance than would be possible with the freisichtigen eye. On the other hand magnifier provides a magnified virtual image. The second effect is also used to determine the increase (see below ) were used.

As a visual aid used, it is similar to hyperopia ( farsightedness) or presbyopia ( presbyopia ), since the eye can accommodate for motion transparency to infinity ( this is often found by those affected as relaxing). It should use a magnifying glass when the observer sees a virtual image enlarged, not to be confused with the correction of hyperopia or presbyopia. When correcting the lens used is also a convex lens, but here with much larger focal length or smaller power ( " diopter " ) and is worn as glasses or contact lenses in front of the eye. The lens of the spectacles and the lens forms with the optical apparatus of the eye, a lens system or functional unit.

In contrast to the glasses magnifying glass must not be adjusted when hyperopia or presbyopia. Therefore, each magnifying glass can be always used as a makeshift for a missing glasses or contact lenses. Myopia ( nearsightedness) can not be corrected with a magnifying glass.

Construction of the virtual image

In the construction of the virtual image ( see Figure ) involves two rays emanating from a point of the object (in this case by the arrowhead) out: The "main beam " - so that beam which passes through the center of the lens - undergoes approximately no refraction. The " parallel beam " ( blue in the figure ) is due to refraction of the lens to " burning ray ", that goes through the focal point. The viewer who is in the picture on the right of the lens is based on the rectilinear propagation of light. For him, that is, the light appears to come from the place where the cross retraced red lines. So the arrowhead of the virtual image is right there.

The subject must be within the focal length to enlarge it through the magnifying glass to see. Optimally, the magnifying glass is held when it is in focus. The rays are then parallel. The object seems to be infinitely far away and the eye can accommodate relaxed at long range. The magnification results from the ratio of the angle of vision, with or without corrective lenses. For reference, the distance of distinct vision is often chosen.

Types

In addition to special magnifiers, such as reading rods (magnifying glass with a direct bearing on the reading material and magnification in one dimension ), rock fragments ( also with direct support but two-dimensional magnification), thread count, watchmakers magnifiers, magnifying glasses, loupes with complex lens systems ( aplanatic, achromatic lenses, etc.), credit card magnifying glass ( Fresnel lens ) and a v. there are two basic types that differ primarily in their application:

Reading Magnifier

Magnifiers enlarge usually 2 - to 6 -fold. Another characteristic is their relatively large field of view due to a large lens diameter. This large field of view allows for a constant distance between the magnifying glass and reading material ( object) a generous and variable distance between the magnifying glass and head ( eyes) take. Reading is therefore possible in a comfortable position. If there is sufficient distance between the magnifying glass and eyes as well as at low magnification three-dimensional vision is possible.

While the classic magnifying glass is a great lens with metal frame and handle (think of the typical appearance of Sherlock Holmes with a magnifying glass - ironically he used in the illustrations is always a magnifying glass instead of a detailed magnifying glass), modern reading magnifiers are often associated with lighting ( magnifiers ), spacers or other devices provided.

Lesesteine ​​and reading rods (the latter larger than the cylindrical lens in one dimension only ) are flat on the bottom and are placed directly on the reading. Thus, the distance between the reading material and lupe effective curved top is set and no hand tremor can interfere. These stones are reading their volume heavy.

Detail magnifying glass

The detailed magnifying glass has a typical magnification of 5 - to 15-fold - with very good quality also something about it. The lens has a small diameter ( about 1-3 cm) so that the field of view is very small.

This small design requires a completely different way of working with the detailed motion as with the magnifying glass. While the distance between the object and the motion is fixed here also, one tries to get through a very small distance between the magnifying glass and eye, a larger field of view. A typical case is the watchmaker's loupe, which is clamped directly in front of the eye. But other details Magnifiers ( eg magnifying glasses to nature observation of rocks and minerals in the ground, or flowers and insects in open field) are directly introduced to the eye, to allow a larger field. Correct handling of such magnifiers usually requires some practice, because you're not used normally to place objects in front of the eye.

Three-dimensional vision is not possible here. At this point it but referred to loupes or special and correspondingly expensive binocular loupes.

Measurement / calculation of focal length

The focal length of a magnifying glass determines the magnification, making it their most important characteristic. To calculate the increase (see below), therefore, the focal length must be known.

Sunlight can be the focal length for everyday sufficiently accurately determine by the circular spot of light from a light source in mapping on a sheet of paper is minimized by using the magnifying glass, while the distance between the magnifying glass and paper is measured. This distance corresponds to the focal length.

Another measuring method is to determine purity of the focal length by means of autocollimation.

Accurately calculate ( a correspondingly good measurement provided ) can be the focal length by several methods:

• With Newtonian imaging equation ( this can be derived from the lens equation )
• With the Bessel process
• With the Abbe method

Determine the magnification of

To estimate the magnification power of a magnifying glass to select the distinct vision as a reference.

An object appears at this distance at the angle α ( in the adjacent Fig φ (phi ) called, also corresponds to the fig distinct vision to the distance d):

The eye is relaxed when it accommodated over considerable distance. This is the case for a large image distance. It increases when the object distance to the focal point F of the motion approaches ( see figure).

In the limit of the magnified image appears under the angle:

For increase follows:

This calculation assumes that the eye is relaxed, ie, infinite- accommodated; then is also no virtual image. Strained to the eye but with on, you can zoom to bring the subject closer to the magnifier, so that a virtual image at the distance of distinct vision. The motion must be close to be at the eye, the refractive powers of motion and near- eye akkommodiertem add up and is the magnification

Example

A magnifying glass with 50 mm focal length allows you to look at an object from a distance of 50 mm, instead of the distance of distinct vision of 250 mm. After defining the magnifier 5x magnified ( v = 250 /50), with near- eye akkommodiertem 6-fold.

Use the magnifying glass as a magnifying glass

The name of a magnifying glass is because the convex lens focuses the sun's rays approximately parallel in the focal distance, and thus the energy released in the absorption of light heat so concentrated that combustible material such as paper or the like can be ignited. In a simple examination, one speaks of a focal point. The biggest influence is played by the lens diameter. The larger, the more light ( power) is bundled. The vastness of the sun ( as imaged object ) causes next to the lens error that the focal point is not an ideal point, but a focal spot.

This property of the lens was the focal point and the focal plane also her name.

Whether or how quickly so that material can be made to burn, depends not only on the size of the focal spot of the ignition temperature of the material and the thermal conditions ( material thickness, thermal conductivity ) at the focal spot from.

About the magnifying effect of the glass ball of Helio Graphs burn time with sunshine in paper.

Warning

The daily and annual course of the sun makes light- objects (this includes all types of magnifying glasses ) next to flammable materials in natural and house a fire hazard. Compared with the direct sun moves her image, the focal spot across the room and can make dried plant parts, wood, paper, upholstery etc.. Verkohlendes material is darker and then absorbs a higher proportion of the heat radiation, the focal spot moves further and further heated material.

Magnifying glasses, which are not in use, must be provided with a protective cover / cap or held in a light- resistant container. There are many cases in which the decoration of magnifiers (including photographic lenses ) have caused a fire with total loss for the company in the shop window of a retail store.

A major hazard is based on spheres of clear, bright glass, solid or filled with water, the collecting mirror - effect of concave bases of ( discarded lying ) glass bottles and beverage cans floors and the back of traffic mirrors made ​​of stainless steel sheet, which is why they are painted on the back of matte gray in order to reduce the unwanted reflection.