Magnifying glasses are a simple optical devices used for viewing details of objects with some magnification. They are sometimes regarded as being the same as loupes, but precisely speaking a loupe is used in a close distance from the eye, while magnifying glasses (or hand lenses) are held at a larger distance. Magnifying glasses are typically used in a larger distance from the eye, are themselves larger and typically have a longer focal length of 125 mm or more.
The imaging properties of a magnifying glass must always be considered in conjunction of those with the human eye. The magnification of a loupe is defined as the ratio of the parent object sizes as observed with and without the glasses. For reading a book with small letters, for example, one may hold a magnifying glass in a significant distance from the eye, closer to the read text. When calculating the magnification based on a comparison of observation with and without the magnifying glass, but each time from the same relatively large distance, one obtains the magnification to be 1 plus the ratio of image distance and focal length.
Usually, the magnification of a simple magnifying glass is between 2 and 6. Higher magnifications would lead to a reduced image quality, as long as a simple lens is used. There are improved multi-lens designs, e.g. triplet magnifiers, which provide substantially higher magnification (up to about 30), but with a small field of view and less light throughput. They normally need to be held close to the eye, and should therefore be called loupes. Four higher magnifications, one uses microscopes.
Interestingly, the eye can accommodate such that either the read text or the glass itself (or its mount) is sharp.
Types of Magnifying Glasses
Magnifying glasses can simply be made as spherical biconvex glass lenses. For a not too large field of view, the solution may be satisfactory. Plastic optics are often used instead of glass, and then it is usually not a problem to make an aspheric lens.
Aplanatic glasses, consisting of two plano-convex lenses, with the convex surfaces on the inner side, are popular as reading glasses, because they exhibit low image distortions.
Particularly large magnifying glasses may be made in the form of Fresnel lenses (e.g. as thin sheet magnifiers), but then with reduced image quality.
The required illumination of the object may be done through the lens and/or from the side. Illumination through the lens can have the advantage that it is brighter, with some concentration of the light by the lens. On the other hand, one may then be disturbed by parasitic reflections, if the lens does not have effective anti-reflection coatings.
There are also magnifying glasses with integrated battery-powered illumination.
Use as Burning Glass; Fire Hazards
A magnifying glass can also be used as burning glass, concentrating sun light such that a material like paper in the focal plane can be incinerated.
The same effect can create a safety hazard; it can happen that a fire inadvertently arises from a magnifying glass, e.g. when displayed in a shop window. It is advisable to appropriately cover magnifying glasses when they are not in use.
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