magnifying glasses (original) (raw)

Definition: optical lenses held above objects for viewing them with some moderate magnification

Alternative term: hand lenses

Categories: general optics, vision, displays and imaging

• lenses
  • achromatic lenses
  • aspheric aspheric
  • ball lenses
  • condensers
  • cylindrical lenses
  • diffractive lenses
  • f–theta lenses
  • fiber lenses
  • field lenses
  • Fresnel lenses
  • gradient-index lenses
  • Kerr lens
  • loupes
  • magnifying glasses
  • microlens arrays
  • microlenses
  • objective lenses
  • ocular lenses
  • rod lenses
  • scanning lenses
  • telecentric lenses
  • zoom lenses
  • (more topics)

Related: loupeslensesmagnificationmicroscopes

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DOI: 10.61835/b12 Cite the article: BibTex BibLaTex plain textHTML Link to this page! LinkedIn

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Contents

What are Magnifying Glasses?

Magnifying glasses are 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 at 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.

Such a device typically consists of a single lens, which modifies the path of light by refraction. The lens is usually mounted in a frame with a handle.

Magnification

Figure 1: A flower seen through a magnifying glass. Note that if one focuses on the object (as done by the autofocus of the camera), the glass itself and its holder are no longer sharp. © Christine Kirchrath Paschotta.

The imaging properties of a magnifying glass must always be considered in conjunction with those of the human eye. The magnification of a loupe is defined as the ratio of the apparent sizes of an object when viewed with and without the lens. For example, when reading a book with small print, a magnifying glass is typically held at some distance from the eye and positioned closer to the text. If the magnification is determined by comparing the apparent sizes of the object as seen with and without the magnifying glass — each time from the same relatively large viewing distance — it can be expressed as: M = 1 + \frac{d_{\rm i}}{f}$$

where ($M$) is the magnification, ($d_{\rm i}$)​ is the image distance, and ($f$) is the focal length of the lens.

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. For 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.

Illumination

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 sunlight 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.

Frequently Asked Questions

What is a magnifying glass?

A magnifying glass is a simple optical device, typically consisting of a single lens in a frame with a handle. It is used to view objects with magnification by modifying the path of light through refraction.

What is the difference between a magnifying glass and a loupe?

A magnifying glass is generally larger, has a longer focal length (125 mm or more), and is held at a significant distance from the eye. In contrast, a loupe is used at a close distance from the eye.

What is the typical magnification of a simple magnifying glass?

A simple magnifying glass typically has a magnification between 2 and 6. Higher magnifications up to about 30 are possible with improved multi-lens designs, but these are generally classified as loupes.

What are aplanatic magnifying glasses?

Aplanatic glasses are magnifiers made with two plano-convex lenses. This design is popular for applications like reading because it produces low image distortions.

Can a magnifying glass start a fire?

Yes, it can be used as a burning glass by concentrating sunlight into its focal plane. This creates a significant fire hazard if the glass is inadvertently left exposed to direct sunlight.

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