Optical InstrumentsOptics

History• The first presence of a magnifying glass is traced

to 11th century when Abu ali al-Hasan Ibn al-Haytham, an Arab scientist, published 'Book of optics' in which he has described in detail the use of a convex lens to develop a magnifying glass.

• The concepts of refraction and  reflection were studied by Euclid in 300 BC and Ptolemy in 150 BC.

• The first real theory about a magnifying glass and its utility for poor vision was put forth by Roger Bacon in 1250.

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General Information

• A magnifying glass (also referred to as a magnifier, magnifying lens, loupe, or hand lens) is a lens or combination of lenses used to magnify (or enlarge) an object.

• A magnifying glass enables people to magnify almost anything (e.g. books, newspapers, fine print, stamps, coins, antiques, art, jewelry, gemstones, foliage, insects, fingerprints, rocks, etc.).

• Today, over 750 years later, it is more popular than ever, assisting people with hobbies and crafts and also helping people with low vision, macular degeneration and other vision conditions to see better.

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Physics

•A magnifying glass is a convex lens.

•Convex means curved outward, like the underside of a spoon or the dome of a sports stadium.

• It is the opposite of concave, or curved inward.

•A lens is something that allows light rays to pass through it and bends, or refracts, them as they do so.

•A magnifying glass uses a convex lens because these lenses cause light rays to converge, or come together.

•Read more : http://www.ehow.com/how-does_4567139_magnifying-glasses-work.html

Image Formation•A magnifying glass, in effect, tricks your eyes into

seeing what isn't there.

• Light rays from the object enter the glass in parallel but are refracted by the lens so that they converge as they exit, and create a "virtual image" on the retina of your eye.

• This image appears to be larger than the object itself because of simple geometry: Your eyes trace the light rays back in straight lines to the virtual image, which is farther from your eyes than the object is, and thus appears bigger.

•Read more : http://www.ehow.com/how-does_4567139_magnifying-glasses-work.html

Convex Lens as a Magnifying Lens

Diopters• Another term used as a measurement of optical power, but

different from magnification is the diopter.

• Diopter refers to the optical power (or strength) of the magnifying lens.

• It is the strength of the magnifying lens measured at one meter.

• Four diopters (4.0D) represent a 100% increase in magnification or 1x power magnification.

• A 20 diopter (20.0D) measurement is approximately equal to 5x power magnification.

• A rough formula to convert from magnification power to diopters is to multiply the magnification power by four.

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Far Point and Near Point

• The far point is the point farthest from the eye at which an object is clearly focused on the retina when accommodation of the eye is completely relaxed. (dictionary.reference.com)

• It is sometimes described as the farthest point from the eye at which images are clear. (en.wikipedia.com)

• The near point is the point nearest the eye at which an object is clearly focused on the retina when accommodation of the eye is at a maximum.

• Accommodation is the automatic adjustment in the focal length of the lens of the eye to permit retinal focus of images of objects at varying distances. (dictionary.reference.com)

Magnifying Glass

Linear Magnification

•Magnification is the ratio of the heights of the object and image.

• It has no units.

• It can also be a ratio of the distance of the object from the lens to the distance of the image from the lens.

• Sign conventions come into play as images pointed down have a negative height and virtual images have a negative distance from the lens (as they are on the same side as the object).

Angular Magnification• the ratio of the angle subtended at the eye by

the image formed by an optical instrument to the angle subtended at the eye by the object being viewed.

Simple Microscope

Compound Microscope

Microscope With Icky Biology Specimen

Reflecting Telescope

Refracting Telescope

Telescope equation

•Different focal length eyepieces really are just moving you closer to, or further from, the image plane of the objective. Link 1

•When you get closer, the image is bigger (higher magnification) and when you get further, the image is smaller (lower magnification). Link 2

•Using radians: =

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• Link 3

Effect of eyepiece on telescope

Angular Magnification Diagram

Aberrations• There are two types you should be aware of:

spherical and chromatic.

•Chromatic aberration is when a lens fails to converge all the colours to a single focal point.

• The effect is caused by dispersion in the glass.

•One way to minimize this effect is to use another lens to correct it (a doublet).

Aberrations• Spherical aberration is when a light rays

refract light differently the further from the optical centre due to the lens curve. (We assumed a thin lens after all!)

• Light rays do not converge at a single point.

• It can be reduced by ensuring the lens shape is the best possible.