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# Optics

Optics is the field of physics that deals with the study of light and its interactions with mater. It is the bases for many of the instruments used by scientists to study the Universe. Optics also helps us to understand vision, the main way we perceive the world. It also helps us improve our vision through corrective lenses where there are defects and to improve are ability to see distant objects and small objects.

## Main principles

The properties of light most often used in optics come mainly from its wave nature, they include reflection, refraction, interference and diffraction. These properties affect the direction and perception of light and it is this aspect that make them useful.

### Reflection

Reflection is the processes were light; or any wave bounces off of a surface. In most cases the light is scattered and only some wavelengths are reflected resulting in variations in color. However when the surface is smooth and reflects all wavelengths of visible light it reproduces the original wave allowing the viewer to see the source of that wave. Such a surface is called a mirror.

### Refraction

Refraction is the bending of light; or any wave; as it passes from medium to another. The phase velocity of a light wave is slower in some media than others, as a result when a change in medium occurs, the wavelength changes so that the frequency remains constant. When entering a faster medium the wavelength gets longer and shorter for a slower medium. When the change in medium occurs at an angle the wave bends so that at the boundary the waves remain aligned.

### Interference

A 2d illustration of the interference of two waves.

Interference is quite easy to understand, since it is simply the adding together of two waves. The math involved in calculating interference is as simple as adding the wave formula of one wave to that of the other. The result is that when the peeks of two waves are at the same point they produce a larger peek, but if they are negative and positive peeks they tend to cancel each other out.

### Diffraction

Diffraction pattern from a square defection grading.

Diffraction occurs when a wave front encounters the edge of a barrier. When encountering such an edge the wave produces secondary wave fronts that bend around the edge. In optics it usually involves one or more slits. When a wave passes through a slit with a width close to the wave length of the original wave the wave fronts that come out on the other side interfere with each other to produce a diffraction pattern as illustrated below.

Illustration of how diffraction works.

The most common use diffraction is measuring the wave length produced by a source.

## Applications

### Lenses

Lenses can be shaped to have single or multiple focal points. These focal points may align with a common optical axis (for example, intraocular or contact lenses), or multiple optical axes within one lens (eg. spectacle lenses).

#### Intraocular Lenses (IOL's)

The most common IOL's used are designed to sit within the bag of the natural crystalline lens, once the nucleus and cortex of the natural lens is removed. This position in the eye is described as posterior, to the iris. Some IOL's are also designed to sit in front of the iris - anterior chamber lenses. IOL's can also be connected to one another, giving additional optical power. These type of lenses are called 'piggyback'. Most IOL's have one focal point and one optical axis, however some IOL's do incorporate multiple focal points through concentric circles of different optical powers - although all aligned along the one optical axis.

#### Contact Lenses

These lenses rest on the anterior corneal surface, lubricated within the tear film. A variety of sizes and materials are used to create contact lenses for different purposes. Soft contact lenses are the most common, their shape and size is such that they fully cover the cornea and slightly overlap the limbus. Hard contact lenses are smaller and more rigid, mostly covering the cornea - these are used for situations such as Keratoconus. Bandage soft contact lenses are much larger and softer than all - their primary function is to protect the cornea - in situations such as corneal injury or transplant.

#### Spectacle Lenses

These are traditionally made out of glass, however the most common material today is CR39. Polycarbonate can also be used in situations where shatterproof protection is of superior importance. Single vision lenses incorporate one focal point, however bifocal and multifocal lenses incorporate multiple focal points through multiple optical axes within the one lens.

## The human eye

Schematic diagram of the human eye
Main Article: Human eye

The human eye contains only a few optical components.[1] When the eye is at rest in its normal state the focus is directed to distant objects. In healthy young people under the age of about 50 years the power of the crystalline lens can be increased to allow near objects to be clearly focused, a process known as accommodation.[1]

## References

1. Bass, Michael, ed. (2010). Handbook of Optics. III - Vision and Vision Optics (3rd ed.). New York: McGraw-Hill. p. 1.3. ISBN 978-0-07-162928-7.