OPTICS and Refraction

LIGHT
Light is the visible portion of the electromagnetic radiation spectrum. It lies between ultraviolet and infrared portions, from 400 nm at the violet end of the spectrum to 700 nm at the red end. The white light consists of seven colours denoted by VIBGYOR (violet, indigo, blue, green, yellow, orange and red). Light ray is the term used to describe the radius of
the concentric wave forms. A group of parallel rays of light is called a beam of light.
Important facts to remember about light rays are :

• The media of the eye are uniformally permeableto the visible rays between 600 nm and 390 nm.
• Cornea absorbs rays shorter than 295 nm.Therefore, rays between 600 nm and 295 nm onlycan reach the lens.
• Lens absorbs rays shorter than 350 nm. Therefore,rays between 600 and 350 nm can reach the retinain phakic eye; and those between 600 nm and 295nm in aphakic eyes.
•  

GEOMETRICAL OPTICS
The behaviour of light rays is determined by rayoptics. A ray of light is the straight line path followed by light in going from one point to another. The rayoptics, therefore, uses the geometry of straight lines to account for the macroscopic phenomena like rectilinear propagation, reflection and refraction. That is why the ray-optics is also called geometrical optics. The knowledge of geometrical optics is essential to understand the optics of eye, errors of refraction
and their correction. Therefore, some of its important aspects are described in the following text.