What is refraction? Refraction is the bending of a light when

it enters a medium where the speed of light is different. When a light ray passes from a fast medium to a slow medium, the light ray bends towards the normal.

Ni * Sin(Ai) = Nr * Sin(Ar) where:Ni is the refractive index of the medium the light is leaving,Ai is the angle of incidence between the light ray and the normal to the medium to medium interface,Nr is the refractive index of the medium the light is entering,Ar is the refractive angle between the light ray and the normal to the medium to medium interface.

A telescope is an amazing device that has the ability  to make faraway objects appear much closer. 

The light from the object passes through the convex lens and converges at the focal point. The light rays then continue and meet directly at another convex lens, where the light is refracted again and this time made parallel for the human eye to see.

Myopia, short sightedness allows us to see objects that are near to us, but what about distant objects?

With glasses, we can see distant objects too.

Lenses, like all glass, refract light. Lenses for eyeglasses are shaped in such a way that they will refract light in a certain direction. They are like two prisms smashed together. Minus lenses have thick outer edges and thin middles. The thin middles are the tips of the prisms all squashed together. This makes light spread out, moving the focal point forward. 

Cuts which are too shallow appear lifeless as the light travelling through the stone is lost out of the bottom of the diamond

Cuts which are too deep will appear lifeless as the light travelling through the stone is lost out of the sides of the stone

The atoms in a diamond are tightly packed with electrons -- so tightly that light travels at less than half the speed through the crystallized carbon as it does through air. In fact, no other transparent material slows light more than diamond. Jewelers use this unique property to temporarily trap light. They cut and polish diamond in its rough, natural state to produce a gem with smooth sides called facets. Light enters from all sides, and once inside, it bounces off several facets before leaving. The farther the light travels before leaving, the more it separates into the rainbow of colors that make it up. A diamond's dazzling sparkle, then, results from light's bouncing travel. 

As the cornea has a larger refractive power, it is more important for focusing light rays on the retina at the back of the eye. However, refraction alone is not enough to obtain a clear image, especially, when we are looking at near objects. From here, the importance of the lens arise, as it can vary its refractive power by changing shape from being spherical to be more oval and back again when needed, in a process known as accommodation.

Myopia, also called near- or short-sightedness, is a refractive error ( vision defect) of the eye in which with accommodation (the ability of the eye to change its refractive power) completely relaxed, parallel light rays entering the eye come to a focus in front of the retina at the, creating a blurred image. 

In the myopic eye, the eyeball is too long or the cornea (the clear front cover of the eye) is too steep (has too much curvature). As a result, the light entering the eye is not focused correctly and distant objects look blurred.

Hyperopia, also known as farsightedness, longsightedness or hypermetropia, is arefractive error (vision defect) of the eye in which with accommodation (the ability of the eye to change its refractive power) completely relaxed, parallel light rays entering the eye come to a focus behind the retina, instead of directly on it, creating a blurred image. 

Hyperopia occurs if the eyeball is too long or the cornea (the clear front cover of the eye) is too flat (has less curvature). As a result, the light entering the eye is not focused correctly and near objects look blurred (Figure 1).

The white light from the projector lamp is split into red, green, and blue components using two dichroic mirrors, special mirrors that only transmit light of a specified wavelength. Each red, green and blue beam then passes through a dedicated LCD panel made up of thousands of miniscule pixels. An electrical current turns the panel's pixels on or off to create the grayscale equivalent of that color channel. The three colors are then recombined in a prism and projected through the projector lens and onto the screen.

Convex in order to magnify the size of the image

http://science.howstuffworks.com/telescope1.htm http://www.ehow.com/how-does_4672671_eyeglasses-work.html http://images.google.com.sg/imglanding?q=diamond

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http://www.lasikguider.com/2009/02/myopia-nearsighted-eye.html

http://www.lasikguider.com/2009/02/hyperopia-farsighted-eye.html