People have been aware of the magnifying properties of a curved piece of glass since at least 2,000 BC. The Greek philosopher Aristophanes in the 5th century BC had used a glass globe filled with water in order to magnify the fine print in his manuscripts. In the middle of the 13th century the English scientist Roger Bacon (1214-1292) proposed that the "lesser segment of a sphere of glass or crystal" will make small objects appear clearer and larger. Due this suggestion, Bacon was branded by his colleagues a dangerous magician and imprisoned for 10 years. Even though spectacles were invented in Italy some time between 1285 and 1300, superstitions were not overcome for another 250 years when scientists discovered the combination of lenses that would lead to the invention of the telescope. There were two types of telescope. The refracting telescope uses lenses to bend light; the reflecting telescope uses mirrors to reflect the light back to the observer.
Chromatic Aberration
When light goes through an ordinary lens, each colour in the spectrum is bent at different angle causing rainbows to appear around the images viewed. The blue end of the spectrum will bend more sharply than the red end of the spectrum so that the two colours will focus at different points. This is chromatic aberration. It can be corrected by adding a second lens (and with a different density), so that all the colours will focus at the same point.
The picture shown in the right is the English optician John Dollond (1706-1761). He was the first to perfect the achromatic lens so that it might be manufactured more easily and solve the problem of chromatic aberration.
An effect of light
Doppler effect can explain one effect of light viewed through a telescope. This explains how wavelength is affected by motion. The light if any object, such as a star approaching the Earth, will be compressed and shifted towards the short wavelength (blue) end of the spectrum. Light from objects moving away from Earth will be elongated and shifted towards the red end of the spectrum. These effects are called "blue shift" and "red shift".
