Monday, February 13, 2012

Humanity learning about light

יהי אור
The Bible tells that God created light as the first command of creation jehi or! (Genesis 1:3)

His Sun rises in the morning and sets in the evening giving daylight, His Moon delights us at nights or raises awe and even fear, the stars twinkle so brightly in their thousands on the velvet night sky.

Humanity has been able to create light by burning things and by reflecting light from surfaces. Since prehistoric times open fires, torches and later on various kinds of lamps have given light to the darkness. Its properties could be studied and used in optics, its beauty shown in high art and shadows utilized for many different kinds of effects. However, for most of our history we have really had no idea what light actually is.

Classical Greece and Hellenism
In the fifth century BC, Empedocles postulated that everything was composed of four elements; fire, air, earth and water. He believed that Aphrodite made the human eye out of the four elements and that she lit the fire in the eye which shone out from the eye making sight possible. If this were true, then one could see during the night just as well as during the day, so Empedocles postulated an interaction between rays from the eyes and rays from a source such as the sun.

In about 300 BC, Euclid wrote Optica, in which he studied the properties of light. Euclid postulated that light travelled in straight lines and he described the laws of reflection and studied them mathematically. He questioned that sight is the result of a beam from the eye, for he asks how one sees the stars immediately, if one closes one's eyes, then opens them at night. Of course if the beam from the eye travels infinitely fast this is not a problem.

In 55 BC, Lucretius, a Roman who carried on the ideas of earlier Greek atomists, wrote: "The light & heat of the sun; these are composed of minute atoms which, when they are shoved off, lose no time in shooting right across the interspace of air in the direction imparted by the shove." – On the nature of the Universe

Despite being similar to later particle theories, Lucretius's views were not generally accepted. Ptolemy (c. 2nd century) wrote about the refraction of light in his book Optics.

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