Atoms & Light

Part 7 - Lenses

Lenses, made from polished crystals of quartz, date from 750 BCE Mesopotamia and both the Romans and Greeks filled glass spheres with water to make lenses. Hero of Alexandria (10–70 CE) extended Euclid's work on geometrical optics by demonstrating that the angles of incidence and reflection were equal and this was the shortest path from the object to the observer. 

Claudius Ptolemy (CE 100 – 170) wrote about the properties of light, including colour, reflection and refraction.

In the 980's CE, Ibn Sahl of Baghdad, commented on Ptolemy's Optics noting how curved mirrors and lenses ("burning instruments") bent and focussed light. He also described a law of refraction equivalent to Snell's law and used this to compute the shapes of lenses and mirrors that focussed light at a single point.

Late in the 13th century, Qutb al-Din al-Shirazi and his student Kamāl al-Dīn al-Fārisī were among the first to give the correct explanations for the rainbow phenomenon.

Sometime between the 11th and 13th century, monks began using crude magnifying glasses made by cutting a glass sphere in half. These were greatly improved when thinner lenses were found to be more effective. 

Eyeglasses were first made about 1286 as optical industries developed in Italy, the Netherlands and Germany. The first ground and polished glass spectacle lenses were convex (magnifying) lenses used for reading but, soon, concave lenses were made to improve distance vision for short sighted people. 

When light travels from one transparent medium to another (for example, from air to water), it is refracted, which is why a straight stick appears bent at a water surface. This is caused by the different speed of light in each medium. Light travels at approximately 300,000 km/s (186,000 mi/s) in a vacuum, (refractive index 1.0),

but it slows down to 225,000 km/s in water (refractive index = 1.3), 

It is 200,000 km/s in glass (refractive index = 1.5),

and 124,000 km/s in diamond (refractive index = 2.42). 

Snell's Law specifies the amount of bending, depending on the refraction indexes of the two media. 

Through most of 1603, the astronomer Johannes Kepler (1571–1630) puzzled over the optical effects of eclipses particularly the red colour of a total lunar eclipse and on January 1, 1604, he published his findings. He described the inverse-square law governing the intensity of light, reflection by flat and curved mirrors, and principles of pinhole cameras, as well as the astronomical implications of optics such as parallax and the apparent sizes of heavenly bodies. Oddly, he did not mention refraction.

In 1621, Willebrord Snellius (1580–1626) described the mathematical law of refraction, (Snell's law). 

Later, René Descartes (1596–1650) used this to show that the angle between the edge of the rainbow and the rainbow's centre was 42°.

Isaac Newton (1643–1727) demonstrated that a triangular glass prism would split sunlight (which is white) into a spectrum of rainbow colours, and that a similar prism could recombine the multicoloured spectrum back into white light. He also separated one of the coloured beams and directed it at various objects to demonstrate that it did not change its colour, whether it was reflected, scattered or again refracted; it remained the same colour and could not be split further. He concluded that the colour of objects is not generated by the object but by their absorption of some of the colours in white light and the reflection of other colours. He also realized that in any refracting telescope, a simple lens would refract each colour at a slightly different angle so that each colour would have a slightly different focal point resulting in chromatic aberration.