Facts and Theories about Light

Light sources, other than those which shine by reflected light, are generally hot; and in the case of solids and liquids (such as filament lamp or the surface of molten iron) the nature of the light emitted depends largely on the t° of the sources - the lower the t° of the source, the greater the preponderance of red light in the emission. The following is an approximate guide to colour t° for surfaces which are good radiators:

very dull red	500-550 ° C	yellow	1050-1150 ° C
dark red	650-750 ° C	yellow-white	1250-1350 ° C
bright red	850-950 ° C	white	1450-1550 ° C

Light of all types travels through space at the rate of 186,000 miles/sec, or 3 × 10¹⁰ cm./sec.

This was first discovered by a Danish astronomer, Roemer, who in 1674 found that light from a celestial source (he observed the eclipse of the light from one of Jupiter's moons) took 1000 seconds to cross the Earth's orbit of 186,000,000 miles. The speed is the same for light of all colours. It is also the same for all types of thermal radiation and for radio waves whether of long or short wave-length. The common speed of such widely different types of radiation is part of the evidence which suggests that they are different aspects of similar phenomena. The name electro-magnetic radiation has been given to all these radiations. Present day theory assumes that light is a disturbance, propagated in space, similar to that sent out from a radio aerial, but whereas the electrical oscillations in an aerial have a frequencv of the order of a million per second, the electrical oscillations set up by the electrons in the atoms of a light-source have a frequency of the order of a thousand million per second.

About the beginning of the 18th century there was much speculation as to whether light consists of minute weightless particles shot out by source (Corpuscular Theory) or a disturbance spreading out from the source as a wave motion (Wave Theory).

Huyghens (1629 - 1695) developed the Wave Theory. Newton (1642 - 1727) recognizing, that both theories could explain all that was then known about light, somewhat favoured the Corpuscular Theory. He was influenced in this by the fact that water ripples and sound waves can bend round obstacles, a phenomenon known as diffraction. Point sources of light cast sharp shadows with no diffraction. Though Newton had observed peculiarities at the edges of shadows he did not consider that they were examples of diffraction. It was not until the beginning of the 19th century that Young (1773 - 1829) and Fresnel (1788 - 1927) showed that diffraction of light does occur, and that the apparentlv straight-line travel of light is the result of its very short wave-length.

The refraction of light passing from air into water was explained by the Corpuscular Theory as due to an increase in speed of the waves.

In 1851 Foucault showed that the speed of light in water is less than in air.

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