(Previous experiments involving electricity largely involved static electricity or, as with Benjamin Franklin and the kite, lightning.) In 1820, Danish scientist Hans Orsted noticed that the needle on a nearby compass deflected when he turned the current on and off a voltaic cell. The mention of magnets in ancient literature goes back to the 4th century B.C., while constructive use of electricity dates to 1800 with the invention of the voltaic pile, the first crude battery. He reasoned that there was an invisible form of light beyond violet, which he named "deoxidizing rays," later known as "chemical rays." Once the nature of light was better established in the late 19th century, the term "ultraviolet" - beyond violet - light was adopted. In 1801, German scientist Johann Ritter noted that the region of the spectrum just beyond the violet edge of visible light was more effective at turning silver halides dark. It was obvious that there was "light" beyond the color red this light was eventually termed "infrared" light - literally, "below red" light.Įxperiments with light showed that some colors could darken certain silver salts - indeed, this is the basis of photographic film. Upon putting the thermometer past the red light, he noted an even larger increase in temperature than when the thermometer was bathed in visible light. In 1800, British astronomer William Herschel was measuring the effect of various colors of light on a thermometer, using a prism to disperse light from the sun.
The energy and momentum of each photon of visible light can be calculated from the earlier equations. For visible light, wavelengths range from 400 to 700 nm, which correspond to frequencies of 7.5 × 10 14 to 4.3 × 10 14 waves per second. (See Figure 2 for an example of the speed of light.)īecause the speed of light is a constant, there is a simple relationship between the wavelength of a light and its frequency. In a vacuum, the speed of light is approximately 3.00 × 10 8 m/s - about seven and a half times around the Earth every second. Unlike that of other waves, the speed of light is a constant for a given medium.
As with any wave, the velocity of light is equal to the product of its frequency and wavelength: Because a photon of light has a certain energy E and momentum p, it acts as a particle:īecause light's properties can be described by values of wavelength λ, frequency υ, and wavenumber υ tilde (see Figure 1), it acts as a wave.
Light has both particle and wave properties, as has been discussed in a recent column (1). The collective range of possible lights is called the electromagnetic spectrum. Although currently the term "light" refers to a lot more than just visible light, it's more common to use the phrase "electromagnetic radiation" when referring to any form of light. However, that changed in the 1800s when it was finally realized that light was a more general phenomenon. When a person says the word "light," a listener - even a spectroscopist - usually interprets that as meaning "visible light." That's not unexpected, because throughout most of recorded history the only light that we recognized was light that we could see.