Emission Vs Absorbtion
The visible spectrum is that portion of the electromagnetic spectrum that is visible to the human eye, which corresponds to wavelengths from about 390 to 700 nm. Colors containing only one wavelength are also called pure colors or spectral colors. Other colors can also be detected by the human eye and brain but are actually combinations of colors. For example, yellow and blue combines to form green. Purple is not near violet but is actually a combination of red and violet.
Isaac Newton discovered that prisms could disassemble and reassemble white light, and described the phenomenon in his book Opticks in 1671. He was the first to use the word spectrum (Latin for “appearance” or “apparition”).
The visible spectrum is actually only a small region of the electromagnetic spectrum. Beyond violet is Ultraviolet (UV), X-rays and gamma rays. All are higher energy than the visible spectrum. Energy is given by E = h\nu (or E=hv) Planck’s Law), which means that violet and therefore UV light has more energy than red and Infrared (IR) light. As an example it is UV light that causes sunburn of our skin. Sun protection agents, either physical or chemical, either refract the UV light or absorb and then re-emit in a less energetic and therefore less damaging portion of the spectrum. The following table illustrates the visible spectrum in relation to the electromagnetic spectrum.
When energy is supplied to an atom the electrons will jump to higher electron shells. When electrons return to their relaxed state a photon of light is emitted. The energy of the emitted photon is equal to the energy difference between the two states. There are many different atoms, each with their own unique electronic states, which means that the photons emitted can be used to identify the atoms present.
Photons are emitted at very discreet wavelengths. A spectrum of photons (light) emitted is said to be an Emission spectrum. Conversely, if energy is supplied to an atom the energy will be absorbed at the same unique and discreet wavelengths. In this case the wavelength will show as black or lack of color since the energy is being absorbed. This is known as an Absorption spectrum. See the following illustration using Hydrogen as the example.
Newton’s color circle:
Newton prismatic colours: