What is white light?

The visible light It is composed of the range of wavelengths of the electromagnetic spectrum that is able to excite the nerve cells of the retina of the human eye, that is why it is called “visible”.

The visible light spectrum can be divided into smaller bands of wavelengths, each of which is interpreted by our brains as a different color. Specifically, within visible light we can find, from the longest to the shortest wavelength, red, orange, yellow, green, light blue, blue and violet, the 7 colors of the rainbow.

When the human eye sees a light made up of the entire range of visible light, that is, all colors superimposed on it, it is interpreted as white by our brain. This light is what is known as white light. In other words, white light is light composed of the superposition of the entire spectrum of visible light.

The composition of white light by different colors was demonstrated by Isaac Newton in the 17th century. Newton began to experiment with the decomposition of the white light of the Sun in 1666 and in his book optics, published in 1704, he brings together his theories about the nature of light and its dispersion, refraction and reflection.

white light sources

Most white light sources are thermal, which means that the type of radiation emitted by a body is a characteristic linked to its temperature. Objects with low temperatures emit infrared radiation, which is not part of visible light. As the object heats up, the wavelengths of emitted radiation become shorter (more energetic) moving to red, yellow, green, etc., until it produces white light if the temperature rises enough to emit waves all over the visible spectrum.

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The main natural sources of white light are the Sun and stars in general. Other glowing bodies, such as glass and some metals, can also generate white light. For example, the incandescent filaments of light bulbs. Some artificial sources of white light, such as fluorescent lamps and white LEDs, are not linked to brightness or high temperatures, but generate white light radiation through other mechanisms (fluorescence S electroluminescence respectively).

white light composition

As described above, white light is a beam of light that contains overlapping wavelengths of the entire visible light spectrum. That is, white light contains electromagnetic radiation with wavelengths from 390 nm to 750 nm, although some people can perceive wavelengths between 380 and 780 nm.

Below 390nm it is ultraviolet while above 750nm it is infrared. Within the spectrum of visible light, and therefore in white light, we find the seven colors of the rainbow, whose wavelengths are:

violet: 390 – 450nm blue: 450 – 475nm cyan or light blue: 475 – 495nm green: 495 – 570nm yellow: 570 – 590nm orange: 590 – 620nm red: 620 – 750nm

Despite the fact that by breaking white light we get seven colors, in practice white light can be obtained, at least interpreted that way by humans, by combining just three, red, yellow and blue, which is why these colors are called primary colors.

refractive scattering

Refractive scattering is the phenomenon of light scattering due to refraction. The refraction of an electromagnetic wave is the change in direction that the wave undergoes when passing from one material medium to another with a different refractive index. Refraction only occurs when the electromagnetic wave strikes the new medium obliquely and does not strike each other perpendicularly.

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The change in direction due to refraction is not the same for all wavelengths, but each wavelength undergoes a different deviation. This makes a beam of light composed of different wavelengths spread out when it undergoes refraction. This is what happens when white light from the Sun passes through water droplets in the atmosphere and a rainbow is formed; each of the different wavelengths that make up white light undergoes a different change of direction, separating itself from the rest. This same phenomenon was used by Newton to describe the visible spectrum using a prism through which he passed sunlight.

Refractive dispersion is the basis of spectrometers (or spectrophotometers), instruments that allow analyzing the emission and absorption spectrum of a material and knowing its composition. This technique is exactly called dispersive spectroscopy.

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