A beam of light passing through a triangular prism deviates to a face lying opposite the refracting angle of the prism. However, if it is a beam of white light, then it, after passing through a prism, will not only deviate, but also decompose into colored beams. This phenomenon is called dispersion of light. It was first studied by Isaac Newton in 1666 in a series of remarkable experiments.
The light source in Newton's experiments was a small round hole located in the shutter of a window illuminated by the rays of the Sun. When a prism was installed in front of the hole, instead of a round spot, a colored strip appeared on the wall, called the Newton spectrum. Such a spectrum consists of seven primary colors: red, orange, yellow, green, blue, blue and violet, which gradually passed one into another. Each of them occupies a space of various sizes in the spectrum. The violet stripe has the greatest length, the red one the smallest.
The next experiment was that from a wide beam of colored rays obtained using a prism, narrow beams of a certain color stood out with a screen with a small hole and were sent to the second prism.
Prism rejecting them does not change the color of these rays. Such rays are called simple or monochromatic (monochromatic).
Experience has shown that red rays experience less deviation than violet rays, i.e. rays of different colors are not equally refracted by the prism.
Collecting a lens with colored beams of rays emerging from a prism, Newton received a white image of a hole instead of a colored strip on a white screen.
Of all the experiments carried out, Newton made the following conclusions:
- white light is inherently a complex light that consists of colored rays;
- in light rays of different colors, the refractive indices of a substance are also different; as a result, when a beam of white light is deflected by a prism, it decomposes into a spectrum;
- if we connect the colored rays of the spectrum, then again we get white light.
Thus, the dispersion of light is a phenomenon that is caused by the dependence of the refractive index of a substance on the wavelength (or frequency).
Light dispersion is noted not only when light passes through a prism, but also in various other cases of light refraction. So, in particular, the refraction of sunlight in drops of water is accompanied by its decomposition into colored rays, this explains the formation of a rainbow.
To obtain the spectrum, Newton sent a rather wide cylindrical beam of sunlight through a round hole made in the shutter to the prism.
The spectrum obtained in this way is a series of multi-colored images of a circular hole, partially overlapping one another. To obtain a cleaner spectrum, when studying a phenomenon such as light dispersion, Newton suggested using not a round hole, but a narrow slit parallel to the refracting edge of the prism. Using a lens on the screen, a clear image of the gap is obtained, after which a prism is installed behind the lens, which gives a spectrum.
The purest and brightest spectra are obtained using special instruments - spectroscopes and spectrographs.
The absorption of light is a phenomenon in which the energy of a light wave decreases as it passes through a substance. This is due to the conversion of the energy of a wave of light into the energy of secondary radiation or, in other words, the internal energy of a substance that has a different spectral composition and other directions of propagation.
The absorption of light can cause heating of the substance, ionization or excitation of molecules or atoms, photochemical reactions, as well as other processes in the substance.