Dispersion of Light and Colors, Physics tutorial

Concept of Dispersion of Light:

Dispersion is the breaking up of or separation of white light into its component colors whenever it is passed via a glass prism.

This is due to the fact that different colors of white light travel at various speeds via the glass. As an outcome of which each and every color is refracted in a slightly different direction or angle via the glass. That is each and every color has its own velocity, wavelength and refractive index of refraction. Therefore, white light is split up into seven colors: Red, Orange, Yellow, Green, Blue, Indigo and Violet.

White Light Spectrum:

Whenever a ray of light is inclined obliquely on a glass prism, it appears deviated and separated into its component colors: Red, Orange, Yellow, Green, Blue, Indigo and Violet (ROYGBIV), due to refraction. Such colors resultant from the separation (that is, dispersion) of white light makes up the spectrum that is usually termed to as the white light spectrum.

Sir Isaac Newton was the first to view in an experiment that if white light is passed via a prism, an elongated colored patch of light is achieved on a screen placed behind the prism. This colored pattern now termed to as the spectrum of white light comprises of the Red, Orange, yellow, Green, Blue, Indigo and Violet (ROYGBIV). An examination of the spectrum represents that the red light is deviated least whereas the violet light is deviated most. Since red is deviated least from its original direction in air, its change of speed on entering glass is least and as such its speed in glass is maximum .Violet is deviated most on entering glass and it consists of the least speed in glass.

1901_white light spectrum.jpg

When another similar prism is placed to intercept the refracted rays, the similar arrangement of colors will emerge on the screen. This time, though, the colors will be more broadly separated. On the other hand, when the second prism is inverted, the colors will disappear and just a patch of white light will be visible on the screen.

The vanishing of the colors was due to their recombination to reproduce white light. Therefore white light spectrum is the band of colors made if white light passes via a prism.

Production of Pure Spectrum:

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The colors in the spectrum generated through a prism alone will not be pure because each color is blurred on the screen and overlies its neighboring colors. A pure spectrum is one in which light of one color just forms each portion of the image or spectrum on a screen.

The apparatus comprises of these components: source of white light, slit, two convex lenses, one triangular prism and a screen.

To generate a pure spectrum the given conditions must be arranged:

The source of white light should be restricted through an extremely narrow slit.

The white light must arrive at the prism in a parallel beam.

The parallel beams of dissimilar colors of light emerging from the prism must be focused through a lens onto a screen in such a way that they don't overlie.

The figure represents an arrangement for generating a pure spectrum. The first converging lens is employed to generate a parallel beam of white light. This is done through making the distance from the slit to the lens equivalent to its focal length.

The second converging lens reverses the consequence of the first in such a way that the parallel beam of each separate color is focused onto the screen.

The joint effect of the two lenses and the prism is to generate a focused image of the slit on the screen in slightly dissimilar positions for each and every color in the spectrum.

This pure spectrum comprises of Red, Yellow, Green, Blue, Indigo and Violet. Red is least deviated whereas Violet is the most deviated.

Primary, Secondary and complementary Colors:

1) Primary Colors:

Primary colors are red, green and blue; they are termed as primary colors as it is not possible to generate them by mixing other colors. Whenever three primary colored lights are mixed they generate white light.

2) Secondary Colors:

Secondary colors are such colors which can be obtained through mixing any two primary colors. Illustrations are yellow, cyan and magenta. They are generated through mixing two primary colors like:

  • Green + blue = cyan (secondary color)
  • Red + blue = magenta (secondary color)
  • Green + red = yellow (secondary color)

3) Complementary Colors:

If two colors, one primary and the other secondary are in such a way that if mixed give white color, then they are stated to be complementary colors. Illustrations are:

  • Yellow + blue = white
  • Cyan + red = white
  • Magenta + green = white

Mixing of Colored Pigments (paints):

In contrary to the mixing of colored light, if blue and yellow paints are mixed, the outcome is green not white. The colors of pigments (that is, paints) are the outcome of the types of light they reflect. For illustration, yellow paint reflects red, yellow and green light and absorbs the rest. On the other hand, blue paint reflects blue and green light and absorbs the rest. Though, if yellow and blue paints are mixed the single color reflected by both components of the mixture is green that is therefore the color of the mixture.

This mixture of paints is termed as color mixing by subtraction. It is dissimilar from mixing of colored lights that are pure colors termed as mixing by addition. 

The primary colors for paints dissimilar to lights are red, yellow and blue. They are the base for all other colors of paint and can't be generated through mixing.

Colors of Objects:

The colors of objects in white and colored lights based on the colors in the light incident or falling on them and as well on the absorption and reflection of such lights through the objects.

A white object seems white in daylight as it reflects uniformly all the colors of the spectrum. As well the colors of opaque objects seen are the colors of light which are reflected. The given are a few illustrations:

a) Rose flower seems red as it reflects red light and absorbs the other colors.

b) White objects reflect all the colors of the spectrum and come out white.

c) Black objects absorb all the colors of spectrum and come out black.

That is, objects come out to be of a certain color as they reflect light of such colors and absorb the rest. Primary colors will reflect or transmit merely their respective colors. Other colors will reflect or transmit other colors all along by their own colors.

Light Filters:

Light filters are transparent-colored sheets that let light of the color of the filter to pass via while absorbing all other colors in the incident light they are made up of gelatin and colored by means of different dyes.

Filter papers are employed to absorb light of different colors from the spectrum of white light. Usually, a filter paper absorbs all other colors apart from that of the filter. For example:

a) If a red filter is put on the path of white light, just red light passes via;

b) If a green filter is put on the path of a red light, no light passes via. Therefore, there is a black appearance.

An unusual condition takes place if a yellow filter is positioned on the path of white light. This time all other colors are absorbed apart from yellow, green and red.

Subtractive color mixing as well outcomes from the super-position of two color filters. The mixing of two filters outcomes in the transmission of only such colors that is common to both of them.

Formation of Rainbow:

This is formed in the similar manner as a spectrum is made by a glass prism. Here, rain drops or drops of water hanged in the air after rainfall act as prisms that disperse the white beam of light into seven colors of the rainbow. The viewer can observe the rainbow only when he is in a position to get the refracted rays of the sunlight. This is at times, two rainbows are seen namely the primary and the secondary rainbows.

The Primary Rainbow:

The primary rainbow is much brighter than the secondary rainbow. This is caused through light rays that experience one internal reflection within the rain drop. If a horizontal light ray enters a water droplet at point, dispersion occurs the colored rays undergo total internal reflection and they exist from the water droplet further refraction takes place. The violet ray refracted the most and the red ray least. The red ray that comes out from each droplet forms a bigger angle of around 42o with the horizontal than the violet ray which makes around angle 40o as well with the horizontal. 

The secondary rainbow:

A secondary rainbow emerges outside of a primary rainbow and develops if light entering a raindrop experiences two internal reflections rather than just one (as in the case of primary rainbow). The intensity of light is decreased even further by means of the second reflection; therefore secondary rainbows are not as bright as primary rainbows. Alternatively: fewer light rays go via the four-step series than the three-step series.

In the secondary rainbow, the red rays are at angle of 50o and the violet rays at 54o to the horizon

The Electromagnetic Spectrum:

You have observed that the spectrum of white light obtained from the sun-light comprises of seven colors. Beyond each end of this colored spectrum (or visible light) are the invisible rays (or spectrum) of radiant energy termed as infra-red and ultraviolet rays. Such rays are as well emitted by the sun. These rays can't be seen however they cause some materials to fluorescence and as well influence photographic plates. They are identified as ultra-violet rays.

Radio-waves, infra-red rays, the visible rays (from red to violet), x-rays, ultra-violet rays and gamma rays are all electromagnetic waves.

Such waves make up the electromagnetic spectrum. They all travel via vacuum having precisely similar speed and differ only in having dissimilar wavelengths.

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