What is Colour?

When Sir Isaac Newton passed a beam of sunlight through a glass prism, he proved that sunlight is made up of colors. As the light was bent by the prism, it formed a spectrum.

Most people can see six or seven colors in the spectrum, but with instruments, more than 100 colors can be seen in it. But white light is really made up of three basic colors which are called “the primary colors.” They cannot be made from any other colors. The primary colors of light are orange-red, green, and violet-blue (RGB).

In the spectrum however, we can also see three mixed colors with the naked eye. These are called “secondary colors.” They are green-blue also called as Cyan (turquoise), yellow and magenta red (CYM). You can make these secondary colors by mixing other colors together.

Colors consist of wave lengths to which the human eye is sensitive. Insects and many other creatures respond to other wave lengths and see other colors too which human beings cannot see say, for example, bees can see and are particularly sensitive to ultraviolet light. In fact, light or color wave lengths are very short ranging somewhere from 350 nm at the shorter end to about 700 nm at the longer red end of the spectrum.

Noted that the so called Paint colors are actually the substances and they are exactly the opposite from light colors. The secondary colors in light are actually the primaries in paint. This simply means that in paint, the primary colors are yellow, green-blue (turquoise) or cyan and magenta red; and the secondary colors are orange-red, green and violet-blue i.e. exactly the other way round of the visible light colors. This is the reason that they are described as the artist’s colors.

What is a Hue?

A color that is brilliant and has no black or white paint in it is called “a hue.” Yellow, red, blue, green, etc., are all hues. In short, a hue is the property that gives a colour its name – for example, red, orange, yellow. A color that is mixed from a hue i.e. having any one of the above colors such as yellow, red, blue or green and a black and mixing them together what that we get is called “a shade.” For example, Deep brown is a shade. On the other hand, a color that is made with a hue and white is called “a tint”. Pink and ivory are tints. A color that is a mixture of pure hue, black and white is described as “a tone.” Tan, beige, straw and grey are nothing, but “tones”.

Red paint inside the can doesn’t look red – it looks black! Why? Because, where there is no light there is no color. For the same reason, not only do we are unable to see any color in a dark room, the color is not there! The color of an object depends on the material of the object and the light in which the object is seen. An orange-red sweater looks orange-red because, the dye with which the wool was treated reflects the orange-red part of the light and absorbs the violet-blue and green parts of the light. 

As noted above, we see that the colour of an object depends on the colour of the light falling on it and the colour(s) it transmits or reflects back onto our eyes. A filter is made of glass or celluloid and lets through light of certain colours only. For example, a red filter transmits or reflects mostly red light and absorbs other colours, it therefore produces red light when white light shines through it. Thus, transparent objects are of that colour which they allow to pass through them. Opaque objects do not allow light to pass but are seen by the light reflected from them. A white object reflects all colours and appears white in white light, red in red light, blue in blue light and so on. A blue object appears blue in white light because, the red, orange, yellow, green and violet colours in white are absorbed and only blue is reflected. It also looks blue in blue light ( an object always reflect back its own color) but in red light it appears black since, no light is reflected and blackness indicates the absence of colour. A black object absorbs all colours and does not reflect any color wavelength.

Mixing of colors: 

In science red, green and blue are called primary colours (they are not the artist’s primary colours) because none of them can be produced from other colours of light. However, they give other colours when suitably mixed. The primary colours can be mixed by shinning beams of red, green and blue light on to a white screen so that they partially overlap.

The colors formed by adding two primaries are called secondary colours; they are as such called as yellow, cyan (peacock blue) and magenta. The three primary colours give white light when they are mixed together, as do the three secondaries. A primary colour and the secondary opposite to it in the colour triangle as shown above – such as blue and yellow – also give white light; any two colours producing white light are called complementary colors. In this sense, blue & yellow, green & magenta or red & cyan are all complementary colors.

Connecting concepts: How do ‘Paints’ & ‘Dyes’ exhibit their color? 

Noted that the paints or dyes are being made from the substances what we call as pigments. It is these pigments that actually give colour to paints and dyes by reflecting light of certain colours only while absorbing all other colours. However, since most of the pigments are impure substances and as such, they reflect not a single, but more than one colour.  Essentially, when they are mixed together, it is always be the colour that is common to all of them that is reflected. For example, mixing blue and yellow paints together gives a green mixture because, blue paint reflects indigo and green (it neighbours in the spectrum) including as well the blue, while yellow paint reflects green, yellow and orange (for the similar reason of their being the neighboring colors). Since green is common to them therefore, only green is reflected by both.

Mixing of colored pigments is done through a process called as ‘subtraction’. Whereas, coloured lights are mixed together by a process of ‘addition’.

“SCATTERING” of Light & Why is the sky blue?

Scattering describes what happens when light rays (indeed, all electromagnetic rays) strike atoms, molecules, or other individual tiny particles. These particles send the rays of light off in new directions – that is, they cause the rays to scatter. Our clear sky appears blue to us because, air molecules scatter more blue rays towards us than they do the other colours in sunlight. Since, violet and blue have shorter wavelengths than the other colors of the spectrum and are thus, scatter the most. But then, our eyes are not very sensitive to violet & secondly, the sun is itself relatively weak in violet light and thus, we see only blue as the scattered light and hence, our sky thus looks to us.

Why does a setting sun appear red to us? When the sun is near the horizon, it looks orange or red because; the light reaching us has lost so much of its other colours by scattering. Whereas, the Red light with its longer wavelength, scatters the least of all. Moreover, the path for the light to traverse is pretty long when the sun is at the horizon and it is only the light of a longer wavelength that could cover this longer path to reach our eyes. Therefore, blue, green and all other colors of the spectrum get almost scattered in the way and only red reaches the observer’s eyes.

“INTERFERENCE” of LIGHT:

In many cases, light can be thought of as being a wave with crests and troughs. When two light waves cross through the same spot, they interfere with each other – that is, they add to or subtract from each other. Suppose that whenever a crest of one wave passes through a particular spot and so does the crest of another wave. The two crests add together to give a larger crest. This process is what that we call as constructive interference. Since, the resulting wave has been formed due to addition of two separate light waves at the same spot; it certainly gives a brighter light than either of the two waves would have given separately. Taking this other way round suppose that instead of crests of two different waves (as given above) crossing at the same spot, there is a crest of one wave and a trough of the other wave that crosses together at the same spot. What will happen? The trough reduces the height of the crest and thus, diminishing the impact of light and hence, leaving the spot dim or even dark. This process is called as destructive interference.

Now the fact that light can interfere in such a way that it can either produce brightness or darkness, provides the strongest argument for the wave nature of the light. All types of waves can in fact, produce a pattern of constructive and destructive interference when they pass through two small, nearby openings.

A useful application of interference is in non-reflecting coating for glass. The surface is covered with a chemical film of just the right thickness to stop most of the light that would ordinarily be reflected and cause glare. When applied to a camera objective this improves the quality and brightness of the image by cutting out reflections from the various lens surfaces.

“DIFFRACTION” of LIGHT:

The spreading of light coming from a source is called as diffraction in its simplest terms. Like interference, diffraction also results from the fact that light behaves as a wave. A light wave always tends to spread slightly when it travels through a small opening or around a small object or past an edge. Water waves also spread similarly, but the openings and objects that cause them to spread must be much larger than those for light. Due to the very small wavelength of the light waves, diffraction of light is thus not easy to detect.

Incidentally, the iridescent rainbow play of colors that you see when white light is reflected almost parallel to the surface from a gramophone record is due to the fact that the various wavelengths of light are diffracted by different amounts when reflected by the regularly spaced ridges with which the surface is covered. In fact, a surface covered by fine, evenly-spaced channels or ridges can be used as a substitute for the prism in a spectroscope.

Diffraction of light can be a problem. Suppose you attempt to see a very small object by using a high quality microscope. As you increase the magnifying power to see the object more and more closely, the object edges begin to blur. Each edge blurs because the light passing by the edge on its way to the eye diffracts.

However, diffraction serves a purpose when a device called a diffracting grating is used to study the colours in a light beam. The grating consists of thousands of thin slits that diffract light. Each colour in the light diffracts by a slightly different amount. The spread of colours can be large enough to make each colour visible. A grating used with a telescope can separate the colours in the light from a star, enabling scientists to learn what materials actually made up the stars…

POLARIZATION of LIGHT:

What kind of waves are light waves – longitudinal, like sound or transverse, like waves in a rope, or a mixture of the two, like sea waves? The answer is that light waves are transverse. The proof comes from a study of what is called polarization of light.

Polarization involves the oscillations (regular variations in strength) of the electric fields that make up a light wave. The directions of the oscillations may be represented by arrows. In most of the light we see, the arrow point in many directions perpendicular to the ray’s path. Such light is unpolarized. If these arrows all point in one direction or just opposite it, the light is polarized. Suppose that when sunlight reflects from a road to you, its arrow point only to your left or right. You can block it by wearing sunglasses with polarizing filters. They block light oscillating left or right.

A manufactured polarizing sheet material, called Polaroid, has now replaced natural crystals for most of these uses and you must be familiar with the use of a Polaroid camera.

Polarized light can be used to find out just how the stresses are distributed in machine parts and this sounds to be a principle application of polarization property of light. In this case, a model of the machine part being scanned is made out of plastic and is then subjected to the kind of stress that the original part would have got in the actual use. When the same is viewed by a polarized light, the colored bands appear or observed which then reveal the exact stress pattern in the piece or that very machine part...