| Contents for this page | Related topics |  |
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Dispersion The visible spectrum Primary and complementary colours The colour of objects Additional questions |
Doppler effect with sound Doppler effect with light 2-D and 3-D wavefronts Wave nature of matter |
 Data Glossary |
| Learning Outcomes | ||
|---|---|---|
| After studying this section, you will (a) be familiar with the phenomenon of dispersion, (b) know what the visible spectrum is about, and (c) understand the principles behind addition and subtraction of light. | ||
Light is a type of energy transmitted by electromagnetic waves. These waves have a velocity (in a vacuum) of 3.0x108 m.s-1. White light, from the sun, for example, is made up light of different wavelengths, which are registered by the human eye as COLOURS. These colours can be separated by means of a prism, as shown in the diagram above. This phenomenon was discovered by Isaac Newton, and is called DISPERSION.
| The reason for this is that the different colours are refracted to different extents. The red light is refracted least, while violet light is refracted the most. |  |
The range of colours into which the light is separated is called the VISIBLE SPECTRUM, so-called because the colours are visible to the human eye. The wavelengths, l, of the different colours increase as we go from violet to red (the frequencies decrease as we go from violet to red). A spectrum that contains all wave-lengths in a specified region of the electromagnetic spectrum, such is the case with the visible spectrum, is called a CONTINUOUS SPECTRUM.
If beams of red, green and blue lights are combined (these are the PRIMARY COLOURS), white light is formed:
If two beams of primary colours are mixed, a colour which is different to the primary colours result. For example, if green light and red lights are mixed, the resulting colour is yellow. This phenomenon is called ADDITION OF LIGHT.
Any two colours, which, when mixed, give rise to white light, are known as COMPLEMENTARY COLOURS. For example, yellow and blue are complementary colours. Any colour which results from the mixing of two primary colours gives the complementary colour of the third primary colour, as shown in the diagram above.
| Primary colour | Complementary colour | Mixture | Resulting colour | |
|---|---|---|---|---|
| Red | Cyan | Red + Green | Yellow | |
| Blue | Yellow | Blue + Green | Cyan | |
| Green | Magenta | Red + Blue | Magenta |
Note: The above discussion refers to light of different colours. It does not hold for pigments of different colours.
Transparent objects:
Transparent objects transmit light to an appreciable extent. If a beam of white light is incident perpendicularly to a sheet of clear glass, most of the light will pass through. Coloured glasses, on the other hand, only allows light of certain wavelengths to pass through (the rest is absorbed by the glass, which heats up to some extent). Such glass objects are often called FILTERS. Thus, a sheet of red glass will only allow the transmission of red light. The diagram below, which assumes glass filters transmitting MONOCHROMATIC LIGHT (that is, light of a unique wavelength), shows what happens when white light passes through such filters. This is called SUBTRACTION OF LIGHT
Opaque objects:
Opaque objects do not transmit light, that is, light does not pass through such materials to any appreciable extent. When light strikes an opaque object, part of the light is absorbed, and part is reflected and scattered. We see the object because the scattered light enter our eyes. Now, if white light is incident on an object which reflects ALL the light, the object will appear white, since all the wavelength components of the visible spectrum are reflected. If ALL the light is absorbed, none will be reflected, and so the object will appear black ("black", strictly speaking is not a colour - it is the absence of light of any colour).
| It turns out that the chemical substances on the surface of opaque objects have the capability of SELECTIVELY absorbing various colours, and thereby reflecting only certain wavelength components of the visible spectrum. The object is then seen in a colour which will result from the combination of the reflected colour(s). |  |
When pigments are mixed, the colour of the mixture will depend on the colour absorption properties of the individual pigments. For example, assume that a mixture of blue and yellow paints is made. The blue paint contains a pigment that strongly reflect blue light, and also reflects some green and some indigo light (light on either side of blue in the spectrum), to a weaker extent. The yellow pigment contains a pigment that strongly reflect yellow light and weakly reflects green and orange light. The light which will be reflected by the mixture will be green, as all other colours will be absorbed, as shown in the table below.
| Strongly reflected |
Weakly reflected |
Absorbed | |||||
|---|---|---|---|---|---|---|---|
| Pigment 1 | |||||||
| Pigment 2 | |||||||
| Mixture | |||||||
If a beam of light falls onto a perfectly smooth surface (as in the diagram above, left), the light will be reflected, and an image of the object will be formed. The angle of incidence of the light will be equal to the angle of reflection.
If however the light falls on a rough surface, the angles of incidence of various part of the beam will vary (above, right). Reflection still occurs, but such a reflection is said to be DIFFUSE, and we say that the reflected light is SCATTERED.
We can summarise why objects have certain colours:
The table below gives a summary:
| Object absorbs | Object reflects | Object appears |
|---|---|---|
| Red | Green and blue | Cyan |
| Green | Red and blue | Magenta |
| Blue | Red and green | Yellow |
| Red and blue | Green | Green |
| Red and green | Blue | Blue |
| Blue and green | Red | Red |
| Red, blue and green | None | Black |
| None | Red, green and blue | White |
