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Light and Colour

Dispersion

What colour is light? The answer lies in a prism (fig.). If a ray of light goes through a prism, it is refracted many times, coming in as well as coming out. Because short-wave light is refracted more than long-wave light, it is fractionalised in its wave components: Colours.

 

 

Lichtbrechung im Prisma


Light is refracted by a prism and fractionalised in the colour range of visible light.

 

Scattering

While dispersion in a prism means that the light is being refracted in equal shares of colour, other light phenomena behave differently. The sky appears blue to us because the blue part of the sunlight is scattered more than the other colours. This is due to the size of the molecules: they are the same size as the wavelength of the blue colour share in sunlight, but smaller than the green or red wavelength.

 

Die unterschiedliche Streuung von Farben führt zum Blau des Himmels

 

Different scattering of colours in water molecules in the atmosphere leads to our blue sky.

Interference

The interference is the third phenomenon of the wave characteristics of light. In daily life, we can observe this phenomenon in oil spills or soap bubbles. It is caused by the overlapping of two waves; this is called interference. Interference can occur constructively or destructively.

 

Interference is constructive if the amplitudes of both waves strengthen each other (fig.).

 

Konstruktive und destruktive Interferenz


Scheme of constructive and destructive interference.

 

Interference is destructive if the phases of both waves are exactly inverse, i.e. wave 1 is in a wave trough and wave 2 in a wave crest. They cancel (Abb.).

 

Interferenz bei Seifenblasen

 

The interference phenomenon is visible in oil spills and soap bubbles.

 

What is the connection between interference and rainbow-coloured oil-water-spills or soap bubbles (fig.)? When light makes contact with a slim oil film on the surface of a puddle, it is refracted multiply (like in a prism). Other tan in a prism, it is refracted in its colour components directly during admission due to the low density of the oil; at discharge, the colour components interfere. At regular intervals, the different colour components are extinct according to the principle of destructive interference; thus, only the remaining colour can be seen. These colours are called interference colours.

 

Why are we able to see colours? This question will be addressed in the following section.

 

Additive und subtractive colour mixing

There are numerous theories about colours and colour mixing. The two most important principles of colour mixing theory are described here. Additive and subtractive colour mixing principles are needed to understand why we can see our world in colour.

 

Additive colour mixing describes the impression of colour which approaches the eye, or, to be precise, the retina. The retina of the human eye has so-called rods and cones. The rods perceive differences in light intensity, the cones are responsible for seeing in colour (fig.).

 

Schema der menschlichen Netzhaut

The retina of the human eye includes rods sensible for differences in light intensity and cones for red, green, and blue colour.

 

3 types of cones can be distinguished: Those sensible for red, green or blue light. Blue, green and red are, therefore, the primary colours of additive colour mixing. Additive colour mixing causes the colour effect by adding colours. If nothing is added, black is perceived. If all equal shares of all colours are mixed, we receive white. If two colours are added, cyan, yellow and magenta are obtained (fig.). Additive colour mixing applies to light emitting bodies like TV or computer. Here, this model is also called R(ed) G(reen) B(lue) model.

 

Additive Farbmischung


Additive colour mixing with the primary colours red, green and blue.

 

Subtractive colour mixing is not about light colours but body colours. Thus, not the colour that reaches the eye is important but the light components absorbed by a non-self-luminous object (i.e. TV or computers are not included). This colour impression is caused, therefore, by subtraction of colours.

 

Durch die Subtraktive Farbmischung sehen wir die Welt bunt

Subtractive colour mixing - because some colours are absorbed and some reflected, we see the world in colour!

 

We conclude: All bodies have colour pigments (colouring particles) which can absorb light. If a surface contains pigments absorbing the blue and green parts of light, we perceive it as red - like an apple. Surfaces with pigments absorbing the blue and red parts of light reflect the green part of light. For the eye, these surfaces appear green (fig.) because the blue and red light has been "subtracted" by the body pigments. Pigments absorbing green light appear magenta, and those absorbing red light seem cyan. If no colour of light is absorbed, the surface appears white, and, accordingly, if all is absorbed we see "black". The objects surrounding us colour the white sunlight and the world appears colourful. The principle of subtractive colour mixing is also present in printing, where it is known as the C(yan) M(agenta) Y(ellow) (blac)K model.

 

 

Subtraktive Farbmischung


Subtractive colour mixing with the primary colours cyan, magenta and yellow (CMYK-Model).

 

Subtractive colour mixing becomes even clearer to you if you look at a lemon which is illuminated by sunlight first, including red, green and blue light, and then by red and blue light, and, finally, by blue light only. Characteristically, the pigments in the lemon peel absorb blue light. The red and green shares of incoming sunlight are reflected, and combine to yellow. If blue and red light illuminate the lemon, the lemon seems to be red. And if we have blue light only, the lemon is black because all light has been absorbed.

 

Eine Zitrone wird mit verschieden farbigen Licht bestrahlt


The animation shows the colour of the lemon when exposed to different light colours (red, green, blue - red, blue - blue).



 


Conclusion:

Light is composed of different colours which have different wavelength. Dispersion, scattering, and interference of light cause incredible colour phenomena. To describe the nature of colour mixing, the additive and subtractive model was explained. The former applies for light colours, the latter for body colours.