# Colour and light  (Page 2/3)

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The wavelength of green light ranges between 500 nm an d 565 nm. Calculate the range of frequencies that correspond to this range of wavelengths.

1. Use

$c=f×\lambda$

to determine $f$ .

2. $\begin{array}{ccc}\hfill c& =& f×\lambda \hfill \\ \hfill f& =& \frac{c}{\lambda }\hfill \\ & =& \frac{3×{10}^{8}\phantom{\rule{0.166667em}{0ex}}\mathrm{m}·{\mathrm{s}}^{-1}}{565×{10}^{-9}\phantom{\rule{0.166667em}{0ex}}\mathrm{m}}\hfill \\ & =& 5,31×{10}^{14}\phantom{\rule{0.277778em}{0ex}}\mathrm{Hz}\hfill \end{array}$
3. $\begin{array}{ccc}\hfill c& =& f×\lambda \hfill \\ \hfill f& =& \frac{c}{\lambda }\hfill \\ & =& \frac{3×{10}^{8}\phantom{\rule{0.166667em}{0ex}}\mathrm{m}·{\mathrm{s}}^{-1}}{500×{10}^{-9}\phantom{\rule{0.166667em}{0ex}}\mathrm{m}}\hfill \\ & =& 6,00×{10}^{14}\phantom{\rule{0.277778em}{0ex}}\mathrm{Hz}\hfill \end{array}$
4. The range of frequencies of green light is $5,31×{10}^{14}\phantom{\rule{0.277778em}{0ex}}\mathrm{Hz}$ to $6,00×{10}^{14}\phantom{\rule{0.277778em}{0ex}}\mathrm{Hz}$ .

## Calculating wavelengths and frequencies of light

1. Calculate the frequency of light which has a wavelength of 400 nm. (Remember to use S.I. units)
2. Calculate the wavelength of light which has a frequency of $550×{10}^{12}$ Hz.
3. What colour is light which has a wavelength of $470×{10}^{-9}$ m and what is its frequency?
4. What is the wavelength of light with a frequency of $510×{10}^{12}$ Hz and what is its color?

## Dispersion of white light

White light, like the light which comes from the sun, is made up of all the visible wavelengths of light. In other words, white light is a combination of all the colours of visible light.

You learnt that the speed of light is different in different substances. The speed of light in different substances depends on the frequency of the light. For example, when white light travels through glass, light of the different frequencies is slowed down by different amounts. The lower the frequency, the less the speed is reduced which means that red light (lowest frequency) is slowed down less than violet light (highest frequency). We can see this when white light is incident on a glass prism.

Have a look at the picture below. When the white light hits the edge of the prism, the light which travels through the glass is refracted as it moves from the less dense medium (air) to the more dense medium (glass).

• The red light which is slowed down the least , is refracted the least .
• The violet light which is slowed down the most , is refracted the most .

When the light hits the other side of the prism it is again refracted but the angle of the prism edge allows the light to remain separated into its different colours. White light is therefore separated into its different colours by the prism and we say that the white light has been dispersed by the prism.

The dispersion effect is also responsible for why we see rainbows. When sunlight hits drops of water in the atmosphere, the white light is dispersed into its different colours by the water.

## Additive primary colours

The primary colours of light are red , green and blue . When all the primary colours are superposed (added together), white light is produced. Red, green and blue are therefore called the additive primary colours . All the other colours can be produced by different combinations of red, green and blue.

## Subtractive primary colours

The subtractive primary colours are obtained by subtracting one of the three additive primary colours from white light. The subtractive primary colours are yellow , magenta and cyan . Magenta appears as a pinkish-purplish colour and cyan looks greenish-blue. You can see how the primary colours of light add up to the different subtractive colours in the illustration below.

#### Questions & Answers

The phenomenon that describes the change in the frequency
Doppler effect
sinelizwi
does the rate of reaction calculated by reactant equal to product
formula of heat reaction
final energy-initial energy of substances
Chantel
what is the difference between ethene and ethane
Ethane is a saturated organic molecule while ethene is unsaturated Ethane has 2 carbon atoms and 6 hydrogen atoms, while ethene has 2 Carbons and 4 hydrogens Ethane has single bond between the 2 Carbon atoms while ethene has double bond
Rito
How do you know that the reaction is in acid medium or basic medium
What formular is used when source and Observer are moving towards to each other
put a + on top n - on the bottom
Sanele
what is doppler effect?
doppler effect is the perceived change in frequency of a sound caused by either the source or the listener
Qaqamba
moving relative to each other
Qaqamba
A Catalyst is a chemical substance that speeds up a chemical reaction.
It does this by lowering the activation energy.
Sheron
what is exothermic
reaction that releases energy
Beauty
what is a catalyst
Joseph
Catalyst is to speed up the reaction
CELO
catalyst is a substance added into a solution to speed up the process
Joseph
As it speeds up the reaction by lowering the activation energy, it does not take part in the reactants or products formed
Rito
reaction that release energy
Qaqamba
what is exothermic
reaction that gives off heat to the surroundings
Thabile
formula for calculating the heat of reaction
Anelisiwe
I need help in electrostatic
in exactly in electrostatic
Beauty
send the question on electrostatic maybe I can help
Beauty
factors affecting the rate of reaction
hi
Xiluva
hi
Mashweme
hey
Musa
I'm xiluva ...do we help those who are having challenges on their studies?
Xiluva
I mean on physical science
Xiluva
I believe it's supposed to be like that. Helping those who are having educational challenges
Rito
Yep physical sciences more specifically
Rito
ok ....nna I need some help in physics electricity ...just hints how we do what n how to solve some of the problem... just hints
Xiluva
The first thing you can do is to understand ohm's law and the difference between a series and parallel connection concerning current, resistance and potential difference
Thendo
Hi Xiluva,not sure if this will be helpful but to understand electricity better I suggest you download Siyavula physical science books there's both a learner book and a teacher's guide I was struggling too they really did help you might be able to do some exercises after reading siyavula material
LUTHO
what diode mean
Ansiwe
diode s a part of electrical current tht allows only moves to one direction ..only ...
Frank
allows current .....to flow to one direction I meant .
Frank
ok thanks
Ansiwe
ok thanks guys
Xiluva
temperature surface area concentration addition of a catalyst
Joseph
thank you mr
Mzukisi
How to calculate e.p
ep=mgh
mudodzwa
Ep=mgh
Maximurm
What is polymer
chemical compound with molecules bonded together in long ,repeating chains
Bukani
hey guys i need help with the topic of forces and their application
A force is a pull or push and it can be applied in rocket launchers moving cars & stoping (breaking them).
Duncan
A force is that Agency which changes the object's state of rest or motion. A push or pull are the effects of force.
Katiso