# 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

can you mix kc nd pressure
Khanyisile Reply
equation for dissociation
Amanda Reply
under which topic
Phila
acids and bases
Amanda
Newton's law
Anna
How does a catalyst affect the rate of chemical reaction
Rebone Reply
it speeds up the reaction without being by the reaction. in other words it increases the Reaction rate by lowering the activation energy of a reaction
Keabetsoe
hey
Siphokazi
ey
Keabetsoe
🙄
Holy
it only speed up the reaction not to affect the reaction
John
what is a compound?
Sithembiso Reply
define Doppler effect
Emihle Reply
It is the change in frequency of a wave in relation to an observer who is moving relative to the wave source.
Junior
Doppler effect is the change in frequency or pitch detected by a listener because the velocity of the sound source is different from that of a listener relative to the medium of sound propagation
John
explain how color is an unreliable clue to.the identify minerals
Camille Reply
Hi I want to know where the topic for electric circuits are?
Zahra Reply
hello.. How to deal with Reactions rate?. i dont Understand them
Simphiwe Reply
Firstly you must know how energy changes during chemical reactions...secondly the collision theory.... Rate and extent of reactions.... And factors that affect the rate of chemical reactions... If u capture all the topic then this chapter will be easy for you the is no way you can fail it...
Pearly
thank you
Petty
Heii.. I'm struggling with chemical equilibrium.
Ronny
how do I attempt chemical equilibrium?
Ronny
doppler effect question
Mabikinyane Reply
How do I differentiate from the formula of a listener who is moving away and who is moving towards the source
Njabulo
if the its away you will subtract on the numerator and add in the denominator but if its towards its the opposite
Ashley
how cn I calculate sound of a source if it's unkown
Simphiwe Reply
Use what you were given. What were you given?
Bandile
where can I find acids and bases
muravha Reply
how to calculate avarage power?
Bongani Reply
good result for bursary
Soluiswayi Reply
Good day Sir
noma
Good day Mam
Aobakwe
good day
clifort
How are you
Aobakwe
4. A builder of mass 75 kg is carrying bricks up a flight of stairs. There are 40 steps, each 25 cm high and he takes 10 s to climb the stairs, carrying 15 kg of bricks at a time. Calculate the total power output of the builder during his climb at constant speed.
Asanda Reply
I want to know how to calculate.
Tuauaa
mm I think it will be zero work an power due to a vertical climbing
CHRISTOPHER
How it was calculated 😕
Nthabeleng
1stly describe Hydrocarbon for me please
Magadi Reply
Compounds that consists of carbon and hydrogen atoms only.
Learner
Compounds that consists of carbon and hydrogen atoms only.
Learner

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Source:  OpenStax, Siyavula textbooks: grade 12 physical science. OpenStax CNX. Aug 03, 2011 Download for free at http://cnx.org/content/col11244/1.2
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 By Yasser Ibrahim By Jonathan Long By Brooke Delaney By Yasser Ibrahim By OpenStax By Brooke Delaney By Brooke Delaney By OpenStax By Katie Montrose By Carly Allen