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Pigments and paints

We have learnt that white light is a combination of all the colours of the visible spectrum and that each colour of light is related to a different frequency. But what gives everyday objects around us their different colours?

Pigments are substances which give an object its colour by absorbing certain frequencies of light and reflecting other frequencies. For example, a red pigment absorbs all colours of light except red which it reflects. Paints and inks contain pigments which gives the paints and inks different colours.

Colour of opaque objects

Objects which you cannot see through (i.e. they are not transparent) are called opaque . Examples of some opaque objects are metals, wood and bricks. The colour of an opaque object is determined by the colours (therefore frequencies ) of light which it reflects . For example, when white light strikes a blue opaque object such as a ruler, the ruler will absorb all frequencies of light except blue, which will be reflected. The reflected blue light is the light which makes it into our eyes and therefore the object will appear blue.

Opaque objects which appear white do not absorb any light. They reflect all the frequencies. Black opaque objects absorb all frequencies of light. They do not reflect at all and therefore appear to have no colour.

If we shine white light on a sheet of paper that can only reflect green light, what is the colour of the paper?

  1. Since the colour of an object is determined by that frequency of light that is reflected , the sheet of paper will appear green, as this is the only frequency that is reflected. All the other frequencies are absorbed by the paper.

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The cover of a book appears to have a magenta colour. What colours of light does it reflect and what colours does it absorb?

  1. We know that magenta is a combination of red and blue primary colours of light. Therefore the object must be reflecting blue and red light and absorb green.

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Colour of transparent objects

If an object is transparent it means that you can see through it. For example, glass, clean water and some clear plastics are transparent. The colour of a transparent object is determined by the colours (frequencies) of light which it transmits (allows to pass through it). For example, a cup made of green glass will appear green because it absorbs all the other frequencies of light except green, which it transmits. This is the light which we receive in our eyes and the object appears green.

If white light is shone through a glass plate that absorbs light of all frequencies except red, what is the colour of the glass plate?

  1. Since the colour of an object is determined by that frequency of light that is transmitted , the glass plate will appear red, as this is the only frequency that is not absorbed.

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Pigment primary colours

The primary pigments and paints are cyan , magenta and yellow . When pigments or paints of these three colours are mixed together in equal amounts they produce black . Any other colour of paint can be made by mixing the primary pigments together in different quantities. The primary pigments are related to the primary colours of light in the following way:

Interesting fact

Colour printers only use 4 colours of ink: cyan, magenta, yellow and black. All the other colours can be mixed from these!

What colours of light are absorbed by a green pigment?

  1. If the pigment is green, then green light must be reflected . Therefore, red and blue light are absorbed.

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I have a ruler which reflects red light and absorbs all other colours of light. What colour does the ruler appear in white light? What primary pigments must have been mixed to make the pigment which gives the ruler its colour?

  1. We need to determine the colour of the ruler and the pigments which were mixed to make the colour.

  2. The ruler reflects red light and absorbs all other colours. Therefore the ruler appears to be red.

  3. Red pigment is produced when magenta and yellow pigments are mixed. Therefore magenta and yellow pigments were mixed to make the red pigment which gives the ruler its colour.

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If cyan light shines on a dress that contains a pigment that is capable of absorbing blue, what colour does the dress appear?

  1. Cyan light is made up of blue and green light.

  2. If the dress absorbs the blue light then the green light must be reflected, so the dress will appear green!

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End of chapter exercises

  1. Calculate the wavelength of light which has a frequency of 570 × 10 12 Hz.
  2. Calculate the frequency of light which has a wavelength of 580 nm.
  3. Complete the following sentence: When white light is dispersed by a prism, light of the colour ? is refracted the most and light of colour ? is refracted the least.
  4. What are the two types of photoreceptor found in the retina of the human eye called and which type is sensitive to colours?
  5. What color do the following shirts appear to the human eye when the lights in a room are turned off and the room is completely dark?
    1. red shirt
    2. blue shirt
    3. green shirt
  6. Two light bulbs, each of a different colour, shine on a sheet of white paper. Each light bulb can be a primary colour of light - red, green, and blue. Depending on which primary colour of light is used, the paper will appear a different color. What colour will the paper appear if the lights are:
    1. red and blue?
    2. red and green?
    3. green and blue?
  7. Match the primary colour of light on the left to its complementary colour on the right:
    Column A Column B
    red yellow
    green cyan
    blue magenta
  8. Which combination of colours of light gives magenta?
    1. red and yellow
    2. green and red
    3. blue and cyan
    4. blue and red
  9. Which combination of colours of light gives cyan?
    1. yellow and red
    2. green and blue
    3. blue and magenta
    4. blue and red
  10. If yellow light falls on an object whose pigment absorbs green light, what colour will the object appear?
  11. If yellow light falls on a blue pigment, what colour will it appear?

Questions & Answers

what does the word emitted mean?
Mwinga Reply
to be ejected or released
Khathutshelo
Ok thanks
Mwinga
what are the hooke laws
Tyriek Reply
what do really asked in exam
Leiyo Reply
hi please help me how to balance redox reactions?
Brian Reply
Which equation do u wanna balance
Rifumo
i tried to write it but my phone just can't write it Cr2 O72-(aq) + H2S (aq) Cr3 (aq) + S(s)
Brian
show is the equation
Pride
us
Pride
hello. what is the difference between a primary, secondary and tertiary alcohol
Thokozani Reply
primary the C=bonded to 1 Carbon atom.... secondary=bonded to 2 carbon atoms tertiary=bonded to 3 carbon atoms
Christina
I hope you're answered
Christina
thanks Christina
Baningi
My pleasure
Christina
Thank you Christina. This is very helpful 😀👍👍💯
Thokozani
💯💯
Thokozani
Anytime Thokozani
Christina
How to calculate pH?
Thabo
thank you guys i didn't know about the primary, secondary and tertiary alcohols
Brian
thank you Christina
Brian
Hey guys which topic does AC nd DC generator falls
Phumelele
electrodynamics
Christina
thank you
Phumelele
can somebody help me with functional isomers
Baningi Reply
function isomer has the same molecular formula but different functional group
Thokozani
what's momentum
Luvelo Reply
the product of an objects mass times velocity. it is mainly prevalent in collisions.
tyrique
momentum
Sesethu
in order to produce an interference pattern, the waves used must be what?
Methane contains C and H. This compound is
Juan Reply
ketones's functional group
Moloi Reply
Why does the carboxyl group have acidc properties?
Evi Reply
carboxylic acids
tyrique
Besides improving appearance what is another reason why medal is electro plated
Elizabeth Reply
it is made of metal
Reunity
thanks
Elizabeth
what is different between Dependent variable and independent variables
Elizabeth
difference between A.c and D.c
Gerrard
AC has slip rings DC has split rings commutator
Mohludi
to avoid rust and to improve strength and conductivity
Shubham
the variable whose value doesn't depend on other is known as independent variable ex time
Shubham
are UV rays dangerous?
Khathutshelo Reply
no
Lianda
what's this?
Lianda
if there is extensive exposure, it can lead to skin cancer
tyrique
yes depend upon their intensity
Shubham
i hv a problem with this chapter because i also have problem with Newton's laws of motion
Kearabetswe Reply
I have a problem with calculation in Newton's law of motion
Vuyokazi
me also VuyokAzi
Amahle
what's the question
Christina
Which area in particular?
Kevo
just get the hang if free body diagrams,know the directions.
Muregu
I know how to draw free diagram I have a problem with calculation in second law
Vuyokazi
what is the wave model of atom ?
Trazy Reply
guys what is momentum
Luvelo
@Luvelo... Momentum is the product of an object's mass and its velocity.
Christina
the model based on wave nature of electron
Shubham
Can you state the Doppler effect in words
Nombuyiselo Reply
State Coulomb's law in words
Nombuyiselo
the magnitude of the electrostatic force exerted point on each other is directly proportional to the product on magnitude of charges and inversely proportional to the square distance between them
Cassiey
the change in frequency of the sound detected by a listener because sound source and listener have different velocity
Elizabeth

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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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