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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

how is ester formed
Aubrey Reply
how is n ester formed
Aubrey
Alcohol reacts with a carboxylic acid
Texas
an athlete with a mass of 70kg runs at a velocity of 45km . determine the athlete's momentum
Lesedi Reply
Is that a velocity or something else
msawenkosi
45km/h i guess
Texas
Change to m/s
Texas
45km/h = 12.5 m/s p=mv =70×12.5 =875 kg.m/s
Thato
what are the measures of the rates of reaction
Lesego Reply
Volume Concentration Temperature Pressure Surface Area
Thato
the principle of superposition of waves
Sfundo Reply
what is work
Kool Reply
is this a group chat
Nobuhle Reply
Hey can y'all define newton's 2nd law
mthebzification
If a resultant force act on an object...the object will accelerate in the direction of a resultant force,the acceleration of the object is directly proportional to the net force and inversely proportional to the mass of the object
mosa
how do you calculate tension force
Bulumko
use the formula Fnet=ma if there is tension connecting two objects
Sboniso
to calculate Tension, usually calculate acceleration first Draw separate free body diagrams for each body. Apply Fnet = ma to calculate Tension
Kevin
Hi people
Paul
how does temperature affect the equilibrium position
Blessing Reply
an increased temperature increases the average kinetic energy thus in turn increases the number of effective collisions........
Lwando
so...which reaction is favored between endothermic and exothermic .when temperature is increased..?
Blessing
exothermic reaction because energy is realised to the surroundings as heat and light energy ....graphical so much energy is realised as reactants to form product and because temperature is high rate of reaction is fast which means there is a successful collision
Code
INTEMENDO - INCREASE IN TEMPERATURE FAVOURS ENDOTHERMIC DETEMEXO - DECREASE IN TEMPERATURE FAVOURS EXOTHERMIC
Thato
an object will continue in a state of rest unless it is acted upon an unbalanced force
Junior Reply
Newton's Law 1
Code
First Newton's Law
Azola
Newton's first law
Surprise
newton first law
Thinavhuyo
Newton's first law
Blessing
when pressure is increased what happen to volume
Siphelo Reply
decreases
Code
care to explain?
Mpati
if pressure is applied to a pistol , the volume will decrease and particles will collide more frequently with the wall of a container .Each time they collide with the wall they exert a force on them .More collision means more force and the pressure will increase , that Boyle's Law
Code
Because the volume has decreased , the particle will collide more frequently with the wall of a container and each time they collide with the wall of a container they exert a force on them.More collision means more force so the pressure will increase , that Boyle's Law
Code
what is the difference between momentum and a change in momentum?
Chavonne Reply
How to name a branched molecule from right or left?
Vadin Reply
What's Free Fall
Senzo Reply
Free Fall means there is no acting force on that object.
Dingaletu
only gravitational force
Dingaletu
no external force acting on an object
Sphiwe
by only force of gravite
Sello
but gravitational force
Sphiwe
true
Lucky
a motion in which the only force acting is gravitational force
Blessing
and an object experiencing free fall is referred as a projectile
Blessing
Do polymers form restrictedly only if compound is saturated, only?
milani Reply
what is a free fall?
Beyanca Reply
is when The Only Force acting On an Object is Gravitational Force
Madman
Thats right
Beyanca
then Why ask when you Know the answer?
Madman
She's just helping those who forgot it...bro
Thato
guys I need help on Getting ready for a last minute test
Kenelioe Reply
what help you need
Neil
we'll I'm in grade 12 so we doing this topic about upac thing
Kenelioe
on What?
Madman
the organic molecule section
Kenelioe
IUPAC NAMING WHICH FUNCTIONAL GROUP YOU CANNOT NAME?SO I COULD HELP YOU
Madman
ester
Sboniso
you should also look at structural isomers. Its crucial that they might add that one. also try and write down the structural formula of all the given compounds on the table
milani

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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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