Types of white Colour

 Light is defined as part of the electromagnetic spectrum which is perceived by our naked

eyes. The visible light which we see consists of different wavelengths of light in the range of

400-700 nm. The different wavelengths of light have different colours. Thus, we can see

various colours around us. This type of light is known as Polychromatic light. Opposed to this

is Monochromatic light which consists of only one wavelength and thus only one colour.

NB:

 Spectrum of colour (red, orange, yellow, green, blue, indigo and violet) is abbreviated as

ROYGBIV .And each of these colours has a different wavelength.

 These coloured lights are refracted differently on passing through the prism

 The velocity of light in a medium (refractive index) depends on the wavelength of incident light.

As a result different wavelengths are refracted by different amounts.

 White colour split due to difference in wave length. Shorter wavelengths have higher refractive

indices and get bent more than longer wavelengths

Recombining Colours of white Light

 Spectrum comes from white light can recombine to form white colour.

 White light is a mixture of lights of seven colours, red, orange, yellow, green, blue, indigo and

violet. The dispersion of white light occurs because colours of white light travel at different

speeds through the glass prism. The amount of refraction depends on the speed of coloured light

in glass.

 When white light consisting of seven colours falls on a glass prims, each colour in it is refracted

by a different angle, with the result that seven colours are spread out to form a spectrum. The

red light bends the least, while violet light bends the most.

Newton’s experiment with two identical prisms:

 When a beam of white light is passed through a glass prism, a band of seven colours is formed

on a white screen. This band of seven colours is called spectrum of white light.

 Newton showed that the seven coloured light of the spectrum can be recombined to give back

white light.

 First he tried to split the colours of the spectrum of white light using a prism.

 He then placed a second identical prism in an inverted position with respect to the first prism.

This allowed all the colours of the spectrum to pass through the second prism. He found a beam

of white light emerging from the other side of the second prism (see the figure below).

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Alternatively; by using Newton’s color disc

 Newton used a colour disc to recombine the colours.

 The disc consists of sectors painted with different colors of the spectrum

 When the disk is allowed to spin about its axis at very high speed, all the colors of the spectrum

recombine to form white light. When it is slowed down, the individual colors of the spectrum are

seen again

NB;

You may see that white light is slightly grey because paints are not pure colors

Colour

 Colour is the property of light that reaches our eyes.

Appearance of coloured objects under white light

 The object seems to have kind of colour due to the fact that it absorbs all colours and reflect

the colour that the object has.

 Example; Green object absorbs the entire colour in white light except green.

Appearance of white objects under coloured light

 When a coloured object is viewed under a coloured light, it takes the colour of that light.

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Example:

 The object will appear blue in blue light and red in red light.

 A colour filter is working on this principle.

Colour filters

 A filter is a coloured sheet of plastic or glass material which allows light of its own type to

pass through it and absorbs the rest of the coloured lights

 For example; a green filter transmits only green, a blue filter transmits only blue, a yellow filter

transmits red, green and yellow lights as seen in the figure below

Appearance of a white object to coloured light

White object

coloured light

Red filters

Yellow filters

Green filter

Blue filter

Colour of object

Red colour

Yellow colour

Green colour

Blue colour

Colors of objects

 The color of an object depends on:

(i) The color of light falling on it

(ii) The color it transmits or reflects e.g

 An object appears blue because it reflects blue light into the eyes and absorbs the other

colours of the spectrum

 Similarly, an object appears red because it reflects red light into the eyes and absorbs all other

colors

 A white object reflects all the colors of the spectrum into the eyes and absorbs none.

Types of Colour

 Primary colour

 Secondary colour

Primary colours

 Primary colours are the colors that cannot be obtained by mixing two different colours of light.

 Example, red, blue and green

Secondary colours

 Secondary colours are the colours which obtained by adding two primary colors together

 Example of secondary colors are cyan (peacock blue), magenta and yellow

 The mixing of these two primary colors to produces a secondary color is given below;

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(a) Green + Blue = Cyan

(b) Green + Red = Yellow

(GBC)

(GRY)

(RBM)

(c) Red

+ Blue = Magenta

Complementary Colour

 These are two different colors which when added produce white light.

 One of them is a secondary color and the other must be a primary color

 The pairs are given below

Red + Cyan

Green + Magenta = White light

Blue + Yellow = White

 The figure below shows the summary of coloured light

= White light

NB:

 From the figure above. When mixing the primary colors (red, blue and green), a white color is

obtained. That is: Red + Blue + Green = White

 When mixing two of the secondary colors, white light is produced

That is: Yellow + Magenta = Magenta + Cyan = Cyan + Yellow = White color

 We are only concerned with colours of light and not with coloured substances (pigments)

 The complementary colour of white light is green, red and blue

 The complementary colour of yellow light is green and red.

Additive and subtractive mixing of colours

Additive mixing of Colours

 Is the mixing of coloured light

 The more colours you add, the closer the result draws to white. Therefore, mixing of coloured

lights

 Additive mixing of colours deals with primary colour of light colour (Red, Green and Blue) not

primary colour of pigments (paints and dyes)

 Adding different colours of light together increases the number of wavelengths present

Subtractive mixing of pigments

 Is the mixing of colours of different paints (pigments)

 Pigments: These are substances which give color to paints by reflecting light of

certain colors only and absorbing all other colors.

 Mixing different paints results in a darker colour because most of the light wavelengths are

absorbed

 Mixing two primary pigments produces a secondary pigment .That is

a) cyan

+ magenta = blue (CMB)

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b) magenta + yellow = red

c) yellow + cyan = green

(MYR)

(CYG)

 The pigments act as filters that subtract one or more colours from the visible spectrum

 Blue, Red and Green are, therefore, the secondary pigments.

 Subtractive complementary colours combine to produce BLACK.

That is

Blue + Green + Red = Black (See the fig below)

N B:

 Each primary pigment absorbs one primary colour:

. Yellow absorbs blue and reflects red and green

. Magenta absorbs green and reflects blue and red

. Cyan absorbs red and reflects green and blue.

 A secondary pigment absorbs two primary colours and reflects one:

. Red absorbs green and blue and reflects red.

. Green absorbs red and blue and reflects green.

. Blue absorbs - red and green and reflects blue.

 The primary pigments are the secondary colours of light and the secondary pigments are the

primary colours of light.

 If you mix a primary pigment with a secondary pigment you get total absorption (black).

ie Blue + Yellow = Black

 The primary pigments are the complementary colours of the three primary colours of light.

Worked example:

1. A plant with green leaves and red flowers is placed in:

(a) Green

(b) Red

(c) Blue light

ANS:

(a) In Green light

 Green leaves will

(b) In Red light

 In red light

(c) In Blue light

 In blue light

Green

appear green

leaves will appear

black

leaves will appear black

 Red flowers will appear

black

 Red

flowers

will

appear black

 Red

flowers

will

appear red

2. Why red light is used for danger signals?

ANS: Because red light is scattered the least by air molecules due to its highest wavelength

so it is able to travel the longest distance through fog, rain and alike

3. A red bus with blue letter on its stops in front of a yellow light at right. Describe the

appearance of the bus

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ANS: In yellow light

 the red bus will appear red because yellow is composed of green and red

 The blue letters will appear black

Class Activity – 4:2

1. With the help of a diagram, show the dispersion of white light of a prism.

2. Fig shows white light falling on a prism.

(a) Name the colour at P and Q

(b) Explain why a prism disperses white light into its component colours

3. State the cause of dispersion of white light passing through a glass prism. How did Newton

show that white light of Sun contains seven colours using two identical glass prisms. Draw a ray

diagram to show the path of light when two identical glass prisms are arranged together in

inverted position with respect to each other and a narrow beam of white light is allowed to fall

4. Explain why an object which appears yellow in day light, appears red when illuminated with red

light and black when illuminated with a blue light?

5. A color printer needs just three colors of ink to print all of the colors that we can see. Which

colors are they?

ANS: The three colors of ink in a color printer are the three primary pigment colors which

are cyan, magenta, and yellow. These three colors can be combined in different ratios to

produce all other colors, so they are the only colors needed for full-color printing.

6. What color do you see if an object absorbs all of the light that strikes it?

7. Describe and explain the appearance of a red tie with blue spots when observed in

(a) Red light

(b) Green light

ANS;

(a) In red light; a red tie will appear red and blue spots will appear black because the red light

reflects the red color while blue color is absorbed and by red light and appears black

(b) In green light; the red tie and blue spots both will appear black. This is because both red and

blue colors are absorbed in red light and non will be reflected

Self Assessment – 4

1 Explain what is meant by a spectrum .Describe with the aid of a diagram how would you obtain

the spectrum of white light

2 Distinguish between a pure and an impure spectrum. Explain with the aid of a diagram how a

pure spectrum can be produced in the laboratory .How are the colours of the spectrum

recombined?

3 A beam of white light is allowed into a dark room through a hole .In the dark room ,the beam

falls on a white screen .How will the screen appear if:

(a) a piece of red glass is placed in the path of the beam

(b) a piece of green glass is placed between the screen and the red glass?

4 Explain, giving examples, what is meant by:

(a) additive combination of colours

(b) subtractive combination of colours

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5 Why does an object appear coloured when light falls onto it?

6 Explain, giving examples, what is meant by primary, secondary and complementary colours.

A flag has a green disc on a yellow background .How will the flag appear in:

(a) green light

7 State , giving a reason for each answer, the colour that results when:

(a) a blue light and a yellow light are mixed (b) blue paint and yellow paint are mixed

8 What color would be seen white light is viewed through:

(a) a red filter? (b) a cyan filter? (c) an orange filter?

(b) red light

(c) blue light

if

9 A book which looks red in white light is viewed in magenta light. In what color does it appear?

10 White light is viewed through a combination of a yellow filter and a red filter held in

contact. What color is seen

11 Explain the meaning of the term ‘’dispersion’’ of white

12 A painter has a blue – green (cyan) paint which she wants to make pure green. What color

pigment should she add to the paint? Explain your answer

13 Briefly explain, why the sky looks blue?

14 Under pure yellow light what will be the appearance of the blue piece of cloth?

15 Danger signs along the road as well as tail and brake lamps of motor vehicles rear are painted

red. Briefly explain the reason behind.

16 Explain each of the following:-

(a) The appearance of a blue flag when viewed in day light through a sheet of red glass

(b) The appearance of a red flag with green stripes when viewed in day light through a sheet of

green glass

(c) The appearance of a man wearing blue shirt and red trousers, holding a handkerchief of

green color when viewed in the pure yellow light

17 (a) How does the angle of deviation produced by a prism change with increase in the angle of

incidence. Draw a curve showing the variation in the angle of deviation with the angle of

incidence at a prism surface.

(b) Using the curve in part (a) above, how would you infer that for a given prism, the angle of

minimum deviation 훿_minis unique for light of a given wavelength.

Answer

(a) It is experimentally observed that as the angle of incidence increases, the angle of

deviation first decreases, reaches to a minimum value for a certain angle of incidence and

then on further increasing the angle of incidence, the angle of deviation begins to increase.

The below curve shows the variation in the angle of deviation with the angle of incidence at

a prism surface:

(b) For a given prism and given colour of light, 훿_min(angle of minimum deviation) is unique

since only one horizontal line can be drawn parallel to i – axis at the lowest point of i – 훿

curve i.e only for one value of angle of incidence i, the refracted ray inside the prism is

parallel to its base.

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