In the previous section we saw primary, secondary and complementary colours. In this section, we will see colours of opaque objects.
Let us do an activity:
1. A sheet of red paper is placed flat on top of a table in a dark room. See fig.13.12(a) below:
2. Arrange a sheet of white paper in such a way that, it is near to the red paper and perpendicular to the table.
3. Now consider fig.13.12(b)
• A dashed line is drawn perpendicular to the red paper.
• This perpendicular line is the 'normal to the surface of the red paper'
4. Allow a ray of white light from a torch to fall on the red paper
• This ray should be at an angle with the normal.
5. A portion of the white light will get reflected from the red paper
• This reflected light will make the same angle with the normal.
• Note that, there will not be any reflection if the ray of light falls along the normal.
6. Now look at the white paper. Its colour has become red.
• This is because, red light is reflected from the red paper
7. We know that white light is composed of seven colours.
• But now, after falling on the red paper, six colours are not available.
• It is obvious that, all those six colours are absorbed by the red paper. In short:
■ The red paper reflects only red light. All other colours will be absorbed
The same activity can be repeated after replacing the red paper with a blue one. It is shown in fig.13.13 below:
We can write:
■ The blue paper reflects only blue light. All other colours will be absorbed
The same activity can be repeated after replacing the blue paper with a green one. It is shown in fig.13.14 below:
We can write:
■ The green paper reflects only green light. All other colours will be absorbed
We can write a summary:
1. White light falls on an opaque object
2. That object reflects its own colour. All other colours are absorbed
3. So we can write:
■ The 'colour of an opaque object' is the 'colour of the light that it reflects'
Some of their uses are:
• For making drawings and paintings
• For making art works
• For painting walls and furniture of our houses
2. Any colour can be made using three basic pigments. They are: cyan, magenta and yellow
• So these three pigments are called primary colour pigments
3. Let us see how they work:
• Consider some cyan pigment. Why does it appear cyan in colour?
Ans: Cyan pigment is an opaque object
• So it is reflecting cyan colour and absorbing all other colours
4. But which are the components of cyan?
Ans: Green and blue
5. So we can write:
The cyan pigment is reflecting only two colours: green and blue
6. That means: cyan pigment is absorbing red.
7. So we can write in simple terms:
■ The cyan pigment:
• Absorbs one colour: red
• Reflects two colour: blue and green
8. Consider some magenta pigment. Why does it appear magenta in colour?
Ans: Magenta pigment is an opaque object
• So it is reflecting magenta colour and absorbing all other colours
9. But which are the components of magenta?
Ans: Red and blue
10. So we can write:
The magenta pigment is reflecting only two colours: Red and blue
11. That means: magenta pigment is absorbing green.
12. So we can write in simple terms:
■ The magenta pigment:
• Absorbs one colour: green
• Reflects two colour: Red and blue
13. Consider some yellow pigment. Why does it appear yellow in colour?
Ans: Yellow pigment is an opaque object
• So it is reflecting yellow colour and absorbing all other colours
14. But which are the components of yellow?
Ans: Red and green
15. So we can write:
The yellow pigment is reflecting only two colours: Red and green
16. That means: yellow pigment is absorbing blue.
17. So we can write in simple terms:
■ The yellow pigment:
• Absorbs one colour: blue
• Reflects two colour: Red and green
■ Now we will see what happens when two primary pigments are mixed together
1. Take equal amounts of cyan and yellow pigments
2. Mix them well.
• Since they are taken in equal amounts, the particles of both the pigments will be distributed uniformly in the mixture.
3. So what happens when white light falls on that mixture?
Ans: The cyan particles will absorb all the red lights
• The yellow particles will absorb all the blue lights
4. So the only remaining light is green and it will be reflected
• So the mixture will appear green
■ Next we will see what happens when all the three primary pigments are mixed together
1. Take equal amounts of cyan, magenta and yellow pigments
2. Mix them well.
• Since they are taken in equal amounts, the particles of all the pigments will be distributed uniformly in the mixture.
3. So what happens when white light falls on that mixture?
Ans: The cyan particles will absorb all the red lights
• The magenta particles will absorb all the green lights
• The yellow particles will absorb all the blue lights
4. So there is nothing left to reflect
• So the mixture will appear black
Now we will see some solved examples
Solved example 13.1
Green and red color lights are allowed to fall on a white cricket ball simultaneously. What will be the color of the ball?
Solution:
1. Green and red are two primary colors.
2. They combine to give the secondary color yellow.
3. So the cricket ball will appear yellow in color
Solved example 13.2
A 'plant with green leaves and red flowers' is kept in a dark room. Light with following colors are allowed to fall on it. What will be the observed colors of leaves and flowers?
1. Green
2. Yellow
3. Red
4. Blue
Solution:
1. Green light
(a) Leaves:
• All the seven colors can fall on the leaves. It will reflect the green. And absorb the other six
• If green alone falls on the leaves, then also, green will be reflected.
• So the leaves appear green
(b) Flowers:
• All the seven colors can fall on the flowers. It will reflect the red. And absorb the other six
• If green alone falls on the flowers, that green will be absorbed. Then there is nothing left to reflect.
• So the flowers will appear black
2. Yellow light
(a) Leaves:
• All the seven colors can fall on the leaves. It will reflect the green. And absorb the other six
• Yellow is a secondary color. If yellow alone falls on the leaves, it means that, (Red + Green) is falling on the leaves
• So the red will be absorbed and green will be reflected
• So the leaves appear green
(b) Flowers:
• All the seven colors can fall on the flowers. It will reflect the red. And absorb the other six
• Yellow is a secondary color. If yellow alone falls on the flowers, it means that, (Red + Green) is falling on the flowers
• So the green will be absorbed and red will be reflected
• So the flowers appear red
3. Red light
(a) Leaves:
• All the seven colors can fall on the leaves. It will reflect the green. And absorb the other six
• If red alone falls on the leaves, that red will be absorbed. Then there is nothing left to reflect
• So the leaves appear black
(b) Flowers:
• All the seven colors can fall on the flowers. It will reflect the red. And absorb the other six
• If red alone falls on the flowers, then also that red will be reflected
• So the flowers will red
4. Blue light
(a) Leaves:
• All the seven colors can fall on the leaves. It will reflect the green. And absorb the other six
• If blue alone falls on the leaves, that blue will be absorbed. Then there is nothing to reflect
• So the leaves appear black
(b) Flowers:
• All the seven colors can fall on the flowers. It will reflect the red. And absorb the other six
• If blue alone falls on the flowers, that blue will be absorbed. Then there is nothing to reflect.
• So the flowers will appear black
Solved example 13.3
The greeting card shown below is kept in a dark room and red light is allowed to fall on it. In which all colors will it appear?
Solution:
(a) White flower:
• All the seven colors can fall on the white flower. It will reflect all those colors
• In our present case, only red light is falling on it. That red will be reflected.
• So the flower will appear red in color
(b) Green leaves
• All the seven colors can fall on the leaves. It will reflect the green. And absorb the other six
• If red alone falls on the leaves, that red will be absorbed. Then there is nothing to reflect
• So the leaves appear black
(c) Blue letters
• All the seven colors can fall on the leaves. It will reflect the blue. And absorb the other six
• If red alone falls on the leaves, that red will be absorbed. Then there is nothing to reflect
• So the letters appear black
(d) Red Signature
• All the seven colors can fall on the signature. It will reflect the red. And absorb the other six
• In our present case, only red light is falling on it. That red will be reflected.
• So the signature will appear red in color
Let us do an activity:
1. A sheet of red paper is placed flat on top of a table in a dark room. See fig.13.12(a) below:
 |
| Fig.13.12 |
3. Now consider fig.13.12(b)
• A dashed line is drawn perpendicular to the red paper.
• This perpendicular line is the 'normal to the surface of the red paper'
4. Allow a ray of white light from a torch to fall on the red paper
• This ray should be at an angle with the normal.
5. A portion of the white light will get reflected from the red paper
• This reflected light will make the same angle with the normal.
• Note that, there will not be any reflection if the ray of light falls along the normal.
6. Now look at the white paper. Its colour has become red.
• This is because, red light is reflected from the red paper
7. We know that white light is composed of seven colours.
• But now, after falling on the red paper, six colours are not available.
• It is obvious that, all those six colours are absorbed by the red paper. In short:
■ The red paper reflects only red light. All other colours will be absorbed
The same activity can be repeated after replacing the red paper with a blue one. It is shown in fig.13.13 below:
 |
| Fig.13.13 |
■ The blue paper reflects only blue light. All other colours will be absorbed
The same activity can be repeated after replacing the blue paper with a green one. It is shown in fig.13.14 below:
 |
| Fig.13.14 |
■ The green paper reflects only green light. All other colours will be absorbed
We can write a summary:
1. White light falls on an opaque object
2. That object reflects its own colour. All other colours are absorbed
3. So we can write:
■ The 'colour of an opaque object' is the 'colour of the light that it reflects'
Colour pigments
1. We find many uses for colour pigments (Some images of pigments can be seen here.)Some of their uses are:
• For making drawings and paintings
• For making art works
• For painting walls and furniture of our houses
2. Any colour can be made using three basic pigments. They are: cyan, magenta and yellow
• So these three pigments are called primary colour pigments
3. Let us see how they work:
• Consider some cyan pigment. Why does it appear cyan in colour?
Ans: Cyan pigment is an opaque object
• So it is reflecting cyan colour and absorbing all other colours
4. But which are the components of cyan?
Ans: Green and blue
5. So we can write:
The cyan pigment is reflecting only two colours: green and blue
6. That means: cyan pigment is absorbing red.
7. So we can write in simple terms:
■ The cyan pigment:
• Absorbs one colour: red
• Reflects two colour: blue and green
8. Consider some magenta pigment. Why does it appear magenta in colour?
Ans: Magenta pigment is an opaque object
• So it is reflecting magenta colour and absorbing all other colours
9. But which are the components of magenta?
Ans: Red and blue
10. So we can write:
The magenta pigment is reflecting only two colours: Red and blue
11. That means: magenta pigment is absorbing green.
12. So we can write in simple terms:
■ The magenta pigment:
• Absorbs one colour: green
• Reflects two colour: Red and blue
13. Consider some yellow pigment. Why does it appear yellow in colour?
Ans: Yellow pigment is an opaque object
• So it is reflecting yellow colour and absorbing all other colours
14. But which are the components of yellow?
Ans: Red and green
15. So we can write:
The yellow pigment is reflecting only two colours: Red and green
16. That means: yellow pigment is absorbing blue.
17. So we can write in simple terms:
■ The yellow pigment:
• Absorbs one colour: blue
• Reflects two colour: Red and green
■ Now we will see what happens when two primary pigments are mixed together
1. Take equal amounts of cyan and yellow pigments
2. Mix them well.
• Since they are taken in equal amounts, the particles of both the pigments will be distributed uniformly in the mixture.
3. So what happens when white light falls on that mixture?
Ans: The cyan particles will absorb all the red lights
• The yellow particles will absorb all the blue lights
4. So the only remaining light is green and it will be reflected
• So the mixture will appear green
■ Next we will see what happens when all the three primary pigments are mixed together
1. Take equal amounts of cyan, magenta and yellow pigments
2. Mix them well.
• Since they are taken in equal amounts, the particles of all the pigments will be distributed uniformly in the mixture.
3. So what happens when white light falls on that mixture?
Ans: The cyan particles will absorb all the red lights
• The magenta particles will absorb all the green lights
• The yellow particles will absorb all the blue lights
4. So there is nothing left to reflect
• So the mixture will appear black
Now we will see some solved examples
Solved example 13.1
Green and red color lights are allowed to fall on a white cricket ball simultaneously. What will be the color of the ball?
Solution:
1. Green and red are two primary colors.
2. They combine to give the secondary color yellow.
3. So the cricket ball will appear yellow in color
Solved example 13.2
A 'plant with green leaves and red flowers' is kept in a dark room. Light with following colors are allowed to fall on it. What will be the observed colors of leaves and flowers?
1. Green
2. Yellow
3. Red
4. Blue
Solution:
1. Green light
(a) Leaves:
• All the seven colors can fall on the leaves. It will reflect the green. And absorb the other six
• If green alone falls on the leaves, then also, green will be reflected.
• So the leaves appear green
(b) Flowers:
• All the seven colors can fall on the flowers. It will reflect the red. And absorb the other six
• If green alone falls on the flowers, that green will be absorbed. Then there is nothing left to reflect.
• So the flowers will appear black
2. Yellow light
(a) Leaves:
• All the seven colors can fall on the leaves. It will reflect the green. And absorb the other six
• Yellow is a secondary color. If yellow alone falls on the leaves, it means that, (Red + Green) is falling on the leaves
• So the red will be absorbed and green will be reflected
• So the leaves appear green
(b) Flowers:
• All the seven colors can fall on the flowers. It will reflect the red. And absorb the other six
• Yellow is a secondary color. If yellow alone falls on the flowers, it means that, (Red + Green) is falling on the flowers
• So the green will be absorbed and red will be reflected
• So the flowers appear red
3. Red light
(a) Leaves:
• All the seven colors can fall on the leaves. It will reflect the green. And absorb the other six
• If red alone falls on the leaves, that red will be absorbed. Then there is nothing left to reflect
• So the leaves appear black
(b) Flowers:
• All the seven colors can fall on the flowers. It will reflect the red. And absorb the other six
• If red alone falls on the flowers, then also that red will be reflected
• So the flowers will red
4. Blue light
(a) Leaves:
• All the seven colors can fall on the leaves. It will reflect the green. And absorb the other six
• If blue alone falls on the leaves, that blue will be absorbed. Then there is nothing to reflect
• So the leaves appear black
(b) Flowers:
• All the seven colors can fall on the flowers. It will reflect the red. And absorb the other six
• If blue alone falls on the flowers, that blue will be absorbed. Then there is nothing to reflect.
• So the flowers will appear black
Solved example 13.3
The greeting card shown below is kept in a dark room and red light is allowed to fall on it. In which all colors will it appear?
Solution:
(a) White flower:
• All the seven colors can fall on the white flower. It will reflect all those colors
• In our present case, only red light is falling on it. That red will be reflected.
• So the flower will appear red in color
(b) Green leaves
• All the seven colors can fall on the leaves. It will reflect the green. And absorb the other six
• If red alone falls on the leaves, that red will be absorbed. Then there is nothing to reflect
• So the leaves appear black
(c) Blue letters
• All the seven colors can fall on the leaves. It will reflect the blue. And absorb the other six
• If red alone falls on the leaves, that red will be absorbed. Then there is nothing to reflect
• So the letters appear black
(d) Red Signature
• All the seven colors can fall on the signature. It will reflect the red. And absorb the other six
• In our present case, only red light is falling on it. That red will be reflected.
• So the signature will appear red in color
In the next section, we will see colour of transparent objects.
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