In the previous section we saw colour of opaque objects. In this section, we will see colours of transparent objects.
■ Transparent objects are objects which allow light to pass through them.
• So what will be the colour of a transparent object?
To find the answer, we must first know what a colour filter is:
■ Colour filter is a special transparent object
♦ It has the ability to absorb certain colours
♦ The other colours will be allowed to pass
Some examples:
Example 1:
• Fig.13.15(a) below shows a 'blue colour filter'.
• All the seven colours are falling on it
• But it allows only the blue light to pass through.
• The other 6 colours will be absorbed.
Example 2:
• Yellow filter will allow only yellow light to pass through it
• But yellow is a secondary colour
♦ It is composed of two primary colours: red and green
• So an yellow filter will allow yellow, red and green to pass through
• All other colours will be absorbed. This is shown in fig.13.15(b) above
Example 3:
• Fig.13.6(c) above, shows a 'red colour filter'.
• All the seven colours are falling on it
• But it allows only the red light to pass through.
• The other 6 colours will be absorbed.
■ Images of some actual colour filters can be seen here.
■ Colour filters are transparent materials which allow the passage of certain colours alone through them.
• Primary colour filters green, blue and red allow only their respective colours to pass through them.
• Secondary colour filters yellow, magenta and cyan allow their component colours also through them.
■ The colour of a transparent object is the colour of the light which it transmits.
• If a material allows all the colours to pass through it, that material will appear colourless.
• Water is colourless because it allows all the lights to pass through it.
Now we will see some solved examples:
Solved example 13.4
What is the colour of a red flower when looked through a yellow filter?
Solution:
1. Consider fig.13.16 below.
• White light is falling on a red flower.
2. Flower is an opaque object. So it will absorb all colours except red.
• The red colour will be reflected
• This reflected red colour should reach our eye. Then only we can see the flower.
3. But there is a filter in between. It is a yellow filter.
• Yellow filter is a secondary filter. It will allow yellow, red and green to pass through.
4. So the red light will easily reach our eye. We will see the red flower as red itself.
Solved example 13.5
In the above problem, if a green filter is used instead of yellow filter, what will be the colour of the flower?
Solution:
1. Consider fig.13.17 below.
• White light is falling on a red flower.
2. Flower is an opaque object. So it will absorb all colours except red.
• The red colour will be reflected
• This reflected red colour should reach our eye. Then only we can see the flower.
3. But there is a filter in between. It is a green filter.
Green filter is a primary filter. It will allow only green to pass through.
4. So the red light will not reach our eye.
• But red is the only colour coming from the flower. Now, it is blocked by the filter
• So the flower will appear as black.
Solved example 13.6
White light is passed through green and red filters and allowed to fall on a white paper what will be the colour of the paper?
Solution:
1. Consider the fig.13.18 below.
• The white light can be represented by the three colours: red, blue and green.
2. The white light first falls on the green filter
• It will absorb the blue and red lights
• Only green will pass.
3. This green will fall on the red filter
• The red filter absorbs all the light except red
• So it will absorb the green light
4. In effect, no light will pass the red filter
• So the paper will appear black
Solved example 13.7
The card shown below is kept in white light. It is viewed through a cyan glass filter. In what colors will it appear?
Solution:
(a) White flower
• White light is falling on the white flower
• This white flower reflect all the seven colors of the white light
• These reflected colors should reach our eyes. Only then we can see the flower
• But a cyan filter is kept between our eyes and the flower
• Cyan filter is a secondary filter. it will allow only cyan, blue and green to pass through
• So, out of the seven colors coming from the flower, we will get blue and green
• So the flower will appear cyan in color
(a) Green leaves
• White light is falling on the green leaves
• These green leaves reflect green color. All the other six colors of the white light will be absorbed
• These reflected green light should reach our eyes. Only then we can see those leaves
• But a cyan filter is kept between our eyes and the flowers
• Cyan filter is a secondary filter. It will allow only cyan, blue and green to pass through
• So, the green light will reach our eyes
• So the leaves will appear green in color
(c) Blue letters
• White light is falling on the blue letters
• These blue letters reflect blue color. All the other six colors of the white light will be absorbed
• These reflected blue light should reach our eyes. Only then we can see those letters
• But a cyan filter is kept between our eyes and the flowers
• Cyan filter is a secondary filter. It will allow only cyan, blue and green to pass through
• So, the blue light will reach our eyes
• So the letters will appear green in color
(d) Red signature
• White light is falling on the red letters
• These red letters reflect red color. All the other six colors of the white light will be absorbed
• These reflected red light should reach our eyes. Only then we can see those letters
• But a cyan filter is kept between our eyes and the flowers
• Cyan filter is a secondary filter. It will allow only cyan, blue and green to pass through
• So, the red light will not reach our eyes
• So the letters of the signature will not be visible
■ Transparent objects are objects which allow light to pass through them.
• So what will be the colour of a transparent object?
To find the answer, we must first know what a colour filter is:
■ Colour filter is a special transparent object
♦ It has the ability to absorb certain colours
♦ The other colours will be allowed to pass
Some examples:
Example 1:
• Fig.13.15(a) below shows a 'blue colour filter'.
 |
| Fig.13.15 |
• But it allows only the blue light to pass through.
• The other 6 colours will be absorbed.
Example 2:
• Yellow filter will allow only yellow light to pass through it
• But yellow is a secondary colour
♦ It is composed of two primary colours: red and green
• So an yellow filter will allow yellow, red and green to pass through
• All other colours will be absorbed. This is shown in fig.13.15(b) above
Example 3:
• Fig.13.6(c) above, shows a 'red colour filter'.
• All the seven colours are falling on it
• But it allows only the red light to pass through.
• The other 6 colours will be absorbed.
■ Images of some actual colour filters can be seen here.
■ Colour filters are transparent materials which allow the passage of certain colours alone through them.
• Primary colour filters green, blue and red allow only their respective colours to pass through them.
• Secondary colour filters yellow, magenta and cyan allow their component colours also through them.
■ The colour of a transparent object is the colour of the light which it transmits.
• If a material allows all the colours to pass through it, that material will appear colourless.
• Water is colourless because it allows all the lights to pass through it.
Now we will see some solved examples:
Solved example 13.4
What is the colour of a red flower when looked through a yellow filter?
Solution:
1. Consider fig.13.16 below.
 |
| Fig.13.16 |
2. Flower is an opaque object. So it will absorb all colours except red.
• The red colour will be reflected
• This reflected red colour should reach our eye. Then only we can see the flower.
3. But there is a filter in between. It is a yellow filter.
• Yellow filter is a secondary filter. It will allow yellow, red and green to pass through.
4. So the red light will easily reach our eye. We will see the red flower as red itself.
Solved example 13.5
In the above problem, if a green filter is used instead of yellow filter, what will be the colour of the flower?
Solution:
1. Consider fig.13.17 below.
 |
| Fig.13.17 |
2. Flower is an opaque object. So it will absorb all colours except red.
• The red colour will be reflected
• This reflected red colour should reach our eye. Then only we can see the flower.
3. But there is a filter in between. It is a green filter.
Green filter is a primary filter. It will allow only green to pass through.
4. So the red light will not reach our eye.
• But red is the only colour coming from the flower. Now, it is blocked by the filter
• So the flower will appear as black.
Solved example 13.6
White light is passed through green and red filters and allowed to fall on a white paper what will be the colour of the paper?
Solution:
1. Consider the fig.13.18 below.
 |
| Fig.13.18 |
2. The white light first falls on the green filter
• It will absorb the blue and red lights
• Only green will pass.
3. This green will fall on the red filter
• The red filter absorbs all the light except red
• So it will absorb the green light
4. In effect, no light will pass the red filter
• So the paper will appear black
Solved example 13.7
The card shown below is kept in white light. It is viewed through a cyan glass filter. In what colors will it appear?
(a) White flower
• White light is falling on the white flower
• This white flower reflect all the seven colors of the white light
• These reflected colors should reach our eyes. Only then we can see the flower
• But a cyan filter is kept between our eyes and the flower
• Cyan filter is a secondary filter. it will allow only cyan, blue and green to pass through
• So, out of the seven colors coming from the flower, we will get blue and green
• So the flower will appear cyan in color
(a) Green leaves
• White light is falling on the green leaves
• These green leaves reflect green color. All the other six colors of the white light will be absorbed
• These reflected green light should reach our eyes. Only then we can see those leaves
• But a cyan filter is kept between our eyes and the flowers
• Cyan filter is a secondary filter. It will allow only cyan, blue and green to pass through
• So, the green light will reach our eyes
• So the leaves will appear green in color
(c) Blue letters
• White light is falling on the blue letters
• These blue letters reflect blue color. All the other six colors of the white light will be absorbed
• These reflected blue light should reach our eyes. Only then we can see those letters
• But a cyan filter is kept between our eyes and the flowers
• Cyan filter is a secondary filter. It will allow only cyan, blue and green to pass through
• So, the blue light will reach our eyes
• So the letters will appear green in color
(d) Red signature
• White light is falling on the red letters
• These red letters reflect red color. All the other six colors of the white light will be absorbed
• These reflected red light should reach our eyes. Only then we can see those letters
• But a cyan filter is kept between our eyes and the flowers
• Cyan filter is a secondary filter. It will allow only cyan, blue and green to pass through
• So, the red light will not reach our eyes
• So the letters of the signature will not be visible
In the next section, we will see the electromagnetic spectrum.
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