In the previous chapter we completed a discussion on power transmission and distribution. In this section, we will see colours of light.
• We have learned about refraction of light in a previous chapter. Based on that discussion, we will now see some advanced topics.
• Consider fig.13.1 below. A ray of white light falls on a glass prism.
• The ray passes through the prism and emerges from the other side.
• But the emerging ray is not white. It is composed of different colours.
• We want to know the reason for the splitting up of white light into different colours. Let us write an analysis. We will write it in steps:
1. First of all, the incident ray should fall on the prism obliquely.
• 'Obliquely' means, 'at an angle'. This can be explained based on fig.13.2 below:
• The magenta line is drawn perpendicular to the surface of the prism.
• Such a perpendicular is called the 'normal of the surface'.
• The ray of light should make an angle with the normal. We shall call this angle as 'θ'. This 'θ' can take any value.
• If there is no angle (that is., if θ = 0), then the ray will be passing exactly along the normal. In that case, we will not get different colours.
2. Now consider the black dashed line in the fig.13.2.
• It is the original path of the light. The light should have passed along that line.
• But it cannot do so. This is because, the glass material of the prism bends the light.
• We have seen the details when we learned about refraction.
3. But the glass is not able to bend all lights to the same extent
• Violet is bent more
• Red is bent less
4. The other colours that fall in between are: indigo, blue, green, yellow and orange
• The order of bending can be arranged in decreasing order:
■ Violet, Indigo, Blue, Green, Yellow, Red
■ Because of this difference in bending, components of the white light gets separated from each other.
■ Any light that is composed of more than one colour is called a composite light
■ So we can write: White light is a composite light.
■ A composite light will undergo dispersion when it passes through a prism.
■ Dispersion is the phenomenon of splitting up of a composite light into it's constituent colours.
■ The components obtained by dispersion will appear as a regular array.
■ This regular array of colours is called visible spectrum
Now we will see the reason for 'violet being bent more than red'
1. Consider the table given below:
• It gives the Wave lengths of different colours.
• We have seen some basics about waves in an earlier chapter (Details here)
2. Wave lengths are arranged in ascending order.
• We can see that, the violet which undergoes most bending is at the top of the list.
• Red which undergoes least bending is at the bottom of the list.
3. So we can write:
• The deviation from the original path depends on the wave length of the colour
• The colour which has more wave length deviates less
• The colour which has less wave length deviates more
• The other colours fall in between in regular order.
Light undergoes refraction when it enters the prism obliquely and when it comes out of the prism. The extent of deviation depends on the wavelength. Therefore waves undergo deviation at different angles and get separated. This is the reason for dispersion.
• We have learned about refraction of light in a previous chapter. Based on that discussion, we will now see some advanced topics.
• Consider fig.13.1 below. A ray of white light falls on a glass prism.
 |
| Fig.13.1 Source: https://en.m.wikipedia.org/wiki/File:Dispersion_prism.jpg |
• But the emerging ray is not white. It is composed of different colours.
• We want to know the reason for the splitting up of white light into different colours. Let us write an analysis. We will write it in steps:
1. First of all, the incident ray should fall on the prism obliquely.
• 'Obliquely' means, 'at an angle'. This can be explained based on fig.13.2 below:
 |
| Fig.13.2 |
• Such a perpendicular is called the 'normal of the surface'.
• The ray of light should make an angle with the normal. We shall call this angle as 'θ'. This 'θ' can take any value.
• If there is no angle (that is., if θ = 0), then the ray will be passing exactly along the normal. In that case, we will not get different colours.
2. Now consider the black dashed line in the fig.13.2.
• It is the original path of the light. The light should have passed along that line.
• But it cannot do so. This is because, the glass material of the prism bends the light.
• We have seen the details when we learned about refraction.
3. But the glass is not able to bend all lights to the same extent
• Violet is bent more
• Red is bent less
4. The other colours that fall in between are: indigo, blue, green, yellow and orange
• The order of bending can be arranged in decreasing order:
■ Violet, Indigo, Blue, Green, Yellow, Red
■ Because of this difference in bending, components of the white light gets separated from each other.
■ Any light that is composed of more than one colour is called a composite light
■ So we can write: White light is a composite light.
■ A composite light will undergo dispersion when it passes through a prism.
■ Dispersion is the phenomenon of splitting up of a composite light into it's constituent colours.
■ The components obtained by dispersion will appear as a regular array.
■ This regular array of colours is called visible spectrum
Now we will see the reason for 'violet being bent more than red'
1. Consider the table given below:
• We have seen some basics about waves in an earlier chapter (Details here)
2. Wave lengths are arranged in ascending order.
• We can see that, the violet which undergoes most bending is at the top of the list.
• Red which undergoes least bending is at the bottom of the list.
3. So we can write:
• The deviation from the original path depends on the wave length of the colour
• The colour which has more wave length deviates less
• The colour which has less wave length deviates more
• The other colours fall in between in regular order.
Light undergoes refraction when it enters the prism obliquely and when it comes out of the prism. The extent of deviation depends on the wavelength. Therefore waves undergo deviation at different angles and get separated. This is the reason for dispersion.
The formation of rainbows in the sky can be explained based on dispersion. We will see it in the next section.
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