13.6 - Sunlight - White or Coloured

Sunlight is a mixture of all the colours present in the visible spectrum. This mixture appears to be white to the human eye. However, when sunlight passes through a prism or droplets of water in the air, it splits into its constituent colours, forming a spectrum. This phenomenon is known as the dispersion of light.

Sunlight is made up of several colours, which can be seen when the light is passed through a glass prism. The different colours that make up sunlight are:

• Red
• Orange
• Yellow
• Green
• Blue
• Indigo
• Violet

These colours can be remembered using the acronym "VIBGYOR" which stands for Violet, Indigo, Blue, Green, Yellow, Orange, and Red. Each of these colours has a different wavelength. The red light has the longest wavelength, while violet light has the shortest wavelength.

Dispersion of Light

Dispersion occurs when white light is passed through a medium like a glass prism. The different wavelengths of light bend by different amounts due to the difference in the refractive index for each colour. This bending causes the colours to spread out and form a spectrum. The sequence of colours formed is:

• Violet (shortest wavelength)
• Indigo
• Blue
• Green
• Yellow
• Orange
• Red (longest wavelength)

In the atmosphere, the sunlight also undergoes scattering, which is why the sky appears blue during the day. The shorter wavelengths (blue and violet) scatter more, and this scattered blue light reaches our eyes from all directions.

Refraction and Dispersion of Light in a Prism

When light passes through a prism, it bends (refracts) at the two surfaces. The amount of bending depends on the wavelength of light. Shorter wavelengths (violet and blue) bend more, while longer wavelengths (red and orange) bend less. This results in the separation of light into its constituent colours.

The formula for the refraction of light is given by Snell's Law, which states that:

$n_1 \sin \theta_1 = n_2 \sin \theta_2$

Where:

• $n_1$ = refractive index of the medium where the light is coming from
• $n_2$ = refractive index of the medium the light is entering
• $\theta_1$ = angle of incidence
• $\theta_2$ = angle of refraction

This bending of light into a spectrum can be used to demonstrate that white light is a combination of multiple colours.

Rainbow Formation

A rainbow is a natural example of dispersion of sunlight. When sunlight passes through water droplets in the atmosphere, it refracts (bends) as it enters the droplet, reflects off the inside surface, and refracts again as it exits. This process causes the sunlight to spread out into its different colours, forming a rainbow. The seven colours seen in a rainbow are a result of the dispersion of sunlight in raindrops.

Key Points

• Sunlight is a mixture of different colours.
• When passed through a prism, sunlight forms a spectrum of colours (VIBGYOR).
• The dispersion of light occurs due to the different wavelengths of light bending by different amounts.
• Refraction and dispersion in a prism show that white light is composed of multiple colours.