Dispersion of Light Through Prism and Formation of Spectrum

Dispersion occurs when white light passes through a prism and splits into its component colours, forming a spectrum. This happens because different colours of light bend (refract) at different angles as they travel through the prism. The result is a rainbow-like spectrum, consisting of red, orange, yellow, green, blue, indigo, and violet (VIBGYOR). This phenomenon proves that white light is composed of multiple colours, which become visible when dispersed.

• Red light bends the least, while violet light bends the most.
• The bending of light depends on its wavelength and the refractive index of the medium.
• Shorter wavelengths (violet, blue) bend more, and longer wavelengths (red, orange) bend less.

Dispersion of light

Sir Isaac Newton demonstrated that white light is made up of seven colours using an experiment with a spinning disc.

• Newton created a disc divided into seven sections, each representing one colour of the spectrum (VIBGYOR).
• He rotated the disc rapidly, and the seven colours merged, appearing white.
• This experiment proved that sunlight is composed of seven colours.
• Newton recorded his findings in his book "Opticks," which explained the nature of light.

Light is a type of electromagnetic radiation, and each colour of light has a different wavelength:

• Red light has the longest wavelength (~700 nm).
• Violet light has the shortest wavelength (~400 nm).
• 1 nanometre (nm) = 10⁻⁹ meters.

In a vacuum, all colours of light travel at the same speed. However, in a medium like glass, different colours travel at different speeds due to their different refractive indices. This variation in speed causes the splitting of white light into its component colours, known as dispersion.

• The refractive index of a medium is different for different colours.
• When white light enters a prism, each colour refracts at a different angle, forming a spectrum.
• Red light travels the fastest and bends the least.
• Violet light travels the slowest and bends the most.

This process of light separation into different colours while passing through a medium is called dispersion.

To observe the dispersion of light through water and the formation of a spectrum (rainbow-like colours).

• Place the glass of water on the white sheet of paper near a window where it receives direct sunlight.
• Observe the light passing through the water. You will see a rainbow-like spectrum on the sheet of paper.

Alternative method (in a dark room): Use a prism and a torch instead of the glass and sunlight. Shine the torchlight through the prism, and a similar spectrum will appear on a nearby surface.

This experiment demonstrates how white light is made up of different colours.  When light passes through water or a prism, it separates into these colours, forming a spectrum. This is called the dispersion of light.

To observe the dispersion of light and formation of rainbow-like colours in soap bubbles.

• Dip the wire loop in soapy water.
• Blow through the loop to create soap bubbles.
• Observe the bubbles, which display rainbow-like colours.

The colours seen in soap bubbles are due to the dispersion of light and interference of light waves, similar to a rainbow. The soap film acts like a prism, separating light into different colours.

To observe the dispersion of light when sunlight hits a CD.

• Hold the CD in direct sunlight.
• Observe the reflection on the CD surface, which creates a rainbow-like pattern of colours.

The grooves on a CD act like a diffraction grating, separating white light into its component colours, similar to a prism. This shows the dispersion of light and the formation of a spectrum.