Interference of Light in the Colour Display of Oil on Water Surface

When a drop of oil is spread on a water surface, it displays beautiful colors in daylight. This phenomenon is due to the interference of light. Let's understand this in detail.

Dispersion of Light
When light enters the oil drop, it refracts and splits into its constituent colors due to the difference in the refractive indices of oil and air. This phenomenon is known as dispersion of light. However, this is not the reason behind the colorful display.

Reflection of Light
When light strikes the oil-water interface, some of it gets reflected back. This reflected light interferes with the incident light, leading to constructive and destructive interference. This interference phenomenon creates a pattern of bright and dark fringes, which appear as colorful rings when viewed from above. But, reflection alone cannot explain the complete phenomenon.

Polarization of Light
The polarization of light occurs when the reflected light is partially polarized. This polarization results in the selective absorption of certain colors. However, this polarization effect is also not the sole reason behind the colorful display.

Interference of Light
The most significant reason behind the colorful display is the interference of light. The thickness of the oil film on the water surface varies at different points. This variation in thickness leads to a difference in the path length of the reflected light, resulting in the interference of light waves. This interference creates a pattern of bright and dark fringes, which we observe as colorful rings.

Conclusion
Hence, the colorful display of oil on a water surface is due to the interference of light, which creates a pattern of bright and dark fringes. The thickness of the oil film on the water surface varies at different points, leading to a difference in the path length of the reflected light, resulting in the interference of light waves.

When a drop of oil is spread on a water surface, it displays beautiful colours in daylight because the oil film is only a few nano meters. Some of the light is reflected off the top surface and some the bottom surface because the thickness of the film is about the same as wavelength of the light the two reflected rays interfere with each other.