The oil film deposited over water surface during rainy days seems to be coloured due to
Interference of Light
Understanding Colours on Oil Films on Water
When you see an oil film on a water surface, especially during rainy days, it often displays a beautiful array of colours. This fascinating phenomenon is primarily caused by the interference of light.
How Interference of Light Creates Colours
The oil film is very thin, typically just a few micrometers or even less. When white light (which contains all colours) from the sun or other sources falls on this thin oil layer, several things happen:
- Some light is reflected from the top surface of the oil film (the boundary between air and oil).
- Some light enters the oil film, passes through it, reflects off the bottom surface of the oil film (the boundary between oil and water), and then travels back up through the oil film and emerges into the air.
The light waves reflected from the top surface and the light waves reflected from the bottom surface travel slightly different path lengths before they recombine and reach our eyes. This difference in path length causes the waves to interfere with each other.
Constructive and Destructive Interference
- If the two sets of waves are in phase when they recombine, they reinforce each other. This is called constructive interference, and the colour corresponding to that wavelength of light will appear bright.
- If the two sets of waves are out of phase, they cancel each other out. This is called destructive interference, and the colour corresponding to that wavelength will be absent or dim.
The path difference between the two reflected waves depends on several factors:
- The thickness of the oil film.
- The refractive index of the oil.
- The angle at which the light hits the film.
Since white light is a mixture of different colours (wavelengths), the conditions for constructive and destructive interference are different for each colour. At any given point on the oil film, a particular colour might be enhanced while another is cancelled out, depending on the local thickness of the film and the viewing angle.
Because the oil film's thickness is usually not uniform across the surface, different areas exhibit constructive interference for different colours, resulting in the iridescent, swirling patterns of colour you observe.
Why Other Options Are Not the Primary Cause
- Dispersion of Light: While oil does cause dispersion, like a prism separating colours, this phenomenon primarily causes colour separation based on refractive index differences for different wavelengths. The vibrant, location-dependent colours on a thin film are better explained by the interference effect.
- Absorption of Light: Absorption involves the oil absorbing certain wavelengths of light. While some absorption might occur, it doesn't explain the bright, varied colours seen. Absorption would generally result in the oil appearing a specific colour (the wavelengths it doesn't absorb) or darker, not the dynamic iridescent display.
- Scattering of Light: Scattering is the deflection of light by particles or irregularities. While scattering contributes to phenomena like the blue sky or the appearance of clouds, it is not the primary mechanism for the colourful patterns observed on a smooth, thin oil film on water.
Therefore, the colourful appearance of an oil film on a water surface is a classic example of interference of light in thin films.
Important Questions from Interference
- A single slit of width $a$ is illuminated by a monochromatic light of wavelength $\lambda_1 = 6000 \text{ Å}$. The angular width of the central maximum observed in the Fraunhofer diffraction pattern is $\theta_1$. When the slit width is increased by $20\%$ and the light source is replaced with another monochromatic light of wavelength $\lambda_2$, the angular width of the central maximum becomes $\frac{3}{5}$ of its initial value, $\theta_1$. Determine the wavelength $\lambda_2$.
A system of three polarizers $P_1$, $P_2$, $P_3$ is set up such that the pass axis of $P_3$ is crossed with respect to that of $P_1$.
The pass axis of $P_2$ is inclined at $15^\circ$ to the pass axis of $P_1$.
When a beam of unpolarized light of intensity $I_0$ is incident on $P_1$, the intensity of light transmitted by the three polarizers is $I$. The ratio $(I_0/I)$ equals (nearly):The interference pattern is obtained with two coherent light sources. If the ratio of their amplitudes is $n$, then in the interference pattern, the ratio $\frac{{{I_{max}} - {I_{min}}}}{{{I_{max}} + {I_{min}}}}$ will be
- Two identical coherent waves are superimposed at a point. If the maximum possible resultant intensity from their interference is $I_{max}$, and the resultant intensity at this point is $I_{max}/4$, then find the phase difference between the two waves at this point.
Which of the following sources gives best monochromatic light?
