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Difficulty: Very hardPhysics Fundamentals and Everyday Applications

When a thin film of oil spreads over water on a road surface, vibrant multi-colored patterns are observed under sunlight. A candidate preparing for a competitive examination confuses this optical effect with the mechanism that causes the clear sky to appear blue. Which of the following statements correctly identifies the physical principle responsible for the oil film's colorful appearance and distinguishes it from the blue sky phenomenon?

  1. The colorful patterns on the oil film are caused by thin-film wave interference between light reflected from the upper and lower boundaries of the oil layer, whereas the blue sky is caused by Rayleigh scattering of sunlight by atmospheric gas molecules.Answer
  2. B
    The colorful patterns on the oil film are caused by chromatic dispersion of sunlight inside the oil, whereas the blue sky is caused by total internal reflection of light within water vapor in the atmosphere.
  3. C
    The colorful patterns on the oil film are caused by Rayleigh scattering of light by microscopic oil molecules, whereas the blue sky is caused by thin-film interference taking place in the upper ozone layer.
  4. D
    The colorful patterns on the oil film are caused by diffraction of sunlight around the edges of water droplets, whereas the blue sky is caused by atmospheric refraction through air density gradients.

Answer

The colorful patterns on the oil film are caused by thin-film wave interference between light reflected from the upper and lower boundaries of the oil layer, whereas the blue sky is caused by Rayleigh scattering of sunlight by atmospheric gas molecules.
The correct response correctly identifies that light reflecting from both the upper (air-oil) and lower (oil-water) boundaries of a thin oil film undergoes wave interference, producing colorful spectral fringes. In contrast, the blue sky is caused by Rayleigh scattering, where air molecules selectively scatter shorter blue wavelengths of sunlight in all directions.

Step-by-Step Solution

1
Analyze the optical mechanism of thin oil films on water
Sunlight strikes the top surface of the thin oil film. Part of the light reflects off the top boundary (air-oil interface), while the rest transmits through the film and reflects off the bottom boundary (oil-water interface).
When these two reflected light waves recombine, they undergo constructive or destructive interference depending on the film thickness and wavelength of light, creating rainbow-like interference patterns.
2
Analyze the optical mechanism responsible for the blue appearance of the daytime sky
Sunlight entering Earth's atmosphere collides with air molecules (N2,O2N_2, O_2) whose sizes are much smaller than the wavelength of visible light.
According to Rayleigh scattering law, scattered intensity I∝1λ4I \propto \frac{1}{\lambda^4}. Shorter blue wavelengths scatter much more efficiently than longer red wavelengths, filling the sky with diffuse blue light.
3
Compare the two phenomena to select the correct choice
Oil film colors stem from wave superposition (thin-film interference), whereas sky color stems from elastic scattering by small particles (Rayleigh scattering).
Differentiating superposition phenomena from scattering processes eliminates all distractor options.

Key Concept

Thin-Film Interference vs. Rayleigh Scattering
Estimated Time:1m 30s
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Topics

Physics Fundamentals and Everyday Applications

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