Question

The brilliant colors in thin films of soap are due to \(\underline{\hspace{1cm}}\).

• A. interference
• B. diffraction
• C. scattering
• D. dispersion

Hint:

Keywords for optical phenomena: - Interference: Thin films, two slits, coherent sources. - Diffraction: Single slit, sharp edges. - Scattering: Small particles, blue sky. - Dispersion: Prism, rainbow.

Answer 1

The Correct Option is A

Step 1: Understand the structure of a soap film.
A soap bubble or film is a very thin sheet of water sandwiched between two layers of soap molecules. Its thickness is comparable to the wavelength of visible light.

Step 2: Analyze how light interacts with the film.
When white light strikes the film, some of it reflects off the top surface, and some of it enters the film and reflects off the bottom surface.

Step 3: Apply the principle of interference.
The two reflected light waves (from the top and bottom surfaces) travel slightly different path lengths. When they recombine, they interfere with each other. For certain wavelengths (colors) and film thicknesses, the interference is constructive (making the color appear bright), and for others, it is destructive (canceling the color out). Since the thickness of the film is not uniform, different colors are seen at different places, creating the brilliant color patterns.

Step 4: Evaluate other options.
- Diffraction is the bending of waves as they pass around an obstacle. - Scattering is the redirection of light by particles (e.g., why the sky is blue). - Dispersion is the splitting of white light into its constituent colors by a prism due to different refractive indices for different wavelengths.