Question

Aryan and Mira are playing with soap bubbles in sunlight. Aryan notices that the bubble appears purple, which is unusual. A scientist explains the phenomenon and gives the following statements:
I. The colour is due to interference of light in a thin film.
II. As the bubble thickness increases, the reflected colour shifts towards higher wavelength.
III. Purple colour appears because shorter wavelengths undergo constructive interference first.
IV. The colour is due to absorption of light by soap.
Which of the above statements correctly explain the observation?

• A. I and II only
• B. I, II and III only
• C. II and IV only
• D. I and IV only

Hint:

In physics multiple-choice questions, be wary of statements that provide an incorrect causal link ("A happens because of B," where B is true but doesn't cause A).
While it's true that shorter wavelengths interfere first, this fact doesn't explain the appearance of a composite color like purple. Stick to the fundamental and undisputed principles.

Answer 1

The Correct Option is A

Step 1: Understanding the Question: We need to evaluate four statements explaining the colors, specifically an unusual purple color, seen in a soap bubble.

Step 2: Detailed Explanation:
Statement I: The colour is due to interference of light in a thin film.
This is the fundamental principle behind the colors of soap bubbles and oil slicks. Light reflects from both the outer and inner surfaces of the thin soap film, and these reflected waves interfere with each other. This is a correct statement.
Statement II: As the bubble thickness increases, the reflected colour shifts towards higher wavelength. The condition for constructive interference is (2nt propto lambda). This means that for a given order of interference, a larger thickness (t) is required to produce constructive interference for a larger wavelength (lambda). So, as thickness increases, the dominant reflected color shifts from blue towards red (higher wavelength). This is a correct statement.
Statement III: Purple colour appears because shorter wavelengths undergo constructive interference first. This statement has two parts. "Shorter wavelengths undergo constructive interference first" is true as thickness increases from zero.
However, this explains the appearance of violet/blue, not purple. Purple is not a spectral color; it's a combination of red and blue/violet light.
For purple to be seen, the bubble thickness must be such that it causes constructive interference for both red and blue light simultaneously, while possibly causing destructive interference for intermediate colors like green.
The reasoning provided in the statement is a poor and misleading explanation for the appearance of purple. Therefore, this statement is considered incorrect in a scientific context.
Statement IV: The colour is due to absorption of light by soap.
The soap solution itself is largely transparent and does not selectively absorb colors to produce the vibrant patterns seen. The colors are almost entirely a result of the interference of light, not absorption. This statement is incorrect.

Step 3: Final Answer:
Statements I and II are correct physical principles governing thin-film interference. Statement IV is incorrect. Statement III provides a flawed explanation for the specific color purple. Therefore, the only consistently correct statements are I and II.