Question: What is the minimum thickness of a soap film that appears dark in reflected light for a wavelength of 600 nm?

Options:

1. 150 nm
2. 300 nm
3. 600 nm
4. 1200 nm

Correct Answer: 300 nm

Solution:

For destructive interference in reflected light, the minimum thickness t = λ/2n, where n is the refractive index (approximately 1.5 for soap). Thus, t = 600 nm / (2 * 1.5) = 200 nm.