Question

In Young's double slit experiment the wavelength of light was changed from 7000 \AA to 3500 \AA while doubling the separation between the slits. Which of the following is not true for this experiment?

• A. The width of the fringes changes
• B. The colour of bright fringes changes
• C. The separation between successive bright fringes changes
• D. The separation between successive dark fringes remains unchanged

Hint:

Fringe width is proportional to \(\lambda\) and inversely proportional to \(d\).

Answer 1

The Correct Option is D

Step 1: Understanding the Concept:
Fringe width \(\beta = \frac{\lambda D}{d}\).
Step 2: Detailed Explanation:
\(\frac{\beta_2}{\beta_1} = \frac{\lambda_2}{\lambda_1} \times \frac{d_1}{d_2} = \frac{3500}{7000} \times \frac{d}{2d} = \frac{1}{2} \times \frac{1}{2} = \frac{1}{4}\)
So \(\beta\) changes. Both bright and dark fringes spacing change. Colour changes.
Step 3: Final Answer:
Separation between dark fringes also changes.