Question

The width of a certain spectral line at 500 nm is 2 × 10^–2 nm. Approximately what is the largest path difference for which interference fringes produced by this light are clearly visible?

• A. 10^–4 cm
• B. 2 × 10^–4 cm
• C. 3 × 10^–4 cm
• D. 4 × 10^–4 cm

Answer 1

Answer: (C)

3 × 10^–4 cm

Answer 2

The coherence length l_c is given by

l_c =$\frac{\lambda^{2}}{\Delta\lambda}$ = 1.25 × 10^–3 cm.

If the optical path difference is about a quarter of l_c, 3 × 10^–4 cm, we can observe the fringes clearly