Question

The coherence length of the He-Ne laser beam is 120 cm. Its coherence time in seconds is:

• A. \( 4 \times 10^{-1} \)
• B. \( 4 \times 10^{-3} \)
• C. \( 4 \times 10^{-5} \)
• D. \( 4 \times 10^{-9} \)

Hint:

Use \( t_c = L_c / c \), where \( L_c \) is coherence length and \( c \) is speed of light.

Answer 1

The Correct Option is D

Coherence time \( t_c = \frac{L_c}{c} = \frac{1.2 \text{ m}}{3 \times 10^8 \text{ m/s}} = 4 \times 10^{-9} \text{ s} \)