Question

If the eighth bright band due to light of wavelength $\lambda_1$coincides with ninth bright band from light of wavelength $\lambda_2$ , in Young's double slit experiment, then the possible wavelengths of visible light are

• A. 400 nm and 450 nm
• B. 425 nm and 400 nm
• C. 400 nm and 425 nm
• D. 450 nm and 400 nm

Answer 1

Answer: (D)

450 nm and 400 nm

Answer 2

Position of nth bright fringe from central maximum is $\frac{n \lambda D}{d}$. $\therefore \frac{8 \lambda_1 D}{d} = \frac{9 \lambda_2 D}{d}$ $\frac{\lambda_1}{\lambda_2} = \frac{9}{8}$ which is correct only option (d).