Question

If yellow light emitted by sodium lamp in Young's double-slit experiment is replaced by a monochromatic blue light of the same intensity

• A. fringe width will decrease.
• B. fringe width will increase.
• C. fringe width will remain unchanged.
• D. fringe width will become less intense.

Answer 1

Answer: (A)

Fringe width is given by, $\beta =\frac{\lambda D}{d}$ , where $\lambda ,D\&d$ represents wavelength, screen distance and distance between the slits respectively.
So for same setup of Y.D.S.E., we can conclude from above formula that,
$\beta \propto \lambda ...\left(1\right)$
And we know from visible light spectrum, that ${\lambda }_{blue}<{\lambda }_{yellow}$
So from $\left(1\right)$ , we can say
${\beta }_{blue}<{\beta }_{yellow}$
$\therefore$ Fringe width $\beta$ will decrease.