Question

Purple light of $400nm$ passes through a pair of narrow slits, forming an interference pattern on a screen. The diagram below is not drawn to scale, but represents path from each of the two slits to the third "bright fringe" formed as part of the interference pattern.
In the diagram how much longer is path $1$ than path $2$?

• A. $400nm$
• B. $1200nm$
• C. $1600nm$
• D. $1000nm$
• E. $600nm$

Answer 1

The correct option is B $1200nm$

Given : $\lambda =400$ nm

Path difference between the lights forming nth bright fringe $\Delta x=n\lambda$

For third bright fringe, $n=3$

$⟹$ $\Delta x=3\times 400=1200$ nm