Question

Please help! Really Really confused!

Answer 1

Coherent red light of wavelength 6*10^-7 is incident normally on a pair of narrow slits 'S' and 'F'. A pattern of bright and dark lines, called fringes appears close to point 'P' on a distant viewing screen.

The distance between the slits and the distant screen is 2.5m and the distance between the 2 slits is 1.2 mm.

State a value of the path difference between the light waves from 'S' and 'F' to the screen to produce a dark fringe on the screen.

Please explain how to answer the question so I know how to do similar ones in the future!

Thank you!