for the first bright fringe, the distasnce from the centre is
`y=(Dlamda)/d`
for violet light `y=2.0. Thus,
`2.0 mm=((2.5m)lamda)/(0.5mm)`
`or lamda=((0.5mm)(2.0mm))/(2.5mm)=400nm`
`Similarly for red light, y=3.5 mm. thus,
`3.5mm=((2.5m)lamda)/(0.5mm)`
`or lamda=700nm`