How do you calculate finge width without the wavelength ?

[AJ786]

Homework Statement

Young's fringes, produced by monochromatic laser light passing through slits 0.60mm apart, are viewed on a screen. The distance accrose 20 fringe spacings on the screen is 58mm. When the screen is moved 0.80m away from the slits, the distance across 20 fringe spacings becomes 74mm.

Homework Equations

Y = D(lamda) / d
Y = Fringe Spacing
D = Distance from slits to screen
d = spacing between slits
(lamda) = Wavelength

The Attempt at a Solution

 

[member 553083]

Y1 = 58mm / 20 = 2.90mm Y2 = 74mm / 20 = 3.70mm (lamda)1 = Y1D1 / d (lamda)1 = 2.90(0.8) / 0.60 (lamda)1 = 4.80(lamda)2 = Y2D2 / d (lamda)2 = 3.70(1.6) / 0.60 (lamda)2 = 9.60