What is the wavelength of the light?

Question

figure below shows the interference pattern from a Young's double-slit experiment, where the maxima are indicated by the yellow patches and the minima are indicated by the black patches. The distance from the slits to the screen displaying the pattern is 4.1 m, the slit spacing is 0.080 mm, and the distance from the extreme maxima, y, is 16. cm. What is the wavelength of the light?

For that question, it is from Youngs Double slit experiment, and there is a figure with 8 black slits and 7 yellow slits. the yellow slits represents the maxima adn black represents the minimum.

If we now shine the same light onto a pair of slits with spacing 0.100 mm, what is the spacing between successive maxima in the interference pattern?

Answer 1

x = lD/a; x is the separation between successive maxima, l is the wavelength, D is the distance from the slits to the screen, a is the slit separation.

l = ax/D = 0.00008*0.16/4.1 = 1.99 x 10^(-5) m

This is not the wavelength of visible light.

Part 2.

x ~ 1/a

x = 16*0.08/0.10 = 12.8 cm