A double slit is illuminated simultaneously with orange light of wavelength 600nm and light of an unknown wavelength. the m=4 bright fringe of the unknown wavelength overlaps the m=3 bright orange fringe. What is the unknown wavelength?
dsin(theta) = m(lambda)
The Attempt at a Solution
I tried finding d for orange light by suing the above formula. I substitued m=3
sin 90 and wavelength as 600 x 10^-9
and got d= 0.0000018m
and then I used this d to find unknown wavelength and used the same above equation but this time substitutes m=4 and sin120 and got wavelength= 0.000000389m or (389nm)
but my answer is wrong .... please tell me where I'm making mistake.