Answer:
[tex]\lambda = 672 nm[/tex]
so this is nearly red colour light
Explanation:
As we know that the interference of light from reflected light then the path difference is given as
[tex]\Delta x = 2\mu t + \frac{\lambda}{2}[/tex]
now we know that for constructive interference of light the path difference is given as
[tex]\Delta x = n\lambda[/tex]
so we will have
[tex]2\mu t + \frac{\lambda}{2} = N\lambda[/tex]
so we will have
[tex]4\mu t = \lambda[/tex]
[tex]\lambda = 2(1.40)(120nm)[/tex]
[tex]\lambda = 672 nm[/tex]
so this is nearly red colour light