What visible wavelengths of light are strongly reflected from a 390-nm-thick soap bubble?

So visible wavelength which is possible here is

416 nm and 693.3 nm

Explanation:

As we know that for normal incidence of light the path difference of the reflected ray is given as

so here we can say that for maximum intensity condition we will have

so we have

now for visible wavelength we have

for N = 1

for N = 2

for N = 3

c. newton's third law of motion