What Determines the Minimum Thickness of a Soap Bubble for Colorful Reflections?

[TomHepp]

Homework Statement

A soap bubble of refractive index 1.33 is illuminated by white light perpendicular to its
surface and viewed normal to its surface. What is the minimum thickness of the bubble for
which brightly-coloured reflected light would be seen?

Homework Equations

$$\lambda=4nt$$

The Attempt at a Solution

How do you solve this without knowing the wavelength?

 

[vela]

What's the range of wavelengths for visible light?

 

[TomHepp]

Ahh I see you use the smallest wavelength for it to be the minimum, thanks.