White light is shining on the surface of a soap bubble, which has the same index of refraction as water. Find the minimum thickness, in nanometers, of a soap bubble which appears red (a wavelength of 725 nm) when the light is perpendicular to its surface.

The minimum thickness is  

Explanation:

From the question we are told that

   The  index of refraction of the bubble is   =  index of refraction of water

    The wavelength is  

Generally the condition for constructive interference is  mathematically represented as

Now t is the thickness and at minimum t,   m(order of maximum ) = 0  

So

=>    

layer a is the exosphere and layer b is the thermosphere.   is the answer i think but i hope this .