Before revision: The path difference between rays reflected from the top and bottom of the surface bumps is: When the wavelength of the light is , the phase difference will be: If is small, the two beams (see Figure 1) are nearly in phase and therefore the specimen surface can be considered smooth.
After revision: The path difference between rays reflected from the top and bottom of the surface bumps is: When the wavelength of the light is , the phase difference will be: If is small, the two beams (see Figure 1) are nearly in phase , resulting in constructive interference; therefore, the specimen surface can be considered smooth.