Answer :
The minimum wavelength required to cause photochromic eyeglasses to darken is just below 400 nm, as this range is in the ultraviolet spectrum which activates the darkening process in these lenses.
These types of lenses contain molecules that change shape and absorb light when exposed to ultraviolet (UV) light, which has a wavelength shorter than the visible spectrum (less than 400 nm).
Since the eye is sensitive to wavelengths between 400-800 nm, it's the UV light, below 400 nm, that causes these lenses to darken.
Photochromic lenses do not usually respond to light that is visible to the human eye because it does not have the energy to activate the darkening process. Therefore, the minimum wavelength required to darken photochromic eyeglasses is just below 400 nm.
