A Photochromic Product for Sunglasses

Photochromic sunglasses should ideally be colourless under low-light conditions, be converted rapidly to a form with an appropriate absorption spectrum under intense light, and revert quickly to colourless under low light. All of this needs to happen efficiently over a wide temperature range and the system must be capable of an almost infinite number of cycles. 

One of the most widely used photochromic compounds for sunglasses is prepared easily as follows reaction of 2,7-dihydroxynaphthalene with a mixture of sodium nitrite and sulfuric acid 

Deduce the structure of the photochromic compound and explain the basis of its action. 

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