Question

You might have seen sunglasses that change to a color when exposed to light, while revert to transpaparency when the light is blocked or dimmed. Thematerical used to make the glasses is called photochromic material that has light sensitive components inside. The technology is similar to that used to make a photographic film.

(a) Explain, using necessary chemical formula, the working principle of photochromic sunglasses.

(b) The color of the photochromic sunglasses is reversible, however, the image on a photographic film once exposed and developed, is fixed and becomes irreversible. Why?

Answer 1

We know that, sunglasses that changes a color when exposed to light while revert to transpaparency when the light is blocked or dimmed. The materical used to make the glasses is called photochromic material that has light sensitive components inside. The technology is similar to that used to make a photographic film.

Photochromic sunglasses which consist of a specialized optical lenses, are known as photochromic lenses that automatically darken in the presence of a certain types of light (like ultraviolet light) with adequate intensity and become 'clear' again in the absence of that particular light.

The working principle of photochromic sunglasses which will be given below as :

They tend to be plastic and instead of silver chemicals. They also contain organic (carbon-based) molecules called naphthopyrans that react to light in a slightly different way. They change their molecular structure when ultraviolet light strikes them and become dark

In the absence of UV light, the molecules reverse to their original shape which making the glass clear once again.

In this altered form, they soak up more ordinary light as it tries to pass by, which is what makes the lenses darken. Imagine, lots of molecules suddenly darkening inside a clear lens. It's a bit like closing the blinds in front of our window on a sunny day as the slats turn, they progressively block out more and more light.

Photochromic lenses filter 100% UVA as well as UVB.

UVB light is more energetic and causes sunburn as well as skin damage including cancers whereas UVA light causes skin cancers but not usually sunburn.

UVB light is blocked by all glass whereas UVA light is not blocked by an ordinary windows or glass lenses.