Chapter U5.116, Problem 7E

Interpretation Introduction

Interpretation:

The difference between sunglasses with UV protection and regular reading glass with no UV protection needs to be explained using a UV-sensitive paper test.

Concept introduction:

The UV sensitive paper is sensitive to UV radiations of the sun. A paper is coated with a chemical that is light sensitive. This chemical reacts with UV light waves when it is exposed to the sun. The color of the paper is blue but after reacting with light waves it turns white.

Explanation of Solution

The chemical coated on the UV sensitive paper is iron (III) hexacyanoferrate (III) $\text{Fe}\left[\text{Fe}{\left(\text{CN}\right)}_{\text{6}}\right]$ which is water-soluble and blue-green in color. It is a photosensitive chemical.

When the chemical is exposed to UV light, the water-soluble compound gets converted into water-insoluble iron (III) hexacyanoferrate (II) $\text{Fe}{\left[{\text{Fe}}_4{\left(\text{CN}\right)}_{\text{6}}\right]}_3$, this is also known as Prussian blue.

Since $\text{Fe}\left[\text{Fe}{\left(\text{CN}\right)}_{\text{6}}\right]$ is water-soluble so when UV sensitive paper is rinsed with water, $\text{Fe}\left[\text{Fe}{\left(\text{CN}\right)}_{\text{6}}\right]$ washes away and water-insoluble $\text{Fe}{\left[{\text{Fe}}_4{\left(\text{CN}\right)}_{\text{6}}\right]}_3$ remains on the paper. The experiment only works in the presence of sunlight.

Thus, this paper can be used to test if the sunglasses are UV protected or regular sunglasses.

This paper can be pasted on the lens of the sunglasses (inner side) and the outer side can be exposed to sunlight.

In the case of UV protected sunglasses, there will be no change in the UV sensitive paper as the glasses protect paper from UV light. In the case of regular glasses, there is no UV protection. Thus, the conversion of $\text{Fe}\left[\text{Fe}{\left(\text{CN}\right)}_{\text{6}}\right]$ to $\text{Fe}{\left[{\text{Fe}}_4{\left(\text{CN}\right)}_{\text{6}}\right]}_3$ which takes place can be seen when the paper is rinsed with water.