Why are spectacles or camera lens so difficult to clean?

Why are spectacles or camera lens so difficult to clean? It seems to get worse when an ordinary cloth is used, which tends to produce mult-coloured marks.

When an ordinary cloth is used to rub grease off the glass, the grease becomes an additional coating with varying thickness. This produces multi-coloured reflections, because light is made up of different colours (i.e. different wavelengths). The 'rainbow' effect is similar to what we see when looking at a wet road with an oil film on it.

What is the proper way to clean the glasses?

Use the special cloth that comes with spectacles or camera lens. It is made of thin fibres that are good at picking up grease, rather than spreading it.

A wet method involves washing with soap or a little liquid detergent. Then rinse, and leave it to dry or dry with a clean linen. You can also go to an optical shop for such a service.

(For more details, please click here to read the article written by Hong Kong Observatory)

(Information provided by Hong Kong Observatory)