See things in a different light!

[Finless]

Originally Posted by Noel Johnson

I have a feeling that magnifying glasses don't work underwater as well as on land due to the refraction of light through water - or something like that.

The below reply was shown on Divernet:-

Where there's muck there's glass

A magnifying glass affords a close-up view of this handsome frogfishI am going to North Sulawesi in Indonesia, where I hope to enjoy some of the world's best muck-diving. Because my eyesight is not as good as it was, I was thinking about buying a large magnifying glass so that I could enjoy better looking at the smallest creatures. However, my local dive shop has told me that magnifying glasses don't work properly under water. Is this the case?

Lenses work through their ability to refract light as it passes from the less dense medium (air) through the more dense medium (the glass).
Because water has a greater density, this effect is not the same in water as it is in air. That's why the prescription lenses are fitted to the inside of a mask and work in conjunction with the air space within it.
If you use a magnifying glass in your hand (to look at minute sea life, for example) with water on both sides you will need one of a very strong (high-dioptre) value to be effective.

[Noel Johnson]

Well I did say "As well as"...

[Finless]

I'm just trying to figure out if I can make some tongue operated device which would enable a magnifying glass to flip into position inside my FFM. I mean, if I'm diving in 6 inch vis I might as well have a proper look at what I can see.

On long (more than 10 mins for me) deco stops I would be able to examine all the little beasties floating past my mask.