There is less difference between the speed of light in glass and the speed of light in water than there is between the speed of light in glass and the speed of light in air. Does this mean that a magnifying glass will magnify more or magnify less when it is used under water rather than in air?

Answer: no, the magnifying glass has nothing to do with the speed of light.

Explanation: When light moves between 2 media (refraction), part of it properties that changes during this process is it speed.

The change in speed is dependent on the refractive index of the 2 media and as given by snell's law, the refractive index is inversely proportional to wave speed.

This implies that moving from dense to a more dense medium reduces wave speed and moving from dense to less dense medium increases wave speed.

For the first statement, light moved from glass to water, it implies that it moves from a dense to a less dense medium, it wave speed increases in water.

For the second statement, light moved from glass to air, it implies that it also moves from a dense to a less dense medium and it wave speed will also increase in air.

Looking at both second medium, for the first statement, the second medium is water, and for the second statement, the second medium is air.

Air is less dense that water, hence light travel faster in air than in water.

Thus we can see that the magnification property of a glass has nothing to do with the wave speed, just the refractive indices of the media.