You should know how to work out magnification in convex lenses for Physics A Level. Magnification happens in convex lens through the formula:
Magnification = image height / object height = V/U
speed of light in a vacuum / speed of light in a material (any material)
This can be represented as n = c/v where c = 3 x 10 to the power of 8 m/s.
The unit is just a number as c and v are measured in m/s. Therefore, the units cancel each other out.
If you are revising Physics Topics, please have a look at other articles I have done on Physics you may be interested in.