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
From the below diagram, we can see the object height and image height as well as the object distance (U) and image distance (V).
To work out magnification, you can divide the image height by the object height or image distance over object distance.
It is important to also know the formula of the refractive index in a material:
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.