I have trouble remembering which muscle contracts and which muscle relaxes during the Iris reflex; could you explain it to me?

The first thing to remember is that muscles always act in pairs; so when one contracts, the other relaxes. Think of the muscles in your arm; when you bend your arm towards you, the bicep contracts so becomes shorter; while the tricep relaxes so becomes longer. In the eye the two types of muscle that control the pupil size are the radial muscles and the circular muscles. The radial muscles are like a radius, so connect the pupil to the iris; while the circular muscles are like a circle, and so are orientated around the pupil. Therefore in bright light, when the pupil needs to be smaller/constricted so less light goes into the eye; the circular muscles would need to contract (to make the pupil smaller), and because muscles work in pairs, the radial muscles would therefore have to relax (and become longer). In dim light the muscles would act in the opposite way.