Is this qualifying examination painful? Do I have to prepare for them? Will I be entitled to a medical leave after the surgery (I work in front of a computer and I have heard that it is not recommended after the surgery).

Qualifying examination is not painful at all.
Before the examination you should restrain from wearing contact lenses for at least a day and you should be driving a car for about 3-4 hours after it (that’s because the eye drops used during the examination temporarily impair vision). No further preparations have to be made. After the surgery you will be given a proper statement entitling you to one week (or longer) medical leave. You can return to work after 2 days, however it is advisable to minimise the time spent in front of the computer during the first week after the surgery.