Your examiner will ask you to read a vehicle's number plate at a distance of 20 metres (for a new style plate) or 20.5 metres (if it's an old style number plate). New style plates start with two letters then have two numbers followed by three letters e.g. AB51 ABC
If you need to wear glasses or contact lenses to do this, then you will need to wear them whenever you drive.
If you can't pass the eyesight test, the driving test will not continue and you will have failed your driving test.