What is a retinoscope?
This is a handheld instrument which allows the optician to pass light into your pupil, so that it passes through it and is projected onto the retina, lining the back of your eye.
The optician twists the retinoscope from side to side, which causes the light patch reflected by your retina to move from side to side.
The optician can estimate from the direction and speed that the light appears to move across the retina whether you are short or long sighted and by how much.
The optician will then place different lenses in front of your eye until the reflected light appears still even when the retinoscope is moved.
The prescription required for each eye can then be determined by a simple calculation.