Light passing through a glass brick
We know that when light passes from one medium to another at an oblique (ie, not perpendicular) angle, the ray of light will change direction. When the light passes into a glass or perspex block and out again the light ray will change direction twice.
The light ray passing through the air towards the glass block will be parallel to the light coming out of the glass block.
If the light enters along the normal line i.e. perpendicular (at 90°) to the block then no refraction (change of direction) occurs.
The light will be slowed down as it travels through the denser medium but it will NOT change direction.