Real Images can be seen on a piece of paper or screen placed because the focal point is in front of the mirror or behind the lens.
Virtual Images can not be seen on a piece of paper or screen, because the focal point is behind the mirror or in front of the lens. Virtual images are images which are formed in locations where light does not actually reach; it only appears to an observer as though the light were coming from this position.
The line of students approach the boundary between the two medium (the masking tape). An arrow is used to show the general direction of travel for the group of students in both medium. Observe that the direction of the students changes at the "boundary."
The optical density of a material relates to the sluggish tendency of the atoms of a material to maintain the absorbed energy of an electromagnetic wave in the form of vibrating electrons before reemitting it as a new electromagnetic disturbance.
The more optically dense which a material is, the slower that a wave will move through the material.