Explanation:

Refraction of Light:
- When light passes from one medium to another, it changes its speed. This change in speed causes the light rays to bend. This phenomenon is known as refraction of light.

Thick Glass Slab:
- When a thick glass slab is placed over printed metal letters, the light rays coming from the letters undergo refraction as they pass from air to glass and then from glass to air.

Perception of Raised Letters:
- Due to refraction, the light rays coming from the printed metal letters change their direction as they pass through the glass slab. This change in direction creates an optical illusion where the letters appear to be raised when viewed through the glass slab.

Explanation Using Ray Diagram:
- A ray diagram can be used to illustrate how the light rays bend when passing through the glass slab. The diagram shows that the light rays converge towards the normal when entering the glass slab and diverge away from the normal when exiting the glass slab, creating the perception of raised letters.

Conclusion:
- In conclusion, the apparent raising of the printed metal letters when viewed through a thick glass slab is a result of the refraction of light as it passes through the different mediums. This phenomenon can be explained by the change in speed and direction of light rays, leading to the optical illusion of raised letters.