Formula for Apparent Depth: When an object inside a denser medium (refractive index $\mu$) is viewed from a rarer medium (air), the relationship between real depth ($d_{real}$) and apparent depth ($d_{app}$) is given by: $d_{real} = \mu \times d_{app}$

Calculation from Surface 1: Apparent depth, $d_{app1} = 5\text{ cm}$. Real depth from surface 1, $x = \mu \times d_{app1} = 1.5 \times 5 = 7.5\text{ cm}$.

Calculation from Surface 2: Apparent depth, $d_{app2} = 3\text{ cm}$. Real depth from surface 2, $y = \mu \times d_{app2} = 1.5 \times 3 = 4.5\text{ cm}$.

Total Thickness: The total thickness of the slab ($t$) is the sum of the real depths of the bubble from both surfaces. $t = x + y = 7.5\text{ cm} + 4.5\text{ cm} = 12\text{ cm}$.