A postage stamp kept below arectangular glass slab of refractive index `1*5` when viewed from vertically above it, appears to be raised by `7*0mm`. Calculate the thickness of the glass slab.

Answer

To solve the problem of finding the thickness of the glass slab, we can follow these steps:

### Step 1: Understand the Problem
We know that a postage stamp appears to be raised by 7 mm when viewed through a glass slab with a refractive index of 1.5. This apparent raise is due to the refraction of light as it passes through the slab.

### Step 2: Identify the Formula
The normal shift (apparent raise) can be calculated using the formula:
\[ \text{Normal Shift} = t \left(1 - \frac{1}{\mu}\right) \]
where:
- \( t \) is the thickness of the slab,
- \( \mu \) is the refractive index of the slab.

### Step 3: Substitute Known Values
Given:
- Normal Shift = 7 mm,
- Refractive Index \( \mu = 1.5 \).

Substituting these values into the formula:
\[ 7 = t \left(1 - \frac{1}{1.5}\right) \]

### Step 4: Simplify the Equation
Calculate \( 1 - \frac{1}{1.5} \):
\[ 1 - \frac{1}{1.5} = 1 - \frac{2}{3} = \frac{1}{3} \]
Now, substitute this back into the equation:
\[ 7 = t \left(\frac{1}{3}\right) \]

### Step 5: Solve for Thickness \( t \)
To find \( t \), rearrange the equation:
\[ t = 7 \times 3 \]
\[ t = 21 \text{ mm} \]

### Step 6: Convert to Centimeters (if needed)
Since 21 mm can also be expressed in centimeters:
\[ t = 2.1 \text{ cm} \]

### Final Answer
The thickness of the glass slab is:
\[ \text{Thickness} = 21 \text{ mm} \text{ or } 2.1 \text{ cm} \]

---