**Solution:**

Given:
- Thickness of the glass slab, h1 = 4 cm
- Thickness of water, h2 = 5 cm
- Refractive index of water, μ2 = 4/3

To find:
- Refractive index of glass, μ1

Let's assume:
- λ is the wavelength of the monochromatic light in vacuum
- n1 is the number of waves in the glass slab
- n2 is the number of waves in the water

**Relationship between wavelength, refractive index, and thickness:**

The relationship between wavelength, refractive index, and thickness is given by the formula:

λ = λ0 / μ

where,
λ0 is the wavelength in vacuum,
λ is the wavelength in the medium,
μ is the refractive index of the medium.

**Number of waves:**

The number of waves in a medium can be calculated using the formula:

n = h / λ

where,
n is the number of waves,
h is the thickness of the medium,
λ is the wavelength in the medium.

**Calculating the wavelength of light in the medium:**

From the given information, we can write the equations for the glass slab and water as:

λ1 = λ0 / μ1
λ2 = λ0 / μ2

Since the same monochromatic light is used in both media, the wavelength in vacuum is the same. Therefore, we can equate the two equations:

λ0 / μ1 = λ0 / μ2

Simplifying the equation, we get:

μ1 = μ2

Therefore, the refractive index of the glass slab is also 4/3.

**Explanation:**

When light passes from one medium to another, its speed and direction change due to the change in the refractive index of the medium. The refractive index determines how much the light bends or refracts when it enters a new medium.

In this question, we are given that the same monochromatic light passes through a glass slab and water. The thickness of the glass slab and water are different, but the number of waves in both media is the same.

Using the relationships between wavelength, refractive index, and thickness, we can determine the refractive index of the glass slab. By equating the wavelength equations for the glass slab and water, we find that the refractive index of the glass slab is also 4/3, the same as that of water.

This means that the glass slab and water have the same ability to bend or refract light. The refractive index of a medium depends on its optical properties, such as the density and speed of light in that medium. Since the light waves pass through both media with the same number of waves, it indicates that the glass slab and water have the same refractive index.

Therefore, the refractive index of the glass slab is 4/3.