Introduction:
When a diamagnetic material is placed in a magnetic field, the flux density inside the material is slightly less compared to the flux density outside. This phenomenon can be explained by understanding the behavior of diamagnetic materials in a magnetic field.

Explanation:
1. Diamagnetism:
Diamagnetism is a property exhibited by certain materials that causes them to create an induced magnetic field in the opposite direction to an applied magnetic field. Diamagnetic materials have all their electrons paired up, resulting in a net magnetic moment of zero.

2. Magnetic Field Induced:
When a diamagnetic material is placed in a magnetic field, the external magnetic field induces a small magnetic field within the material in the opposite direction. This induced magnetic field weakens the overall magnetic field within the material.

3. Flux Density:
Flux density, also known as magnetic flux density or magnetic field strength, is a measure of the strength of a magnetic field. It is defined as the amount of magnetic flux passing through a unit area perpendicular to the field.

4. Effect on Flux Density:
When a diamagnetic material is placed in a magnetic field, the external magnetic field lines are slightly repelled by the induced magnetic field within the material. This repulsion causes the magnetic field lines to spread out slightly, resulting in a decrease in the flux density inside the material compared to the flux density outside.

5. Visual Representation:
Visually, if we imagine the magnetic field lines as arrows, the arrows outside the material would be closer together, indicating a higher flux density. Inside the material, the arrows would be slightly more spread out, indicating a slightly lower flux density.

6. Quantitative Analysis:
The decrease in flux density inside the material compared to outside is relatively small, as diamagnetic materials have weak magnetic properties. The exact amount of decrease depends on the specific properties of the material and the strength of the applied magnetic field.

Conclusion:
In conclusion, when a diamagnetic material is placed in a magnetic field, the flux density inside the material is slightly less compared to the flux density outside. This is due to the induced magnetic field within the material, which repels the external magnetic field lines and causes a slight decrease in the flux density.