Here's why:

* Intensity is defined as the power per unit area. In other words, it's how much energy is flowing through a specific area.

* Power remains constant in a beam, assuming no energy losses.

* If you decrease the area the power is spread over, the same amount of power is now concentrated in a smaller space. This leads to a higher intensity.

Think of it like this:

Imagine shining a flashlight. The beam of light has a certain power. If you cover part of the lens, the beam becomes smaller. Even though the total power of the light hasn't changed, the light appears brighter because it's concentrated into a smaller area.

Formula:

The relationship between intensity (I), power (P), and area (A) is:

I = P / A

This formula clearly shows that as the area (A) decreases, the intensity (I) increases, assuming the power (P) remains constant.