Explanation:

Flat roofs are commonly used in modern architecture, and they provide an additional usable space to a building. However, flat roofs have the disadvantage of accumulating water, which can cause leakage and damage to the building. To avoid this, an opening is left in the flat roof for lightening purposes, which is called a lantern.

A lantern is a small structure that is placed on top of the flat roof to allow natural light and air to enter the building. It is typically made of glass or translucent material that allows light to pass through. The lantern is designed to be weatherproof and to prevent water from entering the building.

The purpose of the lantern is to provide natural light to the interior of the building, which can reduce the need for artificial lighting during the day. It also allows fresh air to enter the building, which can improve indoor air quality and reduce the need for mechanical ventilation.

Lanterns are commonly used in residential and commercial buildings, and they come in various shapes and sizes. They can be customized to match the style of the building and to meet the specific needs of the occupants.

In summary, the opening left in flat roofs for lightening purposes is called a lantern. It is a small structure that allows natural light and air to enter the building, and it is designed to be weatherproof and prevent water from entering the building.