Question

why is a candle been lit off after sometime when covered with a glass

Answer 1

Fire is a chemical reaction that creates light and heat from oxygen and fuel. A litcandle needs to draw oxygen from the air in order to continue burning. If you limit the amount of air available, the candle's flame eventually goes out once it uses up all the oxygen.

Answer 2

In a process of combustion oxygen present in the air is needed. So it's also the type of combustion.
When we cover a candle with a glass ,then the particular amount of oxygen glass takes takes and covers up.when that oxygen is also used then the supply of oxygen is completely stopped. So in this process flame flickers and stop because of no supply of oxygen.

This shows that oxygen is neede for burning....