Concave Mirror Explosion

Two concave mirrors are placed opposite each other. One mirror is equipped with a strong light bulb in its focus point. In the focus point of the other mirror there is a bag of flash cotton.
A couple of seconds after the first lamp is switched on the flash cotton explodes.

How does it work?
The concave mirror reflects the light rays from the light bulb horizontally towards the second mirror, where the light is concentrated on its focus point. Intense heat is produced with enough energy to ignite the flash cotton.

Risk Assessment:
The flash cotton produces a fire ball of maximum 50cm diameter. The apparatus is built out of material of low flammability.