British Airspace has been "focussing on build" European links.

The British needed forests in order to build ships and railways.

A concave lens is placed in the path of convergent rays. The rays are focussed on the axis 15 cm behind the lens. Where would these rays have been focussed in the absence of the lens ? Focal length of the lens is 30 cm.

Concrete has been most widely used as a building material. It is reinforced by rods which make it hard and stronger. However, it has been noticed that some old buildings are showing signs of weakness and cracks. As a student of chemistry how would yu analyse this shocking situation? is there any method to protect these buildings from further damage?