what are the compression forces in tempered glass? is it the atmosphere pressure compressing th surface?

Question

how does it give strength?

Answer 1

No it is not atmosphere.
Tempered glass is made by taking a sheet of ordinary glass and heating it to about the annealing point which a specified temperature at which the molecules can still move to reduce strain but the glass is pretty stiff. Then it is blasted with a lot of cold air from both sides. The outer surface contracts very quickly while the still warm inside cools to a stiffer state more slowly and the glass is put under tension and compression in different parts of the thickness.
The pattern of the tension can be seen with Polaroid glasses against the clear sky because daylight is polarized.
The surface of the glass is tougher than regular glass and the overall strength against breakage is higher, but if the surface is scratched or otherwise broken, the whole sheet breaks up into small cubical pieces. This is a safety advantage because the glass has no big sharp points to puncture people. So the glass is specified for the side windows of cars and for shower door panels. (Laminated glass with a plastic layer is used in windshields to keep people inside the car in an accident.)

Answer 2

The suppression of layers while the glass is setting.