The pain or burning that you feel when you place your hand in the uv light, is called "heat transfer". This is the gel curing which is basically shrinking while it hardens on the natural nail. The UVA light is attracted to the polymers in the gel which harden as the light penetrates them (i.e., curing).

Why does it burn when I cure my gel nails?

The science behind gel heat spikes is well understood. Jim McConnell, president at gel manufacturer Light Elegance, says, “The heat spike is a result of chemical bonds being formed during the curing process; this is called an exothermic reaction. Every time a bond is formed, heat is given off during the curing process.

Why do my fingers burn when I get my nails done?

The burning sensation that occurs during an acrylic nail application is caused by two things: excessive primer used on nails and is not allowed to dry completely or too much acrylic powder applied on thin nails.