What exactly is the thermal lens effect?

[Pericle]

Hello, i just wanted to ask if you can explain to me what thermal lens effect is?

I am supposed to answer why laser beam passing through ink or soy sauce creates this effect and i can't find any information on what actually this thermal lens is. I know of course that it is a lens, but why does heat coming from laser beam create it?

 

[jtbell]

When a liquid heats up, what happens to its density?

 

[sophiecentaur]

1. Energy that's absorbed from the beam will cause local heating of the medium it's passing through. That will cause expansion and a lowering of density - that will increase the speed of the light near the centre of the beam which will produce a beam widening effect. The more optically dense the medium, the greater the effect because there is more heating.