May one use a magnifying glass or solar cooker/oven to cook his food?
It is initially forbidden to cook food using a magnifying glass or solar cooker due to it possibly being considered Toldos Hachamah. Nonetheless, some Poskim rule that in a case of slight illness one may be lenient to ask a gentile to cook through a magnifying glass.
Cooking on a fire that was created through the magnifying glass: Some Poskim rule it is Biblically forbidden to cook food on a fire that was created using a magnifying glass, just as is prohibited by a man-made fire. Other Poskim, however, rule it is only a Rabbinical prohibition of Toldos Haor.
 Kaf Hachaim 318:46 [regarding making a flame with the glass]; See Maor Hashabbos 1:17 footnote 8 [p. 284]
 Shevisas Hashabbos Mivashel 44
 The reason: As perhaps this is considered Chamah, or perhaps it is considered Toldas Chamah, hence through a gentile one may be lenient in a time of illness.
 Lev Chaim 3:68
 Shaar Zekeinim 2 p. 86