Magnifying Visors

How can I calculate the temperature of focal point from magnifying glass?

Assuming I am using the magnifying glass to focusing sunlight. Assuming that this focal point is used to burn/heating up something without knowing any default value of temperature of the object.

Public Comments

1. Beam it onto the bulb of a thermometer.
2. A "point in space" doesn't have a temperature. Whatever object is at that point has a temperature. You can't calculate it easily, because there is a continuous process going on. The glass is adding heat energy (from the sun) to the object, but the object is losing heat by conduction, convection, and radiation. The maximum temperature is when the heat loss = the heat gained. For exmple if you try to heat up a sheet of metal, the metal is good conductor of heat so the focal point won't be much hotter than the surrounding metal. But if you use a material that is a poor conductor of heat (for example wood) the temperature can get hot enough to start a fire. It is fairly easy to get temperatures higher than 1000 C, providing the heat loss is small enough.