Question

An experimenter needs to heat a small sample to 900 K, but the only available oven has a maximum temperature of 600 K. Could the experimenter heat the sample to 900 K by using a large lens to concentrate the radiation from the oven into the sample ?

• A. Yes, if the sample is placed at the focal point of the lens.
• B. No, because it would violate the law of conservation of energy
• C. No, because it would violate the second law of thermodynamics
• D. Yes, if the areas of the front of the oven is at least as much as the area of the front of the sample.

Answer 1

Answer: (A)

Yes, if the sample is placed at the focal point of the lens.

Answer 2

The temperature of the sun is not sufficient of burn a paper. But by focussing by a lens one can concentrate the energy into a small beam. This does not violate the conservation of energy because we are only concentrating the available energy. A lens does not generate energy. The same thing is true when the oven replaces the sun.