On putting both our hand on the red hand mark and wait for a while we will notice that the left palm feels hotter than the right palm although no light is received on the left palm. The lamps are both of the same wattage and both emits visible light rays as well as invisible heat rays. The glass flask on the right contains water, which allows light to pass through it, but blocks heat rays. Behaving as a spherical lens, it focuses light rays on our palm. The other flask contains a chemical compound, which allows heat rays to pass through while blocking light rays. The spherical shape of its container converges the heat rays on to our palm, and hence we fell hot.