Sunday, 30 January 2011
At the focal point
Can you find the focal point of the mirror?
Place your hand in front of the mirror and test the warmth reflected from the mirror at different points on the mirror.
The heat radiation is reflected most strongly at the mirror’s focal point. The focal point can be located with the help of both light and heat.
The light and heat radiating from a heat lamp are reflected off from the concave mirror so that the rays intersect close to the focal point. They do not meet at any one exact point, because the rays hitting the mirror are not parallel. The heat from the rays of the Sun can be utilised effectively with the help of large parabolic mirrors, if a high temperature is required at a specific point. The positive aspect of this system is the cost-free energy. The negative side is that the mirrors would have to rotate at the exact same rate as the Sun in order to ensure that the rays would always be reflected at the same point.
Archimedes is told to have lit Roman ships on fire with the help of concave mirrors at the siege of Syracuse. On the basis of practical research, this story has proven to be a myth. On the other hand, solar cookers that work using mirrors do actually work and can be used, for example, at cabins. As development aid, Finland provides solar cookers to many countries in which they are reducing the use of firewood and logging. They also relieve the workload of women responsible for the collection of firewood.