Elenco Solar Energy - Manual (Page 12)

11
Experiment no. 19 Additional sunlight
The solar reflector faces the sun. It collects
all the sunlight that falls on it and concentrates
it at the F.P.
What would happen if you reflected some
additional sunlight by means of an ordinary
mirror or even several mirrors?
Can you increase the heat at the F.P.
significantly?
Experiment no. 20 The focal point of your lens
The lens enclosed in your kit is a 20.0 diameter double convex converging lens with
a F.P. of 35mm.
All light rays striking our lens on one side will meet at a single point exactly 35mm on
the other side of the lens. Try it.
Hold your lens over any letter E in this booklet. As you increase the height of the lens
over the letter E, the letter grows and grows until a point is reached where the image
seems to become blurred! This is the focal point. Try to measure whether it really is
35mm.
FOCUSSING ENERGY
A converging lens focuses all the energy received on its surface on a considerably
smaller area.
Our lens has a surface of approximately 300 square millimeters. If we focus our lens
on an area 1 mm square, that area receives 300 times the energy per mm.
If our lens receives from the sun one joule of radiant energy per mm then the focal
point of our lens would receive 300 joules per mm.
This concentration of energy increases the temperature of the body placed at the focal
point. While the total energy has not increased, the effect of the energy is localized,
resulting in higher temperatures at the Focal Point.
Experiment no. 21 Concentrated heat
In a normally lit room, place the bulb of your thermometer at the focal point of the lens.
Note the temperature at the start of the experiment and again after 5 minutes. Was
there any noticeable change?
Now take your lens and thermometer outside into direct sunlight. Again, place the tip
of the thermometer into the focal point of the lens. Note temperature at start of
experiment and see how long it takes your thermometer to reach the top (55
o
C).
Be sure to remove the thermometer at that temperature.
mirror