New trends in physics teaching, v.4; The ... - unesdoc - Unesco

New Trends in Physics Teaching IV
Experiments
Real and virtual images can be obtained.
The focal length can be measured.
By reflecting sunlight onto a blackened soft drinks can of water the rate of heat absorbed can
be discovered. (We boiled the water and made tea.) If a solar pyranometer is available the energy
falling on the mirror can be compared with the energy absorbed by the can in the same time and
hence the efficiency of the system can be calculated.
Note
These experiments can also be carried out with a Fresnel lens removed from an unwanted overhead
projector (figure 5).
Figure 6. Solar cells.
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