A lens only redirects the light, so that all of the
energy that hits it comes together at a spot.
This doesn't change the amount of energy that is
involved.
A lens doesn't make energy -- it only concentrates it.
There is a right place to put a lens relative to a viewing
screen, so that all the light that comes from one place
in a light source ends up one place on the screen.
The sequence of pictures below illustrates this.
a large dim spot
a smaller, brighter spot
A very small, very bright spot
Oops! Too close!
The light source in this sequence was a distant light bulb. When
the light source is closer, the lens needs to be a little further
away from the screen; and instead of a spot of light, you may get
a picture of the light bulb!
Here are two activities about light energy:
Place a strip of aluminum foil or a piece of paper on part of a
thermal sensing sheet. Shine light on the system from a flashlight or
an incandescent light. Compare what happens where the light hits the
thermal sensing sheet, to what happens where it is blocked.
Use a magnifying lens to focus incandescent light on the thermal sensing
sheet. Most interesting is the case where there is an image of the light bulb
(instead of just a dot of light). Don't try this with sunlight! The
thermal sensing sheet won't like it at all!
