What do you see?
Notice that in the center of the box there is a glowing vertical filament.
Light spreads out in straight lines radially from this filament.
The box breaks up the spreading light into rays by passing it through a metal comb.
The light also spreads and passes through colored filters.
You can place various mirrors and lenses into these spreading rays of light.
Then, observe the way the light beams reflect or bend when they interact with the mirrors and lenses.
When you place a concave mirror into the spreading beams as shown for the red light in the above image the light reflects off the mirror and comes back together to make an image of the filament. The spreading light comes back together.
The place where the light comes back together is the location of the image of the filament.
Hold a white piece of cardstock at the position of the image.
The place where the light comes back together is not the focal point of the mirror, it is the image point. The focal point is the place where a parallel beam of light comes back together on the axis of the mirror. (If the mirror were a wheel the axis would be the axle of the wheel.)
An eye placed into the spreading beam of light beyond the image receives the light and brings it together once again on the retina. The eye and brain then see the image located where the light comes back together.
Light spreads out from one corner of the heater bounces off the mirror and comes back together at a corner of the image.
The light continues on spreading once again and coming into an eye. The eye and brain sees the light spreading from a point in space and sees the image at that point.
Scientific Explorations with Paul Doherty
30 Oct 2006