Focal Point
Focal Point is the spot where a curved mirror or lens sends all its light together.
A curved bowl lined with foil and held toward the sun acts like a curved mirror. Each part of the bowl bounces a ray of sunlight inward toward the center. All those rays cross at the same spot above the bowl. That spot is the focal point — where all the bounced light meets and the heat is strongest.
Explaining focal point by grade level
A curved mirror can catch sunlight spread over a big area. It bounces all that light to one tiny spot. That spot is called the focal point. It gets so hot you could cook a hot dog there.
Projects that explore focal point
In a solar furnace, every mirror is aimed at the same target — a single spot where all the reflected light arrives together. The more mirrors you aim at that spot, the hotter it becomes. A 4-foot by 8-foot array could create a focal spot 4,000 times brighter than normal sunlight.
A parabolic curve sends all its reflected light to one spot. You can find that spot by attaching posterboard to the opening and looking for the bright area where the light concentrates. Place a skewer through that spot and the sun's focused energy is enough to cook a hot dog.
Both a flat mirror and a parabolic dish redirect sunlight onto a solar cell, but the parabolic shape aims all its reflected light at a single spot on the panel. A flat mirror bounces light toward the cell without that tight focus. As a result, the parabolic reflector produces the highest power output, followed by the flat mirror.