How to Find the Focal Point of a Parabolic Mirror
When visible light (and other forms of electromagnetic radiation) strike the inner surface of a parabolic reflector, every beam is reflected toward a single point: the focal point of the parabola. In this way, satellite dishes are able to focus all of the weaken radio wave signals that strike its surface into a single, powerful signal at the satellite's receiver (which is purposely located at the dish's focal point). Likewise, parabolic mirrors can be used to focus sunlight and generate electricity-producing heat. However, to use a parabolic mirror for this purpose, you must first locate its focal point.
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Things You'll Need
- Flat vacuum suction cup, 0.125-inch fitting diameter
- Shooting marble
- 1/8-inch-wide wooden dowel rod
- Ruler
Instructions
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1
Lay the parabolic mirror on a flat surface, reflector-side up.
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2
Place the shooting marble on the reflective surface of the mirror and release it, letting the marble roll back and forth until it finally comes to rest at the lowest point of the mirror. The purpose of this is to use gravity to find the center of the parabola (which also happens to be its lowest point). The focal is somewhere in space directly above it.
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3
Remove the marble and stick the flat suction cup exactly in its place.
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4
Push the 1/8-inch-wide dowel rod firmly into the suction cup's fitting. Make sure the rod is pointing straight up in the air above the suction cup.
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5
Go outside on a warm, sunny day (at about noon) and point the parabolic mirror directly at the sun. Watch the dowel rod as you experiment with the angle of the mirror. When the angle is right, an intense spot of white light will appear somewhere along the rod.
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6
Continue holding the mirror until you see smoke rising from the white spot on the rod. At this point, turn the mirror away from the sun and carry it back inside. The burnt spot on the dowel rod represents the focal point of the parabola.
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7
Measure the distance from the surface of the mirror to the burned spot on the dowel rod. This is the height of the focal point above the center of the parabola.
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