Carra, we have three responses to your question

Think about a lamp with the base of the shade horizontal, and a vertical wall which the beam of light hits. Figure out why this must be a parabola.

Also do a google search under 'parabolic reflectors', and 'conic sections' to find a number of applications related to the geometry of reflection in these shapes.

BTW, often bridges (hanging chains etc.) are not parabolic.

Walter

There is also a note in the Resource Room of Math Central titled Why are satellite dishes parabolic?

Penny

A few ideas...
The path of anything you throw is shaped like a parabola.
A satellite dish is parabolic.
The curve of a spotlight is parabolic.

Stephen La Rocque>