close
The Strange Reflections
Recently, I visited the Keck Observatory on Mauna Kea. The Keck telescopes are 10 meters in diameter. The primary mirror is comprised of 36 hexagonal mirror segments which fit together to make one large mirror.
They have some extra mirror segments in the coating facility. I stood between to mirrors and took a picture.
Notice how the first several reflections all appear smaller than the previous one. However, the last couple of reflections appear larger. The very last reflection, you can only see part of my chin and collar of my shirt!
Why do the several images appear smaller and the last couple appear larger?
Click to show the solution!
Solution
We had several correct answers this month and selected our winner at random from the correct entries received.
The mirrors are not plane mirrors. Each of the two mirrors makes up part of a parabolic mirror with a focal length of 17.5 meters.
The first several reflections appear to get smaller due to the fact that the light is traveling a greater distance every time it refelects between the two mirrors. Therefore, the image appears farther away and smaller.
When you get close to the focal point of a parabolic mirror, however, the situation changes. As you get close to the focal point, the image becomes magnified. Therefore, in the last image, the light has traveled a total distance of close to 17.5 meters resulting in a magnified image!