Hands-on Science Carnival 2010 Activity Stations: Light & Optics

Fun with Mirrors

[Shopping List: large concave and convex parabolic mirrors]

Convex Mirror

1. Observe the reflected images of nearby and far away objects in the large convex mirror (the one that looks like the outside of a bowl).

When you look into an ordinary flat mirror, the objects you see appear to be behind the mirror (like you’re looking through a window), a little smaller and farther away. Curved mirrors form very different images however. We call mirrors that curve out convex, and convex mirrors form upright images that appear to be smaller than the original objects (or farther away), because the reflected light rays are diverging or spreading out as they reach your eyes. You might also notice that objects at the edges of the mirror appear smaller and more distorted than those near the center. Convex mirrors allow you to see a much wider area than flat mirrors, so they are often used in stores (look up near the ceiling), blind turns in the road, and rear view mirrors on cars.

Concave Mirror

1. Sit on your knees just behind the line marked on the floor and position your body so that you can see your upside-down head in the center of the mirror. Now touch your nose then slowly reach towards the mirror and wiggle your fingers with that same hand. You should see an upside-down image of your hand reaching back towards you, kind of like a creepy 3-D hologram! If you don’t see it, move your eyes around a little until you do.
2. You can also move up very close to- but do not touch- the mirror. Now what do you see? Is the image still upside down? Is it bigger or smaller?

Please do not touch the mirrors- they are very difficult to clean!

What’s Happening: We call a mirror that is curved like the inside of a bowl a concave mirror, and it can form two very different images. A concave mirror converges and focuses all parallel light rays (such as those from the sun) to a sharp focal point in front of the mirror (just like a convex lens focuses the sun’s rays to a point so you can burn a hole in a piece of paper). The focal point of this mirror is about 30” from its center. If your eyes or the object is closer than the focal point the image appears larger and upright. If both your eyes and the object are well beyond the focal point, the image appears inverted and floating in space in front of the mirror, i.e. it’s actually closer than the mirror, and since you can see it from all angles as you move your eyes, it appears to be 3-D, almost like a hologram. Large concave mirrors are used in space telescopes because they’re much cheaper and easier to make than glass lenses, and collect more light for viewing dim stars and galaxies. You also probably have one in your bathroom- it’s called a magnifying mirror and moms like to use them to see their makeup better- you just never held it far enough away to see the creepy upside-down holographic image instead. Try it some time!