Mirror, Mirror on the Wall
Reflections
on Mirrors and Images
Stumble out of bed and
down the hall to the bathroom. Throw some cold water
on your face. Put a dab of toothpaste on the toothbrush
and swish it around in your mouth. OK, the moment
has come. Open your eyes and look in the mirror. Who's
that looking back?
Before reading this article, get a hand mirror ready. Or if you don't have one handy, look at yourself in a wall mirror and make note of how you appear. Ready? Read on.
A mirror is a polished surface that reflects light waves. The mirror that you are holding in your hand or that is hanging on the bathroom wall is called a plane mirror, because it is flat like a geometric plane. The mirror glass should be as smooth as possible, in order not to distort the image. And it should be coated on the back with a highly reflective material, like aluminum or silver, in order to reflect most of the light rays. If the material was not highly reflective, then some of the light rays would pass through the glass, and you would see only a faint image—as you might see when looking through a window at night.
Hold the mirror up in front of you. When the light illuminating your body hits the mirror, it is reflected back. You see your image. If you look closely, you will see that the image appears to be behind the mirror. To see this effect clearly, look at the glass of the mirror — where is your image in relation to the glass surface?
A plane mirror makes an image called a virtual image. Light rays appear to emanate from virtual images, but that is not really the case. A standard "test" for a virtual image is whether or not it can be projected onto a screen. If you placed a screen at the point behind the mirror where the image appears, would you see an image on the screen? By contrast, there are images from which light really emanates, such as an image created by a movie projector lens. A real image can be projected onto a screen.
- You
can explore plane mirrors and their images in
the activity Physics Explorer: Reflections
in a Plane Mirror.
- You
can contrast the two types of images in the activity
Physics Explorer: Real
and Virtual Images.
- If you are having trouble visualizing how light waves are reflected, you may find it helps to study reflection with water waves, since they are visible. See the activity Physics Explorer: Reflection. The "ghost" source in this activity corresponds to the virtual image in a plane mirror.
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Who's the Fairest One
of All Narcissus,
tired from hunting and the heat, lies
down. He stoops to quench his thirst in
the glassy spring. While drinking he beholds
himself reflected in the mirrored pool--and
a new thirst appears: he loves an imagined
body which contains no substance, for
he believes the mirrored image a thing
of life to love...
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More
on Virtual Images
Look again at the schematic above. Rays reflected
from the mirror's surface enter the human eye. The
brain interprets the ray entering the eye as if it
had traveled a straight line. That straight line leads
to the image behind the mirror. The light rays do
not actually pass through the image; it only appears
as if they do.
Here's a little demonstration of how your brain interprets images and what you can do to fool your brain's expectations. Prop a hand mirror up on the desk. When you look at the mirror, the arm in front of the mirror and its reflection should look like two normal, symmetrical arms. Now make two fists and open and close them repeatedly for about 15 seconds. Continue the motion with the hand in front of the mirror, but stop the motion of the hand behind the mirror. Did you feel a sense of surprise, even though you obviously know what was going on? Your brain expected that the virtual image of your hand was a real hand, and therefore your brain felt surprised when the image and the real hand behind the mirror began doing different things.
You may have heard the term "virtual reality." VR is the use of computer modeling and simulations to enable the user to interact with an artificial 3-D environment. For example, architects can use a virtual reality program to have clients "walk through" their house plan. Have you ever experienced a VR environment?
-
Compare
virtual reality to a virtual image, and list the characteristics
that they share.
- Think about how an image is formed in a plane mirror. If you stand in a darkened closet and look at a mirror, why don't you see an image? (Hint: The answer isn't: "Because it's too dark to see anything.")
Through the Looking Glass: Mirror
Images
"Now...I'll tell you all my ideas about Looking-glass House. First, there's the room you can see through the glass — that's just the same as our drawing-room, only the things go the other way. I can see all of it when I get upon a chair — all but the bit just behind the fireplace. Oh! I do so wish I could see that bit! I want so much to know whether they've a fire in the winter: you never can tell, you know, unless our fire smokes, and then smoke comes up in that room too — but that may be only pretence, just to make it look as if they had a fire. Well then, the books are something like our books, only the words go the wrong way: I know that, because I've held up one of our books to the glass, and then they hold up one in the other room."
Alice in Through the Looking-Glass, by Lewis Carroll
Do
you still have your hand mirror? Look into the mirror
and wink your right eye. The image winks back with
its left eye. When you look into a mirror, you are
not seeing yourself as others see you. Your image
is reversed. You can make
a corner reflector to see yourself as others see
you.
- Why does it take more than two mirrors to see
your reflection as others see you directly?
- If you want to paint a self-portrait with the
same orientation as others see you, how should
you organize your mirror(s) and canvas? Can you
offer more than one solution?
- Why is the word AMBULANCE written as a mirror
image on the front of the vehicle but not on its
side?
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It's All Done With Mirrors
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Curved Mirrors and Scopes
Mirrors have many uses besides seeing if you have
chocolate smeared on your mouth. Here are some applications:
- Concave mirrors: These mirrors have reflective
faces that curve inward, and they enlarge the
image, e.g., a makeup mirror magnifies your image.
- Convex mirrors: These mirrors have reflective
faces that curve outward, and they demagnify the
image, e.g., a rearview mirror reduces the image
so you have a wider field of view.
- Telescopes: Astronomical telescopes often use
large mirrors due to their high light-gathering
capability. Sometimes the mirrors are actually
an array of small mirrors that function together
as a large mirror. Read about the Atacama Large
Millimeter Array telescope in the Teaching the
News article, "World's
Largest Telescope to be Built." The Hubble
Space Telescope contains a mirror that is only
2.4 meters (7.9 feet) wide, but its position at
the edge of Earth's atmosphere enables it to obtain
remarkable images. Read more about Hubble in the
Teaching the News article, "A
Hubble View of the Universe."
- Periscopes: Periscopes enable users to view objects
that are not in their direct line of vision, e.g.,
for a submarine captain to view ships on the water's
surface. You can make
a simple periscope to see around corners and over
walls.
- Kaleidoscopes: These favorite childhood toys use multiple mirrors to create beautiful geometric patterns for arbitrary arrangements of colored bits. You can make your own kaleidoscope that reflects your image!