Lasers, and Illusions
of Light & Vision
In order for us to see an object, light must come from it to our
eyes. Mostly we see light from the sun or other sources that
bounces off of objects. Occasionally, objects emit their own
light. Light travels in straight lines. A laser
produces light, but unless the laser light bounces off of something, or
shines directly into your eyes (a very bad idea, since this can damage
your eyes), you cannot see it. The beam is invisible in
normal air. However, you can see the beam if there is
something in the air, like smoke, that the light can bounce off of.
Mostly when light bounces, it does so in all directions, because the
surface is rough. If the surface is smooth enough, like still
water, glass, polished metal, or a mirror, the light will only bounce
in one direction, like a ball. It will bounce off at the same
angle it hit. If you shine a laser on a mirror, you can only
see the light if you are in the direction it bounces, and then it looks
like the light came from behind the mirror. Unlike a laser
beam, light from a normal object, like a light bulb, will hit
on the mirror, so it is much more likely that some of the light will
bounce your way. It will still look like it came from behind
the mirror. This is what we call a reflection.
A popular magician’s trick is called
“Pepper’s Ghost”. Light will
bounce off of clear flat glass as well as pass through it. If
the light is just right, you can see things reflected in the glass as
well as things on the other side of the glass. If two objects
are placed on opposite sides of the glass so the are right where the
reflected image of the other is, what you will see is both objects
together, apparently in the same place. You can use this to
make an unlit candle appear to be lit, or to even burn under
water. If you start with one of the objects in darkness, and
then shine a light on it, its reflection will
“magically” appear in the glass. This
trick is used many times in the “Haunted Mansion”
in Disneyland and Disney World to make “ghosts”
appear and disappear.
When light goes from one material to another, it bends. A
laser beam that hits water, glass, or plastic will change
direction. Under the water, the beam will look like it came
from somewhere else. Light coming up out of water will also
bend, making it appear to come from somewhere else. This is
why straight sticks will appear to be bent when stuck into water, why
fish appear to be farther from shore, and closer to the surface than
they really are, and why streams do not look as deep as they really
are. If you shine a light straight down at water, most of the
light will go into the water, and some will reflect. The same
thing happens if light is coming out of water into air.
However, if the light inside the water hits the air at larger and
larger angles, more and more of the light is bounced, or reflected,
back into the water. If the angle is large enough, all of the
light is bounced back, and none of it will go into the air.
This also happens with clear plastic, glass, and even Jello.
We can use this to make “light pipes” that can
carry signals in the form of light over very long distances with very
little loss of signal.
Stream Light Pipe
Water with a little Pine-Sol will scatter enough light to make a laser
beam visible. If a laser beam enters a glass bottle just
right so that it leaves through a hole, the laser light will bounce
around inside of the water stream coming out the hole without leaving
the water. You can see the laser beam inside the water if
there is enough Pine-Sol, or if the water stream hits something.
Many colorful toys and lights use light pipes made of glass, called
optical fibers. Fiber optic cables are used to send phone and
tv signals in the form of light over long distances.
Wave a white rod or board back and forth through the beam from a slide
or video projector very fast. It will look like the whole
image or screen even though only a bit of the image that hits the rod
or board can be seen at any instant.
of Vision with Lasers
It takes time for our brains to understand what our eyes see.
Because of that, a fast moving laser beam spot will not look like a
spot at all, but like a line of light. If a laser beam is
moved fast enough, the beam spot on a wall can be made to look like a
circle or some other figure. However, if you took a
photograph of it, and if the camera is fast enough, it will clearly
show just a spot. Movies and TV are just a series of still
pictures flashed quickly on the screen. The flashing is fast
enough so that it looks like smooth motion to us.
An inverted, or inside-out mask of a face looks very strange to
us. If we move past it, or the mask moves by us, it will
appear to turn its head to look at us. This is because of
“paralax”. Just like how distant trees
appear to move by slower than closer trees as we drive through a
forest, the nose of the mask will not appear to move as much, because
it is farther away. The sides of the face on the mask will
appear to move more because they are closer. However, our
brain is very good at seeing faces, even where there are none, such is
in clouds. Your brain is easily tricked into seeing the
inside-out mask as a face, with its nose closer to us and the ears
farther away. The only way your brain can match this with the
relatively slow turning nose and more rapidly turning ears is that the
face must be turning to follow you. This trick is also used
in the “Haunted Mansion” in Disneyland and
Nature of Color
Red objects are red because they reflect red light and absorb, or soak
up, all other colors. Blue objects are blue because they
reflect blue light and absorb all other colors. If you only
shine blue light on a red object, it will appear black, and if you
shine only red light on a blue object it will also appear
black. This also works with the other colors. If
the color of the light shining on an object with many bright colors is
quickly changed, it can look pretty freaky.
If you look very closely (think face-plant) at a the image on a TV or
computer screen, you will see red, green, and blue spots very close
together, especially where the image is white. All colors we
can see can be made by mixing red, green, and blue in the right
combinations. An equal mixture of each will look
white. A white rod in red light will look red, and its shadow
will look black. A white rod in green light will look green,
and its shadow will look black. A white rod in blue light
will look blue, and its shadow will look black. A white rod
in front of a red light and a blue light will look magenta, and it will
have two shadows, one blue, and the other red. A white rod in
front of a red and in front of a green light will look yellow, and it
will have two shadows, one red, and the other green. A white
rod in front of a blue and in front of a green light will look cyan (or
aqua), and it will have two shadows, one blue, and the other
green. If the rod is in front of a red light, a green light,
and a blue light, it will appear white and have three shadows, one
magenta, one yellow, and the third one cyan.
Pure white light and sunlight contain all of the colors of the
rainbow. Because different colors of light bend differently
in different materials, white light can be split up to form a rainbow
by prisms, glass balls, and even special plastic sheets.
Rainbows in the sky are from sunlight bent through and off of millions
and millions of raindrops.
the Rainbow - Infrared Light
There are many colors we cannot see. Beyond violet we have
ultraviolet and beyond red we have infrared. However, most
cameras can see quite well into the infrared, and change it to light we
can see. A camera image of a rainbow clearly shows light
beyond red that we cannot see. It can also see the infrared
light signal from a TV remote.
Glasses made with special plastic sheets can split light up into colors
just like a prism or raindrops. Different sources of light
are fun to look at through rainbow glasses.
A white light bulb gives off all colors, while a white florescent bulb
gives off only five distinct colors.
Light given off or absorbed (soaked up) by a gas is unique, like a
fingerprint. We can tell what elements and chemicals are in a
gas by the colors of light (called a “spectra”) it
gives off. Pure elements give off very clear patterns of
colors. We use this to determine what elements and chemicals
are in far off stars, in the atmosphere of distant planets, and in
giant gas clouds in outer space.
Very few things are cooler than lots of lasers shining through fog and
mist to make pretty patterns, especially if lots of students can help