( Ghost) Ooh!
(Host) Okay. Maybe you're not convinced
that you're looking at a real ghost in this image.
(Host) But what are you looking at?
Let's find out the trick behind this illusion
and a whole lot more, in this segment of
"Physics in Motion" on reflection.
(Ghost) Ooh!
♪♪
Reflection is when a wave bounces off of a surface
when it interacts with a material.
Both light and sound waves reflect.
But in this segment, we're gonna focus on light.
We know this behavior intuitively.
When light hits something, it bounces off.
When light rays hit an object,
they obey what we call the Law of Reflection.
The law states that the angle of the reflected ray
equals the angle of the source ray
when bounced off of a surface.
We call the light rays coming from the source--
say, a laser pointer-- the incident rays.
At the point where an incident ray strikes
the surface of an object, we can draw a line
perpendicular to the surface of the object.
This line is known as a normal line--
the one labeled 'N'.
We measure the angle of incidence
between the incident light ray
and the normal line,
and the angle of reflection
between the reflected light ray
and the normal line.
Remember that according to the Law of Reflection,
the angle of incidence equals the angle of reflection.
Because of this, the normal line
divides the angle between the incident ray
and the reflected ray
into two equal angles.
If I shine this laser pointer
at an angle of 45 degrees from the normal line,
the reflected ray will also be 45 degrees
from the normal line. Makes sense, right?
Now, there are two kinds of reflected light.
The first is called specular reflection, which is when
light is reflected off of a shiny or glossy surface.
It can be a lake, or a metal pan, or a mirror.
Light is coming from a single source
and reflected in a single direction.
If the shiny surface is smooth,
you'll get a pretty good approximation
of the object that the light is bouncing off of
in the reflected image, like this plane mirror.
A plane mirror is your basic flat mirror,
like the one you have in your bathroom.
But what do you notice about the reflection?
The letters are reversed. Or are they?
What we call the left-right reversal of an image
is simply a frame of reference phenomenon.
But what if your shiny surface
that the light rays hit is curved?
Your side mirror in your car is made this way.
Curved into a convex shape so that it reflects more
in a smaller space.
The image form shows a larger space
in a smaller reflection. If the curve is inward
into a concave shape,
you get lots of interesting effects,
including magnifying the object.
So that's specular reflection.
The other kind of reflection is called diffuse reflection,
which happens when light rays hit a rough surface.
That could be wood, cloth, or the leaves of a plant.
The roughness of the material means that each individual ray
hits a surface that has a different orientation.
The normal line at the point of incidence
is different for different rays
because of the irregularities in the surface.
That's why the light rays are reflected
in many different directions.
But these rays still obey the Law of Reflection.
When you have a specular reflection in a mirror,
the shiny kind, you see an image of the object.
But it can be one of two different types.
One is a real image,
which is when the light rays
converge on a surface.
The real image doesn't mean you can touch it,
but it can be projected.
The second kind of image
is called a virtual image.
A virtual image forms in locations
where light converges in the virtual world,
behind the mirror, so to speak,
and the image cannot be projected
onto a screen.
For instance,
my face in the mirror is actually
a virtual image. It may look like
the light is originating from the mirror,
but it isn't.
It's coming from in front of the mirror,
from the bulb.
(ghost) Ooh!
How do you think we created it?
And what kind of image do you think it is?
Let's reveal the mystery.
The image is called a Pepper's Ghost,
after the guy who first projected one successfully
back in 1861.
It's been used in amusement parks
and magic shows ever since.
It may look like a hologram,
but it's completely different.
It works like this. You need two rooms--
one to be the stage that the viewer sees
and the other out of sight.
A pane of glass angled between the viewer and both rooms.
The audience can see the stage, but not the glass.
When the lights in the side room are turned on,
light travels from the ghost to the glass.
Some of the light is reflected by the glass
and reaches the viewer. The viewer sees
rays of light from both the stage and the hidden room
projecting the hidden image.
It's an optical illusion, in other words.
What kind of image do you think it is?
Virtual or real?
If you thought virtual, you were right.
The light is converging beyond the mirrored surface.
Over the next few days, look around you
and see if you can figure out
what's specular and what's diffuse,
what's real and what's virtual.
It'll all become crystal clear.
That's it for this segment of "Physics in Motion".
We'll see you next time.
(announcer) For more practice problems,
lab activities and note-taking guides,
check out the "Physics in Motion" toolkit.