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We've all seen this. A mirror that magnifies,
called a concave mirror,
and a mirror that lets us see more of what's around us
called convex.
These are both spherical mirrors,
meaning they have curved surfaces.
Let's look at a diagram of these two kinds of spherical mirrors
and how they affect light rays.
The first one is the concave one,
which looks like this.
The curve is inward, like going into a cave.
See how light rays come together
or are converging?
The second one is convex,
where the mirror curves outward like this.
The rays bounce very differently.
They spread out, or diverge.
We'll look at how these curved surfaces
affect the images we see in a minute,
but first let's talk about how the optics
of spherical mirrors work.
Let's start with the center of curvature.
Think of a spherical mirror as part of a sphere.
The center of curvature is the point in the center
of the imaginary sphere
from which the mirror is cut.
The principal axis is a horizontal line
that connects the center of the spherical mirror
with the center of the imaginary sphere.
By connecting these two points,
the axis serves as a reference line
for any rays of light that hit the mirror.
The focal point of the spherical mirror
is the point in space
where parallel light rays meet
after bouncing off the mirror.
It's equal to half the distance of the center of curvature
from the surface of the mirror.
So that's the center of curvature,
the principal axis, and the focal point.
When light rays hit a reflecting surface,
they can create one of two kinds of images,
depending on the curve of the mirror
and the location of the object.
One kind of image is called a real image,
which is formed when light rays converge
in real space.
Real images can be projected onto a surface.
An example of a real image is what you see on a movie screen.
That's actually created by a spherical lens
rather than a spherical mirror,
but the principle is the same.
You can make a few different kinds of real images
with spherical mirrors,
which is what we're going to do right now.
The image formed will depend on the distance
between the object and the mirror.
I have an LED light, a concave mirror,
and a surface to project the image on.
The LED creates green and blue light,
and the blue light comes from the top half
of the light source.
Right now, the LED is beyond the mirror center
of curvature.
We'll see a real image, and what you'll notice,
it's smaller, and the blue light is at the bottom,
which means the image is upside down, or inverted.
Let's look at why that is.
This is a diagram of what's happening to some of the rays
that are creating the image.
The light doesn't converge at the focal point.
It converges on the other side of the principal axis
between the focal point and the center of curvature,
and that's why the image looks like this.
All real images are inverted because they converge
on the other side of the principal axis.
I'm going to move the light
closer to the center of curvature,
and let's see what happens to the image.
It got bigger, see?
It's the same size as the actual object,
and it's inverted.
That's because the image has moved further away
from the mirror,
so the light is converging
a longer distance from the principal axis.
Now I'm going to move the light
even closer to the mirror,
between the center of curvature and the focal point.
Now, look at the image. It's magnified, right?
Why do you think that is?
It's because the reflected rays converge at a place
even further on the other side of the principal axis.
Remember that these are all real images,
because they can be projected,
and they're formed by light rays
converging in real space.
But what if the light was moved
to the focal point?
What kind of image would you get?
That's kind of a trick question
because the answer is none.
That's because the reflected rays
travel parallel to one another,
and never converge with one another.
An example of this are headlights on a car.
They use a curved mirror to maximize their light.
These mirrors are parabolic, shaped like this,
slightly different than the spherical mirrors
we've been using.
But you can see the same effect.
There's a lamp at the focal point,
which allows all of the light
that strikes the surface of the mirror
to be projected outward. Maximizing the light,
hitting the road in front of the car.
Let's move our light between the focal points
and the mirror.
Now we have another type of image that's been formed
called a virtual image.
A virtual image is one that cannot be projected.
It appears when the light rays converge behind the mirror,
where a real image is formed when light rays converge
on a surface, a virtual image is formed
where they only appear to converge
on the surface of the mirror.
What do you notice about the virtual image?
It's right side up, isn't it?
And, it's magnified.
Virtual images are also formed
by another kind of spherical mirror,
convex mirrors.
In fact, all images produced by convex mirrors
are virtual.
You've seen these in parking decks
and convenience stores.
Let's see what happens when an object is far away
from a convex mirror.
The object's image looks smaller.
As the object moves closer to the mirror,
the image gets bigger,
but not ever as big as the object's actual size.
This is because the reflected rays converge
at a point inside the mirror,
closer to the principal axis.
The side mirrors on our cars are convex.
Like our parking deck mirrors,
they collect a broader field of light
so that our view is expanded.
When we see more of traffic coming up beside us
in our sideview mirrors, it makes the image smaller
than the actual size of the cars beside us,
so it appears farther away,
and that's why they always put
that sticker on the mirrors, saying,
"Objects may be closer than they appear."
So we've talked about real images
which can be formed by concave, spherical mirrors,
and virtual images which can be formed
by either concave or convex mirrors.
Concave and convex mirrors may create very different images,
but they do share certain qualities.
They both follow the Law of Reflection,
which says that the angle of reflection
is equal to the angle of incidence.
What we mean is we're measuring a light ray
from the normal,
which is a line that can be traced out
perpendicular to the surface.
Under that condition,
the angle at which a light ray hits an object
is equal to the angle at which it is reflected.
Does that seem weird for a curved mirror?
It might until you consider each ray of light by itself.
It's hitting a single point on the surface
of the spherical mirror, and bouncing off of it
at the same angle at which it hit.
So we've learned about spherical mirrors,
both concave and convex,
and about real and virtual images
and how they're formed.
Also, how useful they are in our everyday lives.
For a more in-depth look into spherical mirrors,
including the lensmaker and magnification equations,
check out our Closer Looks.
That's it for this segment of "Physics in Motion,"
and we'll see you next time.
(announcer) For more practice problems,
lab activities, and note-taking guides,
check out the "Physics in Motion" toolkit