The refraction of light can create many beautiful optical illusions.
Suppose we change the shape of this lens.
Let us now return to our original shape.
Suppose we change the location of the objects.
The image can either be right side up, or be inverted upside down.
A beam of light is composed of particles called photons.
Photons are very different from the particles that matter is made out of.
The speed of light through empty space is the same for all observers.
But light slows down when it tries to pass through certain materials.
One way to view this is by saying that the speed of light has not really changed,
but the photon is constantly being absorbed and re-emitted by the atoms
of the material, and the delays between the absorptions and re-emissions
is what is responsible for the apparent slowing of the light.
Photons, just like everything else in the universe,
have properties both of particles and of waves.
When a wave enters or exits a material at an angle, the left side and
the right side of the wave will cross the boundary at different times.
The greater the angle, the greater the difference in time when
the left side and right side of the wave change their speed.
This causes the wave as a whole to change direction
both when it enters and when it exits the material.
Parallel beams of light passing through this lens
will exit moving away from each other.
If we trace their paths backwards as straight lines, then the location
where these lines meet is what we will call this lens’s focal point.
Parallel beams of light passing through this other type of lens
will exit moving towards each other.
In this case, we call the focal point the place where their paths meet.
We are now ready to understand why this lens can flip an image upside down.
Consider the paths of the light beams
passing through the lens that come from the objects.
The image appears where the light beams appear to be coming from.
If we change the location of the objects,
the size and location of the image changes as shown.
This image will appear to be in front of the lens,
and it will be inverted upside down.
Let’s now move the objects very close to the lens.
The lens is now no longer able to cause the light beams to converge.
Let’s trace the paths of these light beams backwards as straight lines.
The place where these light beams appear to be coming from
is where we will see the image.
In this case, the image appears to be behind the lens,
and it is no longer inverted upside down.
All this has been done by just moving the location of the objects.
But what if we also change the shape of the lens?
As always, the image will appear where the light beams appear to be coming from.
Here, the image appears to be behind the lens, and it is not inverted upside down.
Suppose we now change the location of the objects.
The image will change as shown.
So far, we have only been talking about lenses,
but what about other types of optical illusions caused by refraction?
Suppose we fill this container with water.
Depending on what angle we look inside,
it may appear that there is more than one yellow cube present.
To understand why this is happening,
let us again consider the paths of the light beams.
Light beams travelling away from the object
will change direction when they exit the water.
Let us consider the light beams exiting the container on two different sides.
Let us now trace the paths of these light beams backwards as straight lines.
The place that these light beams appear to be coming from is different
depending on which side of the container we are looking through.
Therefore, the location of the image appears to be
different depending on which side we look through.
For this same reason, if we insert a rectangle into the water tank,
we can see an optical illusion as shown.
So far, we have been assuming that the amount by which the
material slows down the speed of light is the same for all colors.
For some materials, different colors of light can be
slowed down by significantly different amounts.
Here, violet is slowed down the most, and red is slowed down the least.
A beam of light with all the different colors present
is what we see as the color white.
Photons pass right through each other and many
photons can be at the same place at the same time.
Suppose that white light enters a material at an angle.
And suppose that the speed of light through this material is different for the different colors.
The path of each color will bend by a different amount,
and the colors will separate.
The amount by which a beam of light’s path changes
is larger if there is a larger change in its speed.
This is what will happen if a beam of light
enters a material where its speed is much slower.
If beam of light tries to exit such a material at a sufficiently large angle,
it will turn around completely and 100% of the light will be reflected.
With the exception of the second law of thermodynamics,
almost all the laws of physics work the same way backwards in time
as they do forwards in time.
This means that if our beam of light reverses directions,
it will follow the exact same path as before.
Time symmetry also means that in the case of reflection,
the beam of light comes in at the exact same angle that it reflects.
This is because if the two angles were not equal,
it would not work the same way backwards in time as it does forwards in time.
Whenever a beam of light passes a boundary where its speed changes,
at least some of the photons will always be reflected,
even if many of the photons pass through.
What if light did not actually take that path that it does,
but instead went in a straight line between two points?
In this case,
the light would spend more time in the material where its speed is slower,
and it would actually take light more time for it to reach its destination.
The fastest possible way for light to reach its destination
is for it to travel using the path that it actually does.
Any other path would be slower.
The reason for this,
and the reason for all the other phenomena discussed,
is due to the fact that refraction is a characteristic of all wave propagation
in general, where the speed of the wave is different in different materials.
It was once believed that light is a wave of oscillating electric and magnetic fields,
and that light slows down in certain materials because of the interaction of
the electromagnetic wave with the charged particles inside the material.
We now believe that electric and magnetic fields do not actually exist,
and that all the forces attributed to them are due to the fact that
charged particles exchange photons with one another.
Photons, just like everything else in the universe,
have properties both of particles and of waves,
and it is the wave property of light that is responsible for refraction.
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