1.
Investigating REFRACTION of visible light rays
See also
Refraction of waves and scientific model
air-block prism interface, air-water interface, total internal reflection e.g.
in fibre optic cables
Know and
understand that waves can undergo a change of
direction when they are refracted at an interface.
Refraction: The bending of the
light ray at an interface between two media of different density.
Some sketches of light rays passing through
transparent blocks or prisms (glass or Perspex).
Know that light waves are
not refracted if travelling along the normal (diagram 1 below).
1. No refraction when a light
ray strikes a different medium at 90o to the surface ie 'down'
the normal.
The same applies to 3 and 5 for the
central ray in the diagram.
However, do not assume nothing
happens! There are changes in the wavelength and speed of light, but NOT
the frequency of the light rays.
2. Double refraction through a
rectangular glass block at the air/glass interfaces, note that when the ray
emerges back into air its path is parallel to the original incident ray.
3. Refraction of
light rays at the
two surfaces
of a diverging
concave lens.
4.
Refraction of light rays at two
of the surfaces of
a triangular glass or plastic prism.
5.
Refraction of light rays at the two surfaces of a converging concave
lens.
Two examples of light rays bending
when passing from one medium to another
1.
2.
These are two examples of refraction
- light ray direction changes at a boundary between two transparent
mediums of different density - in this case air/glass/water.
1. The light rays from the graph
paper are refracted at the boundary interfaces by both the glass and
water when entering them from the air and exiting into the air.
Note the magnification as the cylindrical glass of
water is acting like a fat convex lens!
2. The pencil in
the water seems to be bent because the emerging light rays from it
are refracted (bent) at
the interface boundary between the water and air.
The end of the pencil under
water, doesn't seem to be in the right place!
A full explanation of this
phenomena is given further down in this section.
Looking in detail at two refractions situations involving visible light
Be aware that when light rays hit a boundary
between two different mediums (two materials, some of the wave energy is reflected, some
transmitted - refracted and some absorbed -
you should be aware of all three possibilities. (See
reflection
page)
I refer to them as refraction A and
refraction B
The speed of light varies with the medium it
is travelling through and this has important consequences for the behaviour of
light when passing through a boundary between two transparent media of different
densities.
Examples of the speed of light in different
materials:
Vacuum (no material substance) and air
(very low density) ~3.00 x 108 m/s.
Glass is ~1.97 108 m/s,
Perspex ~2.01 x 108 m/s, diamond 1.24 x 108 m/s
Waves travel at different speeds in
different materials and this can result in a change of direction as the
waves pass through a boundary from one material to another.
This change in direction at the boundary
between two media is called refraction.
Refraction A: When light waves passing
through a less dense medium, hit a boundary interface (not at 90o
to it), and on entering a
more dense medium, the light waves 'bend towards the normal' ie refraction
occurs.
Refraction of light rays A from a less dense medium to a more dense medium
This happens because on
entering the more dense medium, the light waves are slowed down causing the change
in wave direction at the boundary interface - ray bent towards the normal.
Diagram above, and the left of the diagram below.
Refraction of light
B is discussed later, but it is the opposite situation to refraction of
light A.
Comparing refractions A and B
The above diagram illustrates the scientific model of the wave theory of refraction
of light.
You can think of the
parallel lines as representing a series of points of maximum amplitude
of the light waves (rather like the crests of waves eg think
waves in a ripple tank, waves on the sea or ripples in a pond on throwing a stone in).
Wave theory of refraction
A (light rays-waves passing from a less dense to a more dense medium):
Refraction A happens because the
wavefronts of the light rays are NOT parallel to the interface boundary
so the first section of a wavefront to hit the interface is slowed down
on entering the more dense medium. BUT, the other section of the
wavefront is moving at the original faster speed and is skewed around
producing the change in direction. So it is this decrease in speed that
causes the change in direction, and, in this case, the skewing round
causes the refracted ray to bend towards the normal.
You can also see that in refraction A the wavelength
is decreased as well as the velocity.
The frequency does NOT change.
speed of light = frequency
x
wavelength,
in 'symbolic shorthand'
v = f x λ
(see wave calculations)
If the frequency (f) does not
change, then the velocity (v) is directly proportional to wavelength
(λ).
The bigger the change in speed the
bigger the change in direction - the greater the angle of refraction.
The obvious examples in your school/college
laboratory are the optics experiments you do in passing light rays passing from air into more dense
transparent triangular or rectangular plastic/glass blocks or triangular prisms.
As long as the material is transparent
and more dense than air you get refraction of light, as long as the incident
light rays strike the interface at any angle other than at 90o
(angle of the normal).
You see this effect in ripple tank
experiments when you abruptly go from deeper water to shallower
water the waves will change direction towards the normal.
The
waves slow down in shallower
water and if they hit the shallower water at an angle, refraction
will occur.
The waves slow down in shallower
water because of increased friction with the bottom surface of the ripple tank.
In the ripple tank the refraction of the
water waves has
nothing to do with density, but is caused by increased friction -
increase in the 'drag' effect.
You can observe the change in speed and
wavelength of water waves in a ripple tank by
placing a rectangular plate in to the water at an angle to the waves and you
can see these changes in wavelength and speed. BUT, by using a stroboscope you can show the frequency does not change.
For ripple tank experiments see
Experiments with water waves
in a ripple tank
Extra notes on refraction A - with refraction experiments, and real life
too, you often get reflection too e.g.
Light rays passing from a less dense medium to a more dense transparent
medium.
You ay 2 refracted. You do get
some reflection too, ray 1. You see reflections on water and in shop
windows.
Note again that when light rays hit a
boundary between two mediums, some of the wave energy is reflected, some
transmitted - refracted and some absorbed - you should be aware of all three
possibilities.
Refraction B: When light
waves from a more dense medium, hit a boundary interface (not at 90o
to it), and on entering a
less dense medium, the light waves 'bend away from the normal' i.e. refraction
occurs.
This happens because on
entering the less dense medium, the light waves can speed up causing the change in
wave direction - light rays bent away from the normal.
The obvious examples you see
in optics experiments are light rays emerging from transparent plastic
blocks or triangular and rectangular glass prisms, and passing out into
less dense air.
Refraction
of light rays B from a more dense medium to a less dense medium
Diagram above and right of diagram below. Diagram
refraction A has been previously discussed, but here refraction
B is
the opposite situation to refraction A.
Comparing refractions A and B
The above diagram illustrates the scientific model of the wave theory of refraction.
Wave theory of refraction
B (light rays-waves passing from a more dense to a less dense medium):
Refraction B happens because the
wavefronts of the light rays are NOT parallel to the interface boundary so
the first section of a wavefront to hit the interface is speeded up on
entering the less dense medium. BUT, the other section of the wavefront is
moving at the original slower speed and is skewed around producing the
change in direction. So it is this increase in speed that causes the change
in direction, and, in this case, the skewing round causes the refracted ray
to bend away from the normal.
You can also see that in refraction B the wavelength
has increased as well as the velocity.
The frequency does NOT change.
speed of light
= frequency x wavelength, in 'symbolic shorthand'
v = f x
λ
See
Experiments with water waves
in a ripple tank
If the frequency (f) does not
change, then velocity (v) is directly proportional to wavelength
(λ).
The bigger the change in speed the
bigger the change in direction - the greater the angle of refraction of
the light rays.
You see this effect in ripple tank
experiments when you abruptly go from shallower water to deeper
water the waves will change direction away from the normal.
The
waves speed up in deeper
water and if they hit the deeper water at an angle, refraction will
occur.
The waves speed up in deeper water because
of decreased friction with the bottom surface of the ripple tank.
In this example the refraction has
nothing to do with density, but is refraction caused by decrease in friction -
reduction of the 'drag' effect.
You can observe this in a ripple tank by
placing a rectangular plate in to the water at an angle to the waves and you
can see these changes in wavelength and speed. BUT, by using a stroboscope you can show the frequency does not change.
For ripple tank experiments see
Introduction to Waves - Ripple Tank
Experiments
Extra notes on refraction B - with refraction
experiments, and real life too, you often get reflection too e.g.
Light rays passing from a more dense medium to a less dense transparent
medium.
The concept of
total internal reflection
needs to be introduced here
Note: Ray 2 refracted. You do get
some reflection too, ray 1.
For glass, if the internal angle of
incidence is over 43o you get total internal reflection and
ray 2 doesn't exist.
This particular angle when the refracted
ray travels along the boundary is called the critical angle.
e.g. when the angle of incidence in a
medium such as water, glass or plastic, reaches a certain critical value,
the refracted ray lies along the boundary, having an angle of refraction
of 90-degrees.
This angle of incidence is known as
the critical angle; it is the largest angle of incidence for which
refraction can still occur - even if it seems strange that some of
the ray travels along the boundary!
The diagram below illustrates these
points.
Situation A: The angle of
incidence i1 is less than the critical angle
Most of the light ray is refracted
at the media boundary.
The angle of refraction is >i1 but
<90o (from more to less dense medium).
The refracted ray bends away from the
normal when entering a less dense medium.
Some of the light is internally
reflected - but not totally.
When the angle of incidence is
less than the critical angle, little reflection takes place.
Situation B: The angle of
incidence i2 equals the critical angle
Although some of the light is still
internally reflected, most of the ray is refracted through an angle
of 90o and travels along the boundary.
The angle of refraction is >i2 but
= 90o.
When the angle of incidence is
equal to the critical angle, much more reflection takes place - but
still not total.
Situation C: The angle of
incidence i3 is greater than the critical angle
No refraction takes place and
the ray is totally internally reflected.
When the angle of incidence is
greater than the critical angle,
total internal reflection occurs.
This phenomenon is exploited when
glass fibres (optic fibres) are used to transmit information using
infrared.
Every transparent material has its own
critical angle.
Glass can range from 30o
to 42o, Perspex plastic 42o, diamond 24o
and water 49o.
You
can investigate all of this behaviour with simple ray box experiments with glass blocks.
The property of total internal reflection
is used to transmit information through glass or plastic fibres using
infrared and visible light beams.
The angle of incidence of light/infrared
beam is always greater than the critical angle, so you always get total
internal reflection and no energy is lost to weaken the signal.
This
sort of internal reflection is part of the explanation of the
formation of a
rainbow.
The origin
of the optical illusion when observing an object at an angle in water.
When you observe an object
half in water and half in air e.g. poking a stick into still water, you
see a 'bent' distorted image, because, the light rays from the object
are bent at the air-water interface because of refraction.
If you think
of the actual object at the start of the incident ray, you think the object is
higher up to the right compared to where it actually is - just follow the line back from the
emerging refracted ray.
You are dealing with a 'real' (deeper) and 'apparent'
(shallower) depth - can be a bit disconcerting! and take care when diving into
swimming pools or off rocks at the seaside - the bottom might not be quite where
you think it is!
You can observe both refraction situations
A and B when doing the ray box light experiments with a transparent
rectangular block of glass or Perspex.
You do the experiment on white paper.
(i) Draw around the block with a
pencil.
(ii) Direct the beam of light through
the block at different angles.
(iii) For each angle mark on dots
where the rays enter and leave the glass block and join them up to
complete the ray diagram. Allow e.g. ~5 cm of dots on each side of the
block.
the green dotted vertical lines
are the two normals.
angles 1 and 3 are angles
of incidence
angles 2 and 4 are angles
of refraction
Remember, when a ray
enters a more dense medium (air ==> glass), the ray bends towards
the normal, and on entering a less dense medium (glass ==> air) the
ray bends away from the normal
there maybe a little
reflection of incident rays 1 and 3, but most of the rays are
refracted.
