Transcript Triple Refraction_and_Total_Internal_Reflection

Refraction
1. Explain how refraction is the result of a
change of speed
2. Explain what critical angle and T.I.R..
mean
3. Explain how optical fibres use T.I.R.
Refraction
Waves are refracted
when they change
speed e.g. when
they travel from
deep (fast) to
shallow (slow) water.
Wavelength λ
When this happens:
1. Wavelength gets
shorter
2. They change
direction
Watch the simulation in this link for two minutes, then answer the following
questions...
1. Do the waves speed up or slow down as they move from
the light blue medium to the dark blue medium?
They slow down as they move from the light blue to the dark blue medium.
2. Do the waves change direction and what is this called?
Yes, the waves do change direction. This is called refraction.
3. Where do the waves change direction?
The waves change direction at the boundary. Then they travel in straight
lines.
4. What can you say about the wavelength of the waves?
The wavelength is shorter in the dark blue medium.
5. Is some of the wave energy reflected as well as refracted?
Yes, some of the wave energy reflects (bounces) off the boundary as well as
being refracted (changing direction at the boundary.)
1.Place the
block in the
middle of a
sheet of A4
paper.
2.Draw an
outline around
it
3. Place the block in the
middle of a sheet of
A4 paper.
4. Draw an outline
around it
5. Take the block away
6. Find the middle of the
straight edge
The NORMAL
Line
7. Draw a 90º line
through this point –
this is the normal line
8.Put the block back
9.Direct a laser
beam as shown
To Answer:
• Is the beam crossing
from glass to air or air to
glass?
• Is the beam bent away
from or towards the
normal?
The NORMAL
Line
• Is this because it’s
slowing down or
speeding up?
Sand (slow)
Concrete (fast)
Sand (slow)
Concrete (fast)
Sand (slow)
Concrete (fast)
Back to start
Sand (slow)
Concrete (fast)
When waves cross from fast to slow, they are
refracted towards the normal.
10.Direct a laser
beam as shown
To Answer:
• Is the beam crossing
from glass to air or air to
glass?
• Is the beam bent away
from or towards the
normal?
The NORMAL
Line
• Is this because it’s
slowing down or
speeding up?
Sand (slow)
Concrete (fast)
Sand (slow)
Concrete (fast)
Sand (slow)
Concrete (fast)
Back to start
Sand (slow)
Concrete (fast)
When waves cross from slow to fast, they are
refracted away from the normal.
11.Gradually make
the angle of
incidence larger
until the angle of
refraction is 90º
i
Note if there is a
reflected ray
present as well
r
The NORMAL
Line
This is called the critical
angle.
Refraction does not
happen after this
point.
c
The NORMAL
Line
After this, the light is
TOTALLY
INTERNALLY
REFLECTED
(T.I.R.ed)
The beam is T.I.R.ed*
when the incident
angle i is larger than
the critical angle c.
i
The reflected angle r is
always equal to the
incident angle i.
r
The NORMAL
Line
* T.I.R.ed = totally
internally reflected
Copy the 2nd para
and 3 diagrams on
p.158
Summary
c
Air into glass
Glass into Air
Critical Angle
T.I.R.
Light bent
towards the
normal
Light bent away
from the normal
Light is refracted
at 90º
Total Internal
Reflection
Light speeds up
This is the last
angle at which
refraction
happens. (T.I.R.
after this.)
Light slows down
Optical Fibres
is guided by T.I.R . . .
Light in at this end . . .
and comes out here.
Optical fibres are used in communications to carry signals.
(The signals are pulses of laser light)
Optical fibres are used in medicine to look inside the body.
An endoscope is made of a bunch of optical fibres to carry
light into and out of the body.