Download Refraction

PHYSICS 3
Saturday, 08 July 2017
Refraction
Lesson objectives
Understand what is meant by
refraction.
► Be able to describe the
refraction of light rays at a
rectangular block.
►
Refraction : effects of refraction
Many visual effects are
caused by refraction.
This ruler appears bent
because the light from one
end of the ruler has been
diffracted, but light from the
other end has travelled in a
straight line.
Would the ruler appear
more or less bent if the
water was replaced with
glass?
More bent, because glass is more dense than water.
Animals and human hunters
Animals (including humans) allow for refraction
when hunting fish in water.
The animals do not aim at
the fish (it is just the
refracted image), instead
they aim at a location
where they know from
experience the fish
actually is.
image
actual location
Key Terms
Incident Ray
Refracted Ray
Emergent Ray
Refraction Through A Glass Block
Animation showing refraction at the air/glass
boundary
Along The Normal
If a ray of light crosses the boundary between two
media, along the normal, it will not be deviated from
its path but the speed of the light will still change.
Less Dense To More Dense
If a ray of light moves from a less dense medium, such
as air, to a more dense medium, such as glass, the ray of
light will slow down and be refracted towards the normal.
The angle of incidence is greater than the angle of
refraction.
More Dense To Less Dense
If a ray of light moves from a more dense medium,
such as glass, to a less dense medium, such as air, the
ray of light will speed up and be refracted away from
the normal. The angle of incidence is less than the
angle of refraction.
Total Internal Reflection
Q1
B
Q2 .. internal ..
.. infrared … reflected
.. .. core … dense ..
.. pulses .. thousands .
Q3 a) True
b) False
D
A
C
E
Total Internal Reflection (cont.)
Q4
-----------------------Q5 The critical angle is the angle above
which only total internal reflection of
light occurs and there is no refraction.
1. When light changes direction as it moves
from one medium to another we call this
effect what?
A.
Reflection
B.
Refraction
C.
Diffraction
D.
Total internal reflection

2. What happens to the speed of light as it
moves from air into glass?

A.
Decreases
B.
Increases
C.
No effect
D.
Decreases and increases
3. If a ray of light moves from air to glass
parallel to the normal what happens?

A.
No change in direction
B.
It bends away from the normal
C.
It bends towards the normal
D.
It stops
Refraction : apparent depth
The rays of light from
the coin get bent
[refracted] as they leave
the water.
Your eye assumes they
have travelled in straight
lines.
Your brain forms an image at the place where it thinks
the rays have come from - the coin appears to be
higher than it really is.
Investigating Light
As the ray of light enters the
Perspex block it CHANGES
DIRECTION. When the ray of
light leaves the Perspex block
it CHANGES DIRECTION
AGAIN. This effect is called
REFRACTION.
1. Place a block of Perspex
on a piece of paper and
draw around the block.
2. Shine a ray of light at the
Perspex block and mark it
on the paper.
3. Mark on the path of the
light ray through the glass
block.
4. Mark on the path of the
light ray as it leaves the
block.
5. What do you observe?
Refraction
Incident ray
Normal
As the alight
When
lightray
rayenters
is incident
the
upon a Perspex
Perspex
THE LIGHT
block;RAY
several things
SLOWS
DOWN.
happen.
The light ray also CHANGES
DIRECTION.
This is called REFRACTION.
Refracted ray
Emergent ray
The light ray BENDS
TOWARDS THE NORMAL
inside the block.
When the light ray leaves the
Perspex it SPEEDS UP AGAIN
and is REFRACTED AWAY
FROM THE NORMAL.
Density and Refraction
DENSITY is the property of a material which tells us HOW
MANY PARTICLES are in a CERTAIN VOLUME. Example:
LIQUID WATER is MORE DENSE than GASEOUS WATER.
When a light ray moves from a LESS DENSE MEDIUM to a
MORE DENSE MEDIUM it SLOWS DOWN and
REFRACTS TOWARDS THE NORMAL.
When a light ray moves from a MORE DENSE MEDIUM to
a LESS DENSE MEDIUM it SPEEDS UP and REFRACTS
AWAY FROM THE NORMAL.
Air (Less dense)
Perspex/Glass/Water
(More dense)
Air (Less dense)
Refraction
Refraction is when waves ____ __ or slow down due to
travelling in a different _________. A medium is
something that waves will travel through. When a pen is
placed in water it looks like this:
In this case the light rays are slowed down by the water
and are _____, causing the pen to look odd. The two
mediums in this example are ______ and _______.
Words – speed up, water, air, bent, medium
Using Refraction : Lenses
F
ƒ

Imagine

The
distance


The
A
lens
lens
can

Draw
normal

Use
the
first
parallel
rays
of
between
the
refracts
be
thought
all
the
of
 Draw
normal
lines
where
theato
refraction
rule
light
from
centre
the
rays
as ato
series
aof
point
ofair
lines
[at
90°
to
rays
enter
the
work
out
the
ray
distant
object
lens
and
F
is
called
prisms.
the
the90º
surface]
for
[at
to the
direction.
hitting
the
called
the
focal
principal
focus
each
ray.
surface].
lens.[].
length
[F].
When light enters a more
less dense
medium
dense
medium
[e.g.
[e.g. air],
it bends
away
glass],
it bends
towards
from
the normal.
the
normal.
Complete the Diagrams Below for Light
Entering a Perspex Block
A.
B.
Experiment
► Follow
the instruction sheet which shows
how to position your rectangular block.
► Measure any angles carefully with your
protractor. Always measure from the nearest
normal.
► Once you have your results, complete the
analysis.
Refraction through a glass block:
Wave slows down and bends
towards the normal due to
entering a more dense
medium
Wave speeds up and bends
away from the normal due to
entering a less dense medium
Wave slows down but is
not bent as it enters
along the normal
Along The Normal
If a ray of light crosses the boundary between two
media, along the normal, it will not be deviated from
its path but the speed of the light will still change.
Refraction Through A Glass Block