The World Communicates – Focus 4

Weekly Reading Chapter 4 – Sections 4.1 and 4.2
(Reflection and Refraction)

Class Quiz on Friday 20th on outcomes
21 – 29. For study you could complete Chapter 3 Review
Questions.

Homework Check – Monday next week
 Detecting the Bands
 Electromagnetic Waves – Wrap Up
 Reflection Prac

When EM waves (including light) interact with matter several things can
happen, the waves maybe:





Transmitted
Scattered
Reflected
Refracted
Absorbed
30. describe and apply the law of reflection and explain the effect of reflection from a plane surface on waves
The ‘normal’ is shown in red
and is perpendicular to the
surface at the point of
reflection.

The “reflections” we are used to seeing occur
off plane highly regular surfaces like a mirror.
These are called specular reflections.

Most objects do not have a
perfect surface that reflects
light uniformly. Most
objects when examined
closely have irregular
surfaces. Light is still
reflected bit in a nonuniform way. This enables
us to see the surface but not
specific images that are
being reflected.

This is called Diffuse
Reflection.
The Law of Reflection is still
obeyed for each ray but
because of the uneven
surface the normals at each
point on the surface are not
parallel.


Concave mirrors reflect waves converging them at
a focus in front of the mirror. Also known as a
converging mirror.
Some terminology:
 The focus is the point where all rays are
concentrated after reflection from a converging
mirror.

If a concave mirror is thought of as being a slice of a
sphere, then there would be a line passing through
the centre of the sphere and attaching to the
mirror in the exact centre of the mirror. This line is
known as the principal axis.

The focal length of the mirror is the distance from
the centre of the mirror to the focus

Concave mirrors produce a
magnified image when the object
being reflected is close to the
mirror

Concave mirrors are often used as
make-up or shaving mirrors so that
details of the face can be seen
more clearly on the magnified
image produced by the concave
mirror.

The photo on the left shows a
concave mirror being used to ignite
a block of wood. Why is careful
placement of the wood required to
make this work?

If a light source is placed in the
focus of a concave reflector the
light rays reflected of the mirror
will be parallel creating a beam of
light.

These dishes collect weak radio
waves from Space.

The reflecting dish collects the
waves and reflects them to the focus
where the detector is located. This
strengthens the weak signal.

Again parabolic reflectors are
used in the transmission of
microwaves.

Convex Mirrors spread light out, they reflect
light in such a way that it appears that the
reflected light is diverging out from a point
behind the mirror. Hence they are known as
diverging mirrors.

The focus is the point behind the mirror
from which the reflected rays appear to
diverge.

Used in safety mirrors – they give a wider field of
view and allow viewers to see around corners.

Reflection of radio waves from the ionosphere can allow radio waves to
be transmitted long distances around the globe.

Weekly Reading Chapter 4 – Sections 4.1 and 4.2
(Reflection and Refraction)

Class Quiz on Friday 20th on outcomes
21 – 29. For study you could complete Chapter 3 Review
Questions.

Homework Check – Tomorrow
 Detecting the Bands
 Electromagnetic Waves – Wrap Up
 Reflection Prac
Notification will be issued later this week for
your first Assessment Task in Week 10.

http://www.brainpop.com/science/energy/refractionanddiffraction/

Refraction is the bending of waves as
they pass from one medium to
another.

It occurs when the waves are incident
on an interface at an angle except for
the normal.

Refraction is caused by the change in
speed of the waves as they cross the
interface.

The density of the medium
determined the speed of the waves. As
density increases, speed decreases.



We have learnt that the speed of
light and all other
electromagnetic waves is very
fast – 3.0 x 108 ms-1
This is the speed of EM waves in
a vacuum.
When EM waves pass through
other mediums such as air, water
and glass they slow down very
slightly as shown in the table.
Medium
Speed of EM waves
Vacuum
3 x 108
Air
2.999 x 108
Water
2.26 x 108
Crown Glass
1.97 x 108
Perspex
2 x 108
Diamond
1.24 x 108

Snell’s law provides a
mathematical relationship
between the angle of incidence
and angle of refraction of waves
crossing an interface.
sin i v1

sin r v2

Where
 i = angle of incidence
 r = angle of refraction
 v1 = velocity of wave in medium 1
 v2 = velocity of wave in medium 2
A ray of light enters water from air at
an angle of incidence of 40.
1.
i.
ii.
2.
Draw a ray diagram (to scale) to show the path
of the ray through the water.
Are the rays refracted towards or away from
the normal.
Light passes from a diamond into the
air. If the angle of incidence of the light
on the boundary was 15. Determine
the angle of refraction.
Medium
Speed of EM waves
Vacuum
3 x 108
Air
2.999 x 108
Water
2.26 x 108
Crown Glass
1.97 x 108
Perspex
2 x 108
Diamond
1.24 x 108

The refractive index of a material is a measure of the velocity of EM waves
in that medium compared to a vacuum.

The absolute refractive index of a medium is determined by the following
equation:
v
n1 
Medium
Vacuum
vac
v1
Speed of EM waves
3 x 108
Air
2.999 x 108
Water
2.26 x 108
Crown Glass
1.97 x 108
Perspex
2 x 108
Diamond
1.24 x 108
Absolute Refractive Index

What does it actually mean:
 Let’s consider diamond – its refractive index is 2.42 this means that light
travels 2.42 times faster in a vacuum compared to diamond.
 For Perspex light travels 1.46 times faster in a vacuum compared to in persex
and so on.....

Question: What is the unit for refractive index?
Medium
Speed of EM waves
Absolute Refractive Index
3 x 108
1.0000
Air
2.999 x 108
1.00028
Water
2.26 x 108
1.46
Crown Glass
1.97 x 108
1.52
Perspex
2 x 108
1.46
Diamond
1.24 x 108
2.42
Vacuum

Snell’s Law can be re-written using refractive index.
sin i v1 n2
 
sin r v2 n1

Light rays travelling through air (n=1.00) strike glass at an incident angle
of 45. The angle of refraction is 25. Determine the refractive index of
the glass.

Identify the conditions necessary for a wave to be refracted:
 Towards the normal
 Away from the normal
 At an angle of zero degrees.

Yes - A difficult concept – requires
lots of practice!

Next lesson – Refraction Prac +
more practice questions

Sample Data*:

* - thank you Nicole and Melanie
Angle of incidence ()
Angle of refraction ()
28
20
35
23
48
32
57
37
Angle of
incidence ()
28
35
48
57
Sin i
0.4695
0.5736
0.7431
0.8387
Angle of
refraction ()
20
23
32
37
Sin r
0.3420
0.3907
0.5299
0.6018
A graph of sin r (y-axis) vs sin i (x axis) should be a straight line
whose gradient = n1 /n2
sin r vs sin i
0.70
y = 0.7109x
R² = 0.991
0.60
sin r
0.50
0.40
0.30
0.20
0.10
0.00
0.00
0.10
0.20
0.30
0.40
0.50
sin i
0.60
0.70
0.80
0.90

The gradient should be used to calculate the refractive index
of the perspex.
nair
n1
gradient

n2 n perspex



From the equation of the line the gradient = 0.7109
We know that the refractive index of air is 1.00.
Hence:
1.00
0.7109
n perspex 
n perspex
1.00
 1.41
0.7109

– refers to whether the experiment was a fair test,
related to how well all the other variables were controlled.
This is a valid experiment, all other variables are easily
controlled e.g. Same prism used, same light source used,
same measuring devices used etc.
– related to repetition. Questions to ask:
 Was the experiment repeated a number of times and an average
taken?
 How close were the values from each repetition?

For this experiment – Ask yourself did you repeat the whole
experiment several times to obtain a few different values of
refractive index and then average them?
– Two questions to ask yourself here...
 How close was your calculated refractive index to the known refractive
index of perspex.
 Were there any sources of inaccuracy in your experiment and how
could they have been eliminated/reduced. E.g errrors in measurement
of angles due to limitation of measuring equipment or spreading out
of rays of light.

Secondary Information – any
information that you do not find
out from carrying out
experiments. Includes
information from:
 Books (including textbooks)
 Journals (New Scientist,
Cosmos etc)
 The internet
 Teacher

Refers to whether the information is on the
topic that is required.
 Important when using search engines as not all hits will be equally
valid for the research you are carrying out.
 Validity can be assessed by considering whether the information from
the source relates to the hypothesis or the problem you are solving.
 For example the following page shows the results of a google search
for GPS....

A student researching
the physics behind GPS
technology – goggles'
“GPS”. The following
websites are returned.

Identify (as best you
can from the blurbs)
potentially valid and
invalid sites for her
task.

Suggest a method for
improving the hit rate
of potentially valid web
sites?
– Refers to whether the
information from a secondary source is
correct.


Accuracy can be assessed by comparing
the information to other sources.
May also be checked by tracing the
information back to the source.
 E.g. If the information is referring to
experimental work conducted by a researcher
– finding the scientists original published
report of this work could confirm the accuracy
of the secondary source.
refers to whether the
information is from a well recognised source.

When looking for reliable source look for:
 Recognised sources in the field, organisation or
scientists specialising in the area you are researching
e.g an astrophysicist or NASA are likely to be more
reliable sources of information on space exploration
compared to Billy Smith’s Space Blog.
 Recent information – scientific knowledge is updated
rapidly. However take care – cutting edge science may
not be as reliable as the tried and tested science.
 Sources with author details and references to other
reliable sources.
 Consider whether the source may be biased.

Weekly Reading Remainder of Chapter 4

Assessment Task – 1st April