Many communication technologies use application of reflection and refraction of electromagnetic waves

The World Communicates – Focus 4

Class Notices 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

Interaction of EM waves with matter 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

Reflection of Waves The Law of Reflection:

The angle of incidence is equal to the angle of reflection: i = r

The incident ray, the reflected ray and the normal are all in the same plane.

The ‘normal’ is shown in red and is perpendicular to the surface at the point of reflection.

Reflection from a plane The “reflections” we are used to

seeing occur off plane highly regular surfaces like a mirror. These are called specular reflections.

Diffuse 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 non-uniform 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.

Diffuse Reflections

Reflections

Concave Mirror 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 Reflectors - Applications

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?

Concave Reflectors - Applications

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.

Satellite Dishes 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.

Microwave Transmission Again parabolic

reflectors are used in the transmission of microwaves.

Convex Mirror 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.

Convex Mirrors Applications

Used in safety mirrors – they give a wider field of view and allow viewers to see around corners.

Reflection of waves of the ionosphere Reflection of radio waves from the ionosphere can allow radio

waves to be transmitted long distances around the globe.

For you to do

Class Notices – Week 8 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.

Refraction – Bending the Light! http://www.brainpop.com/science/energy/refractionanddiffr

action/

Refraction 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.

But isn’t the speed of light, c?

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 Snell’s law provides a

mathematical relationship between the angle of incidence and angle of refraction of waves crossing an interface.

Where i = angle of incidence r = angle of refraction v1 = velocity of wave in

medium 1 v2 = velocity of wave in

medium 2

2

1

sinsin

vv

ri

Practice Questions1. A ray of light enters water from

air at an angle of incidence of 40.

i. Draw a ray diagram (to scale) to show the path of the ray through the water.

ii. Are the rays refracted towards or away from the normal.

2. 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

Refractive index 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:

Medium Speed of EM waves Absolute Refractive Index

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

11 vvn vac

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

IndexVacuum 3 x 108 1.0000Air 2.999 x 108 1.00028Water 2.26 x 108 1.46Crown Glass

1.97 x 108 1.52

Perspex 2 x 108 1.46Diamond 1.24 x 108 2.42

Snell’s Law and Refractive Index Snell’s Law can be re-written using refractive index.

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.

1

2

2

1

sinsin

nn

vv

ri

Challenge: Show this mathematically

Snell’s Law Identify the conditions necessary for a wave to be

refracted: Towards the normal Away from the normal At an angle of zero degrees.

Snell’s Law Yes - A difficult concept –

requires lots of practice!

Next lesson – Refraction Prac + more practice questions

Snell’s Law Prac – Some comments Sample Data*: * - thank you Nicole and Melanie

Angle of incidence () Angle of refraction ()28 2035 2348 3257 37

Snell’s Law Prac – Some comments

Angle of incidence

()

Sin i Angle of refraction

()

Sin r

28 0.4695 20 0.342035 0.5736 23 0.390748 0.7431 32 0.529957 0.8387 37 0.6018

A graph of sin r (y-axis) vs sin i (x axis) should be a straight line whose gradient = n1 /n2

Snell’s Law Prac – Some comments

0.45 0.50 0.55 0.60 0.65 0.70 0.75 0.80 0.85 0.900.000.100.200.300.400.500.600.70

f(x) = 0.710874478247294 xR² = 0.999569461509052

sin r vs sin i

sin i

sin

r

Snell’s Law Prac – Some comments The gradient should be used to calculate the

refractive index of the perspex.

From the equation of the line the gradient = 0.7109

We know that the refractive index of air is 1.00. Hence:

perspex

air

nn

nngradient 2

1

perspexn00.17109.0

41.17109.000.1

perspexn

Snell’s Law Prac – the discussion Some confusion re: validity, reliability and

accuracy.

VALIDITY – 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.

Snell’s Law Prac – the discussion RELIABILITY – 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?

Snell’s Law Prac – the discussion ACCURACY – 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.

Validity, Reliability and Accuracy of Secondary Sources 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

Secondary Sources VALIDITY – 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?

Secondary Sources ACCURACY – 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.

Secondary Sources RELIABILITY – 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.

Class Notices – Week 9/10 Weekly Reading Remainder of Chapter 4

Assessment Task – 1st April

Total Internal Reflection