Waves – Topic 4Waves – Topic 4Chapters 26 Chapters 26

Reflection & RefractionReflection & Refraction

ReflectionReflection Law of ReflectionLaw of Reflection

– The angle of The angle of incidence equals incidence equals the angle of the angle of reflectionreflection

The incident and The incident and reflected rays lie in reflected rays lie in the same plane the same plane with the normal.with the normal.

Image positionImage position

You think in You think in straight straight lines!lines!

The image is The image is located located behind the behind the mirror.mirror.

Diffuse vs Specular Diffuse vs Specular ReflectionReflection

Diffuse ReflectionDiffuse Reflection– Light incident upon Light incident upon

a rough surfacea rough surface– Law of reflection Law of reflection

still holds; Normals still holds; Normals are not parallel.are not parallel.

Specular ReflectionSpecular Reflection– Mirror like reflectionMirror like reflection– All Normals are All Normals are

parallelparallel

RefractionRefraction Sudden change in direction of a wave as Sudden change in direction of a wave as

it changes speed.it changes speed.– It must enter obliquely to change direction!It must enter obliquely to change direction!

Which direction does it bend as it slows?Which direction does it bend as it slows?

RefractionRefraction In both cases the speed of the wave has In both cases the speed of the wave has

decreased. This is indicated by the decreased. This is indicated by the decrease in wavelength! decrease in wavelength!

RefractionRefraction

In which medium does light travel In which medium does light travel faster? (glass rod appears bent)faster? (glass rod appears bent)

Speed of lightSpeed of light v is the speed of light v is the speed of light

in the new medium.in the new medium.

c= 3.0 x 10c= 3.0 x 1088 m/sm/s

As the index increases As the index increases the speed decrease.the speed decrease.

Draw a graph for index Draw a graph for index vs. speed.vs. speed.

nn is the absolute index is the absolute index of refraction. This is a of refraction. This is a measure of optical measure of optical density. density. nn is defined is defined as the ratio of the as the ratio of the speed of light in a speed of light in a vacuum to the speed vacuum to the speed of light in a new of light in a new medium.medium.

Relative Index of RefractionRelative Index of Refraction n is the relative index of n is the relative index of

refraction. If air is not refraction. If air is not used, then rememberused, then remember

nnrelrel = n = n22/n/n11

What is the relative index What is the relative index when going when going fromfrom diamond diamond intointo lucite? lucite?

If nIf nrelrel < 1 ; < 1 ; speeds up speeds up

If nIf nrelrel > 1 ; > 1 ; slows down slows down

RefractionRefraction

VVww = 2.26 x 10 = 2.26 x 1088m/sm/s

VVgg = 2.00 x 10 = 2.00 x 1088m/sm/s

Calculate the Calculate the speed of light in speed of light in water and glass.water and glass.

nn(water)(water)=1.33; n=1.33; n(glass)(glass)=1.50; n=1.50; n(air)(air)=1.00=1.00

RefractionRefraction

When a wave slows down it bends closer to the When a wave slows down it bends closer to the normal. {less to more – toward} n2>n1normal. {less to more – toward} n2>n1

When a wave speed up it bends away from the When a wave speed up it bends away from the normal. {BLA – Big ―› Little – Away} n2<n1normal. {BLA – Big ―› Little – Away} n2<n1

nn11- from- from

nn2 2 - into- into

RefractionRefraction

If light rays bend closer to the normal If light rays bend closer to the normal when slowing down, why does the glass when slowing down, why does the glass rod seem to bend away form the normal?rod seem to bend away form the normal?

Apparent DepthApparent Depth

                  

Diverging rays Diverging rays enter your eyes.enter your eyes.

You “think” in You “think” in Straight Lines.Straight Lines.

A virtual image A virtual image appears to come appears to come from point from point yy

R – Real Depth

A – Apparent Depth

Apparent DepthApparent Depth

If the chest is 20 m below the surface If the chest is 20 m below the surface at what depth will the image appear? at what depth will the image appear? Assume nAssume nsea watersea water = 1.34 = 1.34

Snell’s LawSnell’s Law

nn11sinsinθθ11 = n = n22sin sin θθ22

vv11/v/v22 = = λλ1 1 // λλ22

ExampleExampleA monochromatic light ray f= 5.09 x 10A monochromatic light ray f= 5.09 x 1014 14 Hz Hz

is incident on medium X at 55˚. The is incident on medium X at 55˚. The absolute index of refraction for material X absolute index of refraction for material X is 1.66is 1.66

1.1. What is material X?What is material X?2.2. Determine the angle of Determine the angle of

refraction.refraction.3.3. Determine the speed of Determine the speed of

light in medium X.light in medium X.

Ex: SolutionEx: SolutionThe index of 1.66 is Flint GlassThe index of 1.66 is Flint Glass

To find the angle of refraction To find the angle of refraction use Snell’s Law.use Snell’s Law. θθ22= 30˚= 30˚

To find the speed use n=c/v.To find the speed use n=c/v. v = 1.8 x 10v = 1.8 x 108 8 m/sm/s

DispersionDispersion

The breaking up of white light into its The breaking up of white light into its component frequencies.component frequencies.

Refractive & FrequencyRefractive & Frequency

The refractive index The refractive index depends on the depends on the medium & the medium & the frequency of light.frequency of light.

Each frequency Each frequency “color” propagates at “color” propagates at a different speed and a different speed and bends a different bends a different amount.amount.

Critical AnglesCritical Angles

Def: The angle of incidence when the Def: The angle of incidence when the angle of refraction is 90 degrees.angle of refraction is 90 degrees.

Only exist when you have the BLA’sOnly exist when you have the BLA’s– Big to Little AwayBig to Little Away

When a light ray exceeds the critical When a light ray exceeds the critical angle it undergoes total internal angle it undergoes total internal reflection (TIR) reflection (TIR)

Snell’s Law LabSnell’s Law Lab

What factors effect the index of What factors effect the index of refraction of a liquid?refraction of a liquid?

Write up a planning A and B for this Write up a planning A and B for this question. (Last IB Assessment)question. (Last IB Assessment)

Research any procedures you might Research any procedures you might want to use when investigating.want to use when investigating.– I may modify your methods based on I may modify your methods based on

materials we have available.materials we have available.