WavesWaves

Six lessons of fun withSix lessons of fun with

sound and lightsound and light

1. Sound Waves1. Sound Waves

Wave MotionWave Motion Physicists think of light and Physicists think of light and

sound as wavessound as waves The idea of waves explains The idea of waves explains

reflection and refractionreflection and refraction Both light and sound can be Both light and sound can be

reflected and refractedreflected and refracted When a wave passes When a wave passes

through the medium, parts through the medium, parts of the medium vibrate of the medium vibrate around equilibriumaround equilibrium

For sound, the medium is a For sound, the medium is a gas or liquidgas or liquid

Sound cannot travel in a Sound cannot travel in a vacuumvacuum

Wave PropertiesWave Properties

Waves have an amplitude, wavelength, Waves have an amplitude, wavelength, frequency and speedfrequency and speed

Put together the worksheet on wave Put together the worksheet on wave propertiesproperties

Sound Wave PatternsSound Wave Patterns

Amplitude = loudness (volume)Amplitude = loudness (volume) Frequency = pitchFrequency = pitch Shape = timbre (quality of note)Shape = timbre (quality of note)

Transients

Starter QuestionStarter Question Find the frequency and amplitude of this soundFind the frequency and amplitude of this sound Volunteer – draw a quieter sound with Volunteer – draw a quieter sound with twicetwice the f the f

-50

-40

-30

-20

-10

0

10

20

30

40

50

0 2 4 6 8 10 12 14 16

Time / ms

So

un

d P

ress

ure

/ d

Ba

UltrasoundUltrasound

• Sound with pitch above the hearing of Sound with pitch above the hearing of humans is called humans is called ultrasound …ultrasound …

• … … just as just as ultraviolet ultraviolet is light with a higher is light with a higher frequency than we can seefrequency than we can see

• Humans can hear from Humans can hear from 20Hz – 20kHz20Hz – 20kHz• As we age we lose the ability to hear high As we age we lose the ability to hear high

frequency notesfrequency notes

UsesUses

The high-speed vibrations caused by The high-speed vibrations caused by ultrasound can be used to ultrasound can be used to cleanclean delicate delicate objects (objects (e.g.e.g. watch mechanisms) watch mechanisms)

Ultrasounds are reflected at the boundary Ultrasounds are reflected at the boundary between materials – so they are used for between materials – so they are used for pre-natal scans pre-natal scans andand industrial quality industrial quality controlcontrol

Video clipVideo clip

ReflectionsReflections

Ultrasound echoes can be used to Ultrasound echoes can be used to determine the position and type of materials determine the position and type of materials presentpresent

Amplitude => type of interfaceAmplitude => type of interface Time => distance to interfaceTime => distance to interface

WorksheetWorksheet

3. Rays and Images3. Rays and Images

Drawing a ray diagramDrawing a ray diagram

Light sources emit rays in all directionsLight sources emit rays in all directions

Drawing a ray diagramDrawing a ray diagram

We are usually only interested in rays in We are usually only interested in rays in some particular directionssome particular directions

We might use a slit to cut out the others, We might use a slit to cut out the others, or we might just ignore themor we might just ignore them

Drawing a ray diagramDrawing a ray diagram

When a ray hits a surface we draw a When a ray hits a surface we draw a normal (‘normal’ = ‘at right-angles’)normal (‘normal’ = ‘at right-angles’)

Mirrors are usually shown like this

Normal shown dashed

Drawing a ray diagramDrawing a ray diagram

Angle of incidence = angle of reflectionAngle of incidence = angle of reflection

Incidence

Reflection

Drawing a ray diagramDrawing a ray diagram

Finding the image in a mirrorFinding the image in a mirror

Drawing a ray diagramDrawing a ray diagram

Head reflects in all directions – choose Head reflects in all directions – choose twotwo

Drawing a ray diagramDrawing a ray diagram

Draw-in the normal(s) for each rayDraw-in the normal(s) for each ray

Drawing a ray diagramDrawing a ray diagram

Draw-in the changed (reflected) raysDraw-in the changed (reflected) rays

Drawing a ray diagramDrawing a ray diagram

Locate image (where changed rays meet)Locate image (where changed rays meet)

So top of headappears here

Drawing a ray diagramDrawing a ray diagram

Can repeat for another point, or just see …Can repeat for another point, or just see …

Image

Same distance behind mirror

ImagesImages

You describe images by their:You describe images by their: Size (Size (magnified, diminishedmagnified, diminished)) Orientation (Orientation (upright, invertedupright, inverted)) Nature (Nature (real, virtualreal, virtual))

A A real real image shows up if you put a screen image shows up if you put a screen in its place (real light rays pass through it)in its place (real light rays pass through it)

Is the mirror image real or virtual?Is the mirror image real or virtual?

PracticalPractical

Form images using the mirrorsForm images using the mirrors Use the three terms to describe the image Use the three terms to describe the image

in each casein each case

Does it make a difference if the object is at Does it make a difference if the object is at different distances from the mirror?different distances from the mirror?

4. Spherical Mirrors4. Spherical Mirrors

StarterStarter

Complete the diagram showing the image Complete the diagram showing the image and describe the nature of the imageand describe the nature of the image

More NormalsMore Normals

You can still draw a normal to a curved You can still draw a normal to a curved boundaryboundary

Concave MirrorsConcave Mirrors Concave = bellies inConcave = bellies in All reflected rays pass through the focusAll reflected rays pass through the focus

Optic AxisFocus

F

Convex MirrorsConvex Mirrors Convex = sticking outConvex = sticking out All reflected rays extend back to the focusAll reflected rays extend back to the focus

Optic AxisFocus

F

Special Rays for MirrorsSpecial Rays for Mirrors

Object

F

Parallel ray

Direct ray

Image: diminished, upright, virtual

Now your turn for a concave mirror(a) object within F, (b) object beyond F

5. Refraction and 5. Refraction and LensesLenses

Starter – construct the imageStarter – construct the image

Object

RefractionRefraction

Light waves travel Light waves travel slowerslower in in denserdenser materials materials

When they change speed When they change speed their their directiondirection changes changes

When they When they slow slow they they move move towardstowards the normal the normal

Glassblock

slower

faster

Ray Diagram 2Ray Diagram 2

Appears shallower

Ray Diagram 3Ray Diagram 3

ROYGBIV

• Different frequencies are refracted by different amounts• “Blue bends best”

Practical WorkPractical Work

Observe refraction in the glass blockObserve refraction in the glass block What special case causes no change in direction?What special case causes no change in direction?

Observe refraction in waterObserve refraction in water Does the penny appear deeper or shallower?Does the penny appear deeper or shallower?

Observe refraction in the prismObserve refraction in the prism What happens to light of different frequencies?What happens to light of different frequencies?

Observe refraction at a curved interfaceObserve refraction at a curved interface What happens to the three rays?What happens to the three rays?

Look through lenses and describe the images Look through lenses and describe the images (as last time)(as last time)

6. Lenses6. Lenses

Starter – complete the ray dig.Starter – complete the ray dig.

Convex LensesConvex Lenses Convex = sticking outConvex = sticking out All refracted rays pass through the focusAll refracted rays pass through the focus

Optic AxisFocus

F

Concave LensesConcave Lenses Concave = bellies inConcave = bellies in All refracted rays extend back to the focusAll refracted rays extend back to the focus

Optic AxisFocus

F

Special rays for lensesSpecial rays for lenses

Image: diminished, upright, virtual

Optic AxisObjectF

Parallel ray

Direct ray

Now your turn for a convex lens(a) object within F, (b) object beyond F, (c) object beyond 2F

ProcessProcess

1.1. Draw lens/mirrorDraw lens/mirror

2.2. Add focus (on both sides if necessary)Add focus (on both sides if necessary)

3.3. Draw direct rayDraw direct ray Goes straight through lensesGoes straight through lenses Reflected like a plane mirror for mirrorsReflected like a plane mirror for mirrors

4.4. Draw parallel rayDraw parallel ray Goes through / extends back to focusGoes through / extends back to focus

5.5. Find image and describe itFind image and describe it