Waves 1

What is a progressive wave?

What is a progressive wave?

• An oscillation that travels through matter (or sometimes a vacuum)

• All progressive waves transfer energy from one place to another but not matter

• Although particles in the matter vibrate, they do NOT move along the wave

Explain why sound is a progressive wave?

When you hear someone talking to you, vibrations travel to your ears. But the air particles do NOT move.

Instead they vibrate in a plane parallel to the direction of energy transfer as the wave passes through the air

What is a transverse wave? Include diagram

The direction of oscillation of the medium are perpendicular to the direction of energy transfer

Give 4 examples of a transverse wave?

• Waves on the surface of water

• Any Electromagnetic wave

• Waves on stretched springs

• S-waves produced in earthquakes

What is a longitudinal wave? Include diagram

The direction of oscillation of the medium are in parallel to the direction of energy transfer

Give 2 examples of a longitudinal wave?

• Sound waves

• P- waves produced in an Earthquake

Longitudinal waves can be described as having regions of compression and rarefaction.

What do these 2 terms mean?

• Compression- Where the medium of particle are closer and there is a momentary decrease in volume of medium

• Rarefaction- Where the medium of particles are further apart and there is a momentary increase in volume of medium

Sketch a displacement- distance of a transverse wave?

Sketch a displacement- distance of a longitudinal wave?

What is meant by displacement?

Distance from the equilibrium position in a particular direction ; a vector, so it can be positive or negative value (m/meters)

What is meant by amplitude?

Maximum displacement from the equilibrium position (can be positive or negative) (m/meters)

What is meant by wavelength?

Minimum distance between 2 points in phase on adjacent waves. The distance from one peak to the next peak or from one compression to the next compression

Define the time period of an oscillation? Include units and symbols

The period of oscillation and the frequency of a wave are reciprocals of each other

f (frequency/ Hz) = 1/ Time period (s)

Define frequency of an oscillation? Include units

The amount of complete waves each second

v (wave speed/ms^-1) =

f (frequency/Hz) X λ (wavelength/m)

How are Time period and Frequency related?

They are reciprocals of each other

State the wave equation. Define all terms and Units

v (wave speed/ms^-1) =

f (frequency/Hz) X λ (wavelength/m)

Derive the wave equation?

1. v= x/t = λ/T

2. v= λ/T = 1/T X λ

3. 1/T = frequency

4. v= F X λ

What is a wave profile?

A graph seeing the displacement of the particles in the wave against the distance along the wave. Can be used to determine wavelength and amplitude

How do the particles displace overtime in waves?

They oscillate from their equilibrium position to a maximum positive position, back to the equilibrium and then to the maximum negative position and then repeat

What is a phase difference? Include diagram

The difference between displacements of particles along a wave or the difference between the displacement of particles on different waves

What are the units of phases difference?

Measured in degrees representing 360° for a complete wave

OR

Measured in radians representing 2π for a complete wave

How can you convert degrees to radians?

° × π/180

How can you convert radians to degrees?

radians × 180/π

What does it mean if 2 particles are in phase?

If particles are oscillating perfectly in step with each other (they both reach maximum positive displacement at the same time)

What does it mean if particles are antiphase?

If particles are oscillating completely out of step of each other (one reaches maximum positive displacement when the other reaches maximum negative displacement)

Phase difference of 180° or π radians

How can phase difference be calculated for 2 points separated by a distance x on a wave?

Phase difference= Φ

Φ = x/λ × 360°

Draw a displacement- time graph with the Time period labelled?

5 = T/2

10= T

15= 3T/2

What piece of scientific equipment can be used to determine the frequency of a wave?

Oscilloscope

What is the reflection of a wave?

When a wave is travelling through a medium and meets a boundary and reflects back through the medium

Sketch a diagram of reflection of a wave? Show incident and reflected wave

Angle of incident= Angle of reflection

What does a ray show?

Rays are drawn 90° to the wave fronts

State the law of reflection?

The angle of incidence= the angle of reflection

(Measured from the normal)

When a wave is reflected, do the wavelengths or frequency change?

The wavelength and frequency do NOT change as it is still travelling through the same medium

What is refraction?

When a wave changes its speed as it passes from one medium to another

Sketch a diagram of refraction?

Why does the speed of a wave change when entering a new medium?

The frequency must remain constant

The wavelength is changing as it is a new medium

Wave speed must change as:

v= f ×λ

Describe what happens if a wave slows down upon entering a different medium?

The wavelength decreases as the frequency must remain constant

Describe what happens if a wave speeds up upon entering a different medium?

The wavelength increases as the frequency must remain constant

Generally, do waves speed up or slow down when entering a denser medium?

Speed up

Generally, do light waves speed up or slow down when entering a denser medium?

Slow down

What happens to water waves when they enter shallower waters?

When water enters shallower water, it slows down and the wavelength gets shorter

What is diffraction? Include a diagram

Diffraction is the spreading out of a wave front as it passes through a gap.

Do the speed, frequency and wavelength change when a wave is diffracted?

Yes

Under what conditions are diffraction most significant?

• When the gap is the same size of the wavelength → Maximum diffraction occurs

• When the gap is much smaller than the wavelength → little diffraction occurs

Explain why we need a smaller gap to observe diffraction in light compared to sound?

Light has a much smaller wavelength compared to sound, so it does not diffract through large area

Diffraction needs the gap to be the same size of the wavelength to be most effective

What does polarisation mean?

Particles oscillate along the direction of the wave (e.g. up and down in the vertical direction) which means it is confined to a single plane

What does plane polarisation mean?

When the plane of oscillation contains the oscillation of the particles and the direction of travel of the wave

Sketch a diagram of plane polarised and unpolarised light

Explain why a longitudinal wave cannot be plane polarised?

The oscillation are always parallel to the direction of energy transfer. Their oscillations are already limited to only one plane (direction of energy transfer)

How can transverse waves become plane polarised?

When transverse waves reflect off a surface they become partially polarised. This means more waves oscillate in one particular plane but the wave is not completely plane polarised

How is intensity defined?

The intensity of a progressive wave is the radiant power passing through a surface per unit area

How is intensity of a wave calculated? Include units and terms

Intensity (Wm^-2) = Power ( W/ Watts) / Cross- sectional area (4πr²)

I= P/A

Explain why the intensity of a wave reduces as it spreads out from a point source?

The radiant power spreads out

Show that for a point source intensity and distance have an inverse square relationship? (diagram)

What happens to Intensity if distance doubles?

The intensity will be quartered

I= 1/ (2)²

What is the relationship between intensity and amplitude?

Intensity is directly proportional to amplitude²

I = k X A²

I/A² = k

(I/A²) 1 = (I/A²) 2

Is an electromagnetic wave transverse or longitudinal?

An EM wave is a transverse wave

What is the unique property of electromagnetic waves that allow energy to be transferred from the Sun to the Earth?

It does not require a medium to transfer energy (It can travel through a vacuum)

What is an EM wave (diagram)?

List the different types of EM waves in order of decreasing wavelength? (Include a range)

• Radio waves= 10^6 → 10^-1

• Microwaves= 10^-1 → 10^-3

• Infrared= 10^-3 → 7×10^-7

• Visible light= 7×10^-9 → 4 × 10^-7

• Ultra violet= 1×10^-7 → 4×10^-7

• X rays= 10^-8 → 10^-10

• Gamma Rays= 10^-13 → 10^-16

How are X- rays and gamma rays classified?

By their origin, not their wavelength

List the key properties of an EM wave?

• They can be reflected, refracted and diffracted

• They can be plane polarised

• They can travel in a vacuum

State the wave equation for EM waves? Define all terms and units

C (speed of light/ 3×10^8) = f (frequency/Hz) X λ (wavelength/m)

What does it mean if light is unpolarised?

The electric fields oscillate in random planes, all at 90 degrees at the direction of energy transfer

What does it mean if light is polarised?

Light only oscillates in one plane (only for transverse waves)

Polarising filters can be used to polarise light, Sketch a diagram to show the basic principle behind a polarising filter?

Describe an experiment that can be performed to show the polarisation of light and describe the expected results?

• Take 2 polaroid filters, place them together and rotate them, you can observe the plane polarisation.

• Unpolarised light passing through the 1st filter becomes polarised, if the 2nd filter is in the same place as the 1st then the light passes through.

• However when it has turned 90 degrees, no light is transmitted and the intensity falls to 0

Describe an experiment that can be performed to show the polarisation of microwaves. Include a diagram

After the metal grill, less microwaves would be transmitted

How is refractive index calculated? Define all terms and units

n (refractive index) = c(speed of light) / v (speed of light through material)

refractive index is a ratio, no units

What does it mean if a material has a refractive index, n equal to 1?

It is the same speed of light through a vacuum

What factor decides how much the angle of refraction is through a material?

The density of material

• More dense- slows down → further away from the normal

• Less dense- speeds up → closer to the normal

From which point do you measure the angle of incidence, and the angle of refraction on a diagram?

From the normal

State the law of refraction? Define all terms and units

n1 X sinθ1 = n2 X sinθ2

n= refractive index

θ= angle between normal and incident ray

What is total internal reflection?

When the angle of incidence is bigger than the critical angle when the light moves from a higher refractive index to a lower one

State 2 conditions required for total internal reflection?

• Angle of incidence must be bigger than critical angle

• The material change must be from a higher refractive index to a lower one

What is the equation for the critical angle?

n (refractive index) = 1 / Sin C(critical angle)

Describe the basic principle of an optic fibre. Include a labelled diagram?

The reflective index changes suddenly between the core and the cladding