4.4 Waves

What is a progressive wave?

A wave which transfers energy from one place to another with a wave front that travels through the material.

What is a longitudinal wave?

Waves in which the particles oscillate in the same direction as energy propagation, with areas of rarefaction (low pressure) and compression (high pressure).

What is a transverse wave? Give examples.

Waves where particle oscillations are perpendicular to the energy propagation; examples include electromagnetic waves.

Define frequency and give its units.

The number of waves passing through a point per second; units are Hertz (Hz) or s-1.

What is phase difference and its measurements?

The amount one wave lags behind another as a proportion of the wavelength, measured in radians or degrees.

What do the vertical and horizontal axes on an oscilloscope measure?

Vertical divisions measure voltage/amplitude; horizontal divisions measure time.

Define refraction.

Refraction is when a wave bends at a boundary between two materials due to the difference in density, causing it to speed up or slow down.

True or false: A wave can either be refracted or reflected at a boundary, but never both.

False. Most will be refracted, but some will reflect.

True or false: Diffraction is most noticeable when the wavelength is much larger than the gap the wave is travelling through.

False. Most diffraction occurs when the gap and the wavelength are the same size.

What is the difference between a polarised and unpolarised wave?

Polarised waves oscillate along one axis; unpolarised waves can oscillate in any direction perpendicular to the axis of propagation.

How can a ripple tank be used to investigate diffraction?

Create water waves in the tank, vary the size of a gap for them to pass through, and note how the direction of the waves changes.

What happens to the intensity of light passing through a rotating polarisation filter?

It varies from maximum (all light passes) to minimum (no light passes) depending on the alignment of the axes.

True or false: All electromagnetic waves have the same time period.

False. They all travel at the same speed, but their wavelengths, frequencies, and time periods vary.

True or False? The magnetic field and electric field in an electromagnetic wave are parallel to each other.

False. The electric and magnetic fields are at right angles to each other.

List the following in order from highest to lowest frequency: X-rays, Radio, Microwaves, UV, Visible.

Highest: X-rays; UV; Visible; Microwaves; Lowest: Radio.

True or false: Microwaves can be polarised using a metal grid rather than a polarising filter.

True. The wavelength of microwaves is sufficiently large for the grid to act as a polarising filter.

What is the refractive index of a material?

A measure of how fast light travels in a material compared to its speed in a vacuum, found using n = c/v.

What happens when a beam of light hits a boundary between air and glass at varying angles of incidence?

At 0°, all light passes into the material; as the angle increases, light refracts and eventually reflects.

What is the critical angle?

The angle of incidence where light will reflect off a boundary rather than refract into the medium, calculated with sinC = 1/n.

What happens to light at angles greater than the critical angle?

Total internal reflection occurs.

What does it mean for waves to ‘superpose’?

Two waves in the same place combine when one is on top of the other.

Describe an experiment to investigate the principle of superposition using sound.

Use two speakers connected to the same signal generator, walk along a line perpendicular to them to hear alternating loud and quiet points.

Define coherence.

Coherent waves have the same frequency, wavelength, and fixed phase difference.

True or false: Path difference and phase difference are the same thing.

False. They refer to different measurements in wave characteristics.

What is Young's double-slit experiment?

A single source of light directed towards a double slit creates two coherent beams of light that interfere and create a pattern.

Describe the interference pattern created using white light.

A repeating colored spectrum along the screen, with a bright white point directly in front of the slit.

What is the equation relating wavelength to slit spacing and distance to the screen?

λ = ax/D, where λ is wavelength, a is slit spacing, x is fringe spacing, and D is distance to the screen.

What two properties of light can only be explained if it is a wave?

Diffraction and interference.

How do you find the maximum number of fringes through a diffraction grating?

Use nλ = Dsinθ, rearranging for n using θ=90 to find the maximum.

What is a stationary wave?

A stationary wave has alternating fixed patterns of nodes and antinodes, with no energy transfer.

What are the conditions for a stationary wave to be produced?

The waves must be coherent and travel in opposite directions.

Give an example of an experiment to show a stationary wave.

Use an oscillator to pass a wave along a fixed-end string; a stationary wave forms upon reflection.

What is a similarity and difference between stationary and progressive waves?

Similarity: both have wavelength, frequency, amplitude; Difference: stationary waves don’t transmit energy.

How could you use stationary waves in a resonance tube to find the speed of sound?

Create a closed-end pipe in a measuring cylinder, use a tuning fork, and find the length causing resonance.

What are harmonics?

Points where the stationary wave form doesn’t change due to reinforcing waves in each direction.

A stationary wave at its fundamental frequency has how many nodes and antinodes?

Nodes: 2; Antinodes: 1.