Waves - Physics

Transverse waves

Oscillations are perpendicular to the direction of energy transfer

Examples of transverse waves

Electromagnetic and water

Longtitudinal waves

Oscillations parallel to the direction of energy transfer

Example of a longitudinal wave

Sound

Longtitudinal waves

Transverse waves

Wavelength

The distance between 2 identical successive points on a wave such as a peak to the next peak, or a trough to the next trough

Amplitude

The maximum displacement or distance moved by a point on a wave from its undisturbed rest position

Frequency

The number of complete waves passing a fixed point per second, measured in Hz

Mechanical waves

Vibrations that transfer energy through a material medium (solid, liquid or gas) by making particles oscillate

Waves that transfer energy

All transverse and longtitudinal waves

Waves that travel at the speed of light

Specific types of transverse waves like electromagnetic waves

Waves that can be heard

Longtitudinal waves are always heard (unless in a vaccum), but transverse waves are only heard when travelling through a solid

Waves that can travel through a vaccum

Electromagnetic transverse waves can travel through a vaccum, whereas mechanical waves cannot, and longtitudinal waves cannot

Period of a wave

How long it takes for one wave to pass a point

Equation for wavelength

wavelength = wave speed / frequency

Speed of sound in air

343m/s

Speed of light in a vaccum

3×10^8m/s

Echo-ranging

A process used to detect how far beneath the surface the sea bed is from a ship, using the reflection of a sound wave (echo) to determine the distance (range) to an object, such as the seabed.

Why can the equation used to find the distance of the sea bed beneath the surface be written as d = ½ (v x t)?

Because the time (t) measured is the total time for the signal to travel to the seabed and back again, so since the wave travels the distance twice, the desired answer for distance is only ½ of the total distance.

Translucent

A material that allows objects to be seen through them but not as clearly or sharply as a transparent material. eg. tracing material, frosted glass

Transparent

A material that allows objects behind it to be seen as if nothing is in the way, eg. glass, perspex

Opaque

A material that may or may not allow light through to the object behind it, and so it would be hard to determine what object was behind it, eg. wood, paper

Law of reflection

The angle of incidence is equal to the angle of reflection

Specular reflection

Reflection from a smooth surface in a single direction so give a shiny, reflective appearance, eg. from a mirror

Diffuse reflection

Reflection from a rough surface causes scattering which gives the surface a dull or matte appearance.

What type of wave are light waves?

Electromagnetic transverse waves

Virtual image

An image where light rays appear to diverge from, but do not actually pass through

Refraction

The change in direction of a wave, typically light, as it passes from one medium to another

Absorption

The process by which a material takes in energy from a wave, such as radiation, light or sound, causing the energy to be transferred to the material’s internal energy store

Transmission

The process where a wave, such as light, sound, or electromagnetic, passes through a material or medium

What happens when light enters a glass block? Mention density

It refracts because glass is more dense than air

What type of wave is sound?

Longtitudinal

How do we hear things?

Sound waves are funnelled by the pinna down the ear canal down to the ear drum, which cause the ear drum to vibrate, and these are passed via the ossicles (anvil, hammer and stirrup) to the cochlea. Here the vibrations are converted to electrical signals, which pass down the auditory nerve to the brain.

Range of human hearing

20Hz to 20 KHz

Why is the speed of sound faster in solid compared to in air?

In air, the particles are spread out more, and the vibrations are passed on more slowly

Features of ultrasound waves

They have a higher frequency than the upper limit of human hearing, and they are partially reflected when they meet a boundary between 2 different media. The time taken for the reflections to reach a detector, and the speed of the waves, can be used to determine how far away such a boundary is. This allows for ultrasound waves to be used for both medical and industrial imaging.

How are ultrasound waves used in pre natal scans?

If the amplitude is converted into the brightness of an image, we obtain a 2 dimensional image known as a brightness scan. (B scan)

What are seismic waves produced by?

Earthquakes

P-waves

Longtitudinal seismic waves that travel at different speeds through solids and liquids

S-waves

Transverse seismic waves that cannot pass through liquid

Seisometer

Tool used for the detection of P and S waves

State of the inner core

Solid

State of the outer core

Liquid

State of the mantle

Mostly solid, but it can flow

State of the crust

Solid

Why are P waves called primary waves and S waves called secondary waves?

P waves move faster, while S waves move slower

What does a frequency of 440 Hz mean for a note?

The sound oscillates 440 times per minute.

RP method for activity 1: determining the wave speed through a liquid

1. Set up a ripple tank with an oscillating wooden rod connected to a motor in a shallow tank of water with a light above it to illuminate the waves

2. Count the number of waves to pass a given point every 10 seconds

3. Measure the length of 10 waves

4. Change the frequency and retake the measurements

5. Repeat until you have at least 6 sets of results, finding the frequency and wavelength for each

6. Calculate the speed for each result set using the equation: speed = frequency x wavelength

RP method for activity 2: determining the wave speed through a solid

1. Set up a vibration generator connected to a power supply, where a stretched string/elastic cord hangs over a wooden bridge and off the end is connected to a weight.

2. Put on eye protection and turn on the vibration generator

3. Adjust the length of the spring which vibrates by moving the wooden bridge if you are unable to see a stable wave pattern produced by the string

4. Measure the length of 1 wavelength

5. Record the frequency of the vibration generator

6. Calculate the speed, using the equation: speed = frequency x wavelength

7. Change the frequency and repeat twice more

Why, in RP activity 1, do we measure the length of 10 waves to calculate the wavelength, instead of just measuring 1?

It is easier to measure a larger distance more accurately