Physics 1.5 JaskiratK

Physics - P1.5Waves

A wave is something that transfers energy from one point to another. There are two types of waves:Transverse and longitudinal waves.

What Are Waves?

• Transverse Waves -Most of the waves that you should know about are transverse waves. With this type of wave, the particles of the medium vibrate at right angles to the direction that the energy travels. This is where the name transverse comes from - it means 'across'. All of the electromagnetic waves are transverse waves, as are water waves.

• Longitudinal Waves -With sound waves, the energy travels along in the same direction as the particles vibrate. This type of wave is known as a longitudinal wave, so named because the energy travels along the direction of vibration of the particles.

These words are important as they can tell you many things about a wave.

Waves Definition

• Wavelength - The distance from a

point to another point where the wave begins

to repeat itself. This could be the distance from peak to peak or

trough to trough. Wavelength is

measured in metres (m) and has the

symbol λ.

• Amplitude -The amplitude of a

wave is half the distance from peak to trough. It can also be

thought of as the height of the wave from the

rest position (rest position = in the middle when the wave is not moving up or down).

Amplitude is measured in metres (m).

Amplitude is the letter “a” in the diagram

below.

• WaveSpeed -The distance covered

by a wave in 1 second. Speed is

measured in metres per second (m/s or

ms-1).

• Frequency -This is the number of waves produced in 1 second by the source producing the wave.

Frequency is measured in Hertz (Hz) and has

the symbol f..• Period-

The period of a wave is the time taken for 1 wave to be produced. It is also

the time taken for one whole wave to pass a

point. Period is measured in seconds.

Energy TransferWaves are vibrations that transfer energy from place to place without mater (solid, liquid or gas) being transferred. Think of a Mexican wave in a football crowd – the wave moves around the stadium, while

each spectator stays in their seat, only moving up the down when its their turn.

Some waves must travel through a substance. The substance is known as the medium and it can be solid, liquid or gas. Sound waves and seismic waves are like this. They must travel through a medium,

and it is the medium that vibrates as the waves travel through.

Other waves do not need to travel through a substance. They may be able to travel through a medium, but they do not have to. Visible light, infrared rays microwaves, and other types of

electromagnetic radiation, are like this. They can travel through empty space. Electrical and magnetic fields vibrate as the waves travel.

Wave SpeedThe speed of a wave is related to its frequency and wavelength, according to this equation:

v = f x λ• v = Wave speed in metres per second, m/s.

• f = Frequency in hertz, Hz.• λ (Lambda) = Wavelength in metres, m.

v

f λ

v

f λ

v

f λ

v = f x λ

f = v / λ

λ = v / f

Frequency & Time PeriodThe frequency of a wave can be calculated using this equation:

Frequency = f =

Where:• f = Number of waves produced by a source per second, measured in Hertz, Hz.

• T = Time it takes for one complete oscillation, measured in seconds, S.

All waves, including sound waves and electromagnetic waves, following this equation. For example, a wave with a time period of 2 seconds has a frequency of 1 ÷ 2 = 0.5 Hz.

Light & SoundLight travels as transverse waves and can travel through a vacuum. Sound travels as longitudinal

waves and needs to travel through a solid, liquid or gas: It cannot travel through a vacuum.

Light and sound can be reflected and refracted, just like water waves. Light and sound can also be diffracted, just like water waves, but diffraction in light is less obvious than sound.

Light and sound both travel as waves, but they are not identical. This table summaries the similarities and differences between them.

Property Light Sound

Type Of Wave Transverse Longitudinal

Travel Through Vacuum Yes No (Only Solid, Liquid & Gas)

Reflected Yes Yes

Refracted Yes Yes

Diffracted Yes Yes

Interfere Yes Yes

Go Round Corners No Yes

ReflectionSound waves and light waves reflect from surfaces. Remember that they behave just like water waves

in a ripple tank. This is called the ‘Law Of Reflection’.You can investigate the law of reflection using a light box of 36°, it will be reflected at the same angle

of 36°.The Angle Of Incidence = The Angle Of Reflection

An incident ray of light hits a plane mirror at an angle and is reflected back off it. Both angles are measured from the normal. The normal is

an imaginary line at right angles to the plane mirror.

Smooth surfaces produces strong echoes then sound waves hit them, and they can act as mirrors when light waves hit them. The waves are

reflected uniformly and light can from images.

The waves can:- Be focused to a point, e.g

sunlight reflected off a concave telescope mirror.

- Appear to come from a point behind the mirror, e.g looking at

glass.Rough surfaces scatter sound and

light in all directions. However, each tiny bit of the surface still

follows the rule:The Angle Of Incidence = The

Angle Of Reflection

Ray DiagramsIn a ray diagram, the mirror is drawn a straight line with thick hatchings to show which side has the

reflective coating. The light rays are drawn as solid straight lines, each with an arrowhead to show the direction of travel. Light rays that appear to come from behind the mirror are shown as dashed straight

lines.

Make sure that the incident rays (the solid lines) obey the law of reflections: The Angle Of Incidence = The Angle Of Reflection. Extend two lines behind the mirror. They cross where the image appears to

come from.The image in a plane mirror is:

- Virtual (It cannot be touched or projected onto a screen).- Upright (If you stand in front of a mirror, you look the right way up).

- Laterally Inverted (If you stand in front of a mirror, your left side seems to be on the right in the reflection).

RefractionSound waves and light waves change speed when they pass across the boundary between two

substances with different densities, such as air and glass. This cause them to change direction and this effect is called refraction. We can use water waves in a ripple tank to show this effect.

Refraction doesn’t happen if the waves cross the boundary at an angle of 90° (called the normal) – in this case, they carry straight on. The refraction

follows a regular pattern.

When light passes from air into semi-circular blocks or Perspex, as seen as the next page.

The light enters the curved face of the block directly, so no refraction is seen here. As you

increase the angle of incidence you see a greater angles of reflection.

At a specific angle, the light ray will no longer leave the block. At this point the angle of

incidence is called the critical angle. Any further increase in the angle of incidence will mean the

ray is reflected, not refracted.

When white light passes from air into a triangular prism, it is refracted as it enters, and then again as it exits. As it leaves the prism, the different

wavelengths of the individual colours of light result in different angles of refraction.

This splits white light into the seven colours of the rainbow. This process is called dispersion. Red light is refracted the least and violet is refracted

the most. Each colour of light can be called ‘Monochromatic’.

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2.

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DiffractionWhen waves meet a gap in a barrier, they carry on through the gap. However, the waves spread out to

some extent into the area beyond the gap. This is diffraction.

The extent of the spreading depends on how the width of the gap compares to the wavelength of the waves. Significant diffraction only happens when the wavelength is of the same order of

magnitude as the gap.

So, for example:- A gap much larger than the wavelength causes

little spreading and sharp shadow, e.g light through a doorway.

- A gap similar to the wavelength causes a lot of spreading with no sharp shadow, e.g sound

through a doorway.

Diffraction can sometimes be seen in waves in the sea when they pas into a harbour opening as

shown in the diagrams. The wavelength in these diagrams is represented as the distance between

the blue vertical lines: Diffraction through a narrow gap.

Diffraction through a wide gap.

Measuring WavesWaves can be measured in two different ways:

- Wavelength = Peak to Peak.- Amplitude = Centre to Top

Frequency represents the number of waves per second.- Higher the Frequency = Shorter Wavelength- Lower the Frequency = Longer Wavelength

WaveSpeed Equation:Frequency (Hz) x Wavelength (m) = WaveSpeed (m/s)

Frequency Equation:WaveSpeed (m/s) ÷ Wavelength (m) = Frequency (Hz)

Wavelength Equation:WaveSpeed (m/s) ÷ Frequency (Hz) = Wavelength (m)

Frequency (Hertz):- 1mHz = 0.001 Hz

- 1Hz = 1 Hz- 1KHz = 1,000 Hz

- 1MHz = 1,000,000 Hz

Oscilloscopes An oscilloscope is a machine that shows the wave shape of an electrical signal. When connected to a microphone they can show the wave shape of sounds. These diagrams show oscilloscope traces of

three sounds:

Sounds 1 & 2:- The sound waves have the same frequency, so

the sounds have the same pitch.- Sound 3 has a greater amplitude than sound 1,

so sound 2 is louder.

Sounds 2 & 3:- The sound waves have the same amplitude, so

the sounds have the same loudness.- Sound 3 has a greater frequency than sound 2,

so sound 3 is higher pitched.

The frequency of a sound is the number od oscillations (waves) per second as is measured in hertz (Hz). It can be calculated by:

𝐹𝑟𝑒𝑞𝑢𝑒𝑛𝑐𝑦=1

𝑇𝑖𝑚𝑒𝑃𝑒𝑟𝑖𝑜𝑑The time period is the time taken to complete one oscillation and is measured in seconds. So if the

time period of a sound wave is 0.01 seconds, then the frequency is calculated as;Frequency = 1 ÷ 0.01 = 100 Hertz

This would make a low pitched sound (Low Frequency) as this is towards the bottom of our

audible range.

Big Amplitude = Big VolumeHigh Frequency = High Pitch

Amplitude has NO impact on pitch.Frequency has NO impact on volume

Using Light- Largest Waves = Radio Waves

- Shortest Waves = Gamma Rays- Largest Wavelength = Lower

Energy- Shortest Wavelength = Higher

Energy

Red-ShiftMoving Away (Stretched Out):

- Longer Wavelength - Lower Frequency

- Lower Pitch

Moving Towards (Squashed Together):- Shorter Wavelength- Shorter Frequency

-Higher Pitch

The position of the lines have changed because of the Doppler effect. Their wavelengths have increased (and their frequencies have decreased.

Astronomers have found that the further from us a star is, the more its light is red-shifted. This tells us that distant galaxies are moving away from us, and the further away a galaxy is, the

faster it’s moving away.Since we cannot assume that we have a special place in the Universe, this is evidence for a

generally expanding universe. It suggests that everything is moving away from everything else.

Red Shift(ed):Moving away, at the speed of light, lower pitch.

Blue Shift(ed):Moving towards, at the speed of light, higher pitch.

The Big Bang TheoryScientists have gathered a lot of evidence and information about the Universe. They have used their observations to develop a theory call the Big Bang. The theory states that about 13.7 billion years ago all the matter in the Universe was concentrated into a single incredibly tiny point. This

began to enlarge rapidly in a hot explosion, and it is still expanding today.

Evidence for the Big Bang theory:- All the galaxies are moving away from us.

- The further away the galaxy is, the faster it’s moving away.

Scientists have also detected a cosmic microwave background

radiation or CMBR. This is received from all parts of the Universe and is thought to be the heat left over from

the original explosion.

These two features are found in explosions – the fastest moving

objects end up furthest away from the explosion.