p6 - waves

Question 1

transverse waves

Answer 1

A transverse wave is one where the direction of vibration is perpendicular to the direction of energy transfer.
e.g. water waves, Electromagnetic waves, S waves

Question 2

longitudinal waves

Answer 2

A longitudinal wave is one where the direction of vibration is parallel to the direction of energy transfer.

e.g. sound waves (in air), p waves

Question 3

Wave properties

Answer 3

For both ripples on a water surface and sound waves in air, it is the wave and not the water or air itself that travels.

Question 4

amplitude

Answer 4

The amplitude of a wave is the maximum displacement of a point on a wave away from its undisturbed position.

for transverse waves

Question 5

wavelength

Answer 5

The wavelength of a wave is the distance from a point on one wave to the equivalent point on the adjacent wave.

for transverse waves

Question 6

The Wave Equation

Answer 6

𝑤𝑎𝑣𝑒𝑠𝑝𝑒𝑒𝑑=𝑓𝑟𝑒𝑞𝑢𝑒𝑛𝑐𝑦×𝑤𝑎𝑣𝑒𝑙𝑒𝑛𝑔𝑡ℎ
𝑣=𝑓𝜆
v: m/s
f: Hz
λ: m

Question 7

Time Period

Answer 7

The time period of a wave is the time it takes for one complete wave (length) to pass a point.

𝑓𝑟𝑒𝑞𝑢𝑒𝑛𝑐𝑦 (Hz) =1/(𝑡𝑖𝑚𝑒 𝑝𝑒𝑟𝑖𝑜𝑑 - s)
𝑓=1/𝑇

Question 8

specular reflection

Answer 8

Question 9

diffuse reflection

Answer 9

Question 10

how is an image formed by a mirror

Answer 10

Question 11

Describe a method that can be used to measure the speed of sound in air:

Answer 11

two people (person A and person B) are placed a distance apart, eg 100 metres
person A fires a starter’s pistol (or hitting two blocks of wood)
person B times the difference between seeing the flash of the gun and hearing the sound - this is measured in seconds
The speed of sound can be calculated using this equation:

𝑠𝑝𝑒𝑒𝑑=𝑑is𝑡𝑎𝑛𝑐𝑒/𝑡𝑖𝑚𝑒

Question 12

Sound waves

Answer 12

Sound waves can travel through solids causing vibrations in the solid.
Within the ear, sound waves cause the ear drum and other parts to vibrate which causes the sensation of sound. The conversion of sound waves to vibrations of solids works over a limited frequency range of 20 to 20 000 Hz. This restricts the limits of human hearing.

Question 13

Ultrasound

Answer 13

• consists of longitudinal waves
• consists of particle vibrations in a material medium, so it cannot travel through a vacuum
• consists of sound waves with frequencies above 20 kHz
• typically cannot be heard by humans because ultrasound frequencies are higher than the upper
  limit of human hearing for most humans.

Question 14

Ultrasound uses

Answer 14

SONAR (sound navigation and ranging):
* Medical scanning
* Measuring the depth of the sea
* Crack detection
* Prenatal scanning

Question 15

Medical scanning

Answer 15

An ultrasound transmitter and detector is used to send pulses of ultrasound into the body (e.g. in
prenatal scanning) and some of the ultrasound reflects from each internal boundary. This results in
several returning pulses from which an image of the internal structures (e.g. an unborn baby) can be
constructed.

pros of ultrasound:
* it can be used to image soft tissue
* it does not damage human tissue.

Question 16

Measuring the depth of the sea - sonar

Answer 16

This diagram shows how a ship’s SONAR can be used to measure the depth of the sea. SONAR can also be
used to locate other vessels and shoals of fish.

A pulse of ultrasound (a ‘ping’) is emitted and the time t between emission and the detection of the
reflected pulse is measured. The distance (d) to the remote object is calculated:

2d = vt
so d = vt/2 as pulse travels to object and back again

Question 17

Crack detection

Answer 17

Ultrasound can be used to detect cracks and flaws inside a solid material (e.g. a pipeline). Partial
reflections occur at the crack. The time for these to return is used to determine the depth of the crack.

Question 18

Prenatal scanning

Answer 18

The principle of prenatal scanning is identical to that of crack detection. Each change of tissue type inside the body partially reflects the incident ultrasound pulse. This results in a series of returning pulses that can be used to locate the depths of each boundary. If an array of transmitters and detectors is used then a detailed image can be constructed.

Question 19

reflecion and refraction of light with water waves

Answer 19

Question 20

a ripple tank set up

Answer 20

Question 21

Differences between water waves and light waves

Answer 21

The analogy between water waves and light waves cannot be taken too far. There are important differences between the two types of wave:
* Light waves are electromagnetic and water waves are mechanical.
* Light can travel through a vacuum and water waves cannot.
* Light waves consist of vibrations of electric and magnetic fields and water waves consist of vibrations of particles.

Question 22

The Electromagnetic Spectrum

Answer 22

Electromagnetic waves are transverse waves that transfer energy from the source of the waves to an absorber.
Electromagnetic waves form a continuous spectrum and all types of electromagnetic wave travel at the same velocity through a vacuum (space) or air.

The waves that form the electromagnetic spectrum are grouped in terms of their wavelength (long to short) and their frequency (low to high frequency):

Radio waves, Microwaves, Infrared, Visible Light, Ultra Violet, X-rays, Gamma Rays

Our eyes only detect visible light and so detect a limited range of electromagnetic waves.

Question 23

Radio

Answer 23

Radio waves can be produced by oscillations in electrical
circuits. When radio waves are absorbed they may create an alternating current with the same frequency as the radio wave itself, so radio waves can themselves induce oscillations in an electrical circuit. Radio waves are reflected by ionosphere (and ground) so can travel around the Earth.
uses: television and radio

Question 24

Microwave

Answer 24

When microwaves are absorbed by water they cause the water molecule to vibrate faster. Because they are not easily absorbed by water they are able to penetrate several cm into food. They can penetrate the ionosphere
uses: satellite communications, cooking food

Question 25

Infrared

Answer 25

All object emit infrared radiation, the hotter they are the more radiation is emitted When the surface of an object absorbs infrared radiation it causes the molecules on the surface to vibrate faster. uses: electrical heaters, cooking food, infrared cameras

Question 26

visible light

Answer 26

Light reflects off the internal edges of fibre optic cables.

Question 27

Ultra Violet

Answer 27

UV is produced inside energy efficient bulbs. This is absorbed by the white coating on the surface of the bulb, which then emits the energy as visible light. (Fluorescence). uses: energy efficient lamps, sun tanning, dangers: ionising - can cause skin cancer

Question 28

X-ray

Answer 28

X-rays can penetrate soft tissue (such as muscle) but not bone (absorbed by bone). X-rays are ionizing radiation that can cause the mutation of genes and cancer, but can also kill cancer cells uses: medical imaging and treatments dangers: ionising - can cause cancer

Question 29

Gamma Ray

Answer 29

Gamma are ionizing radiation that can cause the mutation of genes and cancer, but can also kill cancer cells and can kill bacteria (e.g. they can sterilize surgical equipment) uses: medical imaging and treatments, dangers: ionising - can cause cancer

Question 30

Refraction

Answer 30

refraction, is due to the difference in velocity of the waves in different substances. If a wave slows down it bends towards the normal if a wave speeds up it bends away from the normal When a wave changes speed is wavelength changes, but its frequency always remains constant.

Question 31

refraction wavefronts

Answer 31

The wave travels faster in the first medium. Side A of the wave (front) enters the new medium first so slows down. But side B continues at a higher speed, so travels a greater distance in the same timw. This causes a change in direction of the wave fronts. The wavelength is shorter after it has crossed the boundary meaning that the wave must be slowing down

Question 32

Visible Light

Answer 32

Each colour within the visible light spectrum has its own narrow band of wavelengths and frequencies. Colour filters work by absorbing certain wavelengths (and colours) and transmitting other wavelengths (and colours). The colour of an opaque object is determined by which wavelengths of light are more strongly reflected. Wavelengths that are not reflected are absorbed. If all wavelengths are reflected equally the object appears white. If all wavelengths are absorbed the objects appears black.

Question 33

Global Warming

Answer 33

A body at constant temperature is absorbing radiation at the same rate as it is emitting radiation. The temperature of a body increases when the body absorbs radiation faster than it emits radiation. Carbon dioxide (and other greenhouse gases) reduces the rate at which radiation is emitted into space, which means that the Earth absorbs more radiation than it emits causing it to increase in temperature. As the Earth becomes hotter it emits more radiation until, eventually, the amount emitted into space becomes equal to the amount being absorbed.

Question 34

Black Body Radiation

Answer 34

All bodies (objects), no matter what temperature, emit and absorb infrared radiation. The hotter the body, the more infrared radiation it radiates in a given time. A perfect black body is an object that absorbs all of the radiation incident on it. A black body does not reflect or transmit any radiation. Since a good absorber is also a good emitter a perfect black body would be the best possible emitter. graph to show the distribution of wavelengths emitted by a black body for two objects at different temperatures:

Question 35

Echo Sounding

Answer 35

Echo sounding, using high frequency sound waves is used to detect objects in deep water and measure water depth.

Question 36

frequency

Answer 36

The frequency of a wave is the number of waves passing a point each second.

Question 37

Doppler effect

Answer 37

When there is relative motion between a source of waves and an observer, the wavelength and frequency of the waves detected by the observer is different from the wavelength and frequency of the waves received when there is no relative motion. This change in wavelength and frequency is called the Doppler effect

Question 38

Doppler effect - direction of motion

Answer 38

Note that the Doppler effect depends on motion along the line from the source to the observer. The greater the speed along that line, the greater the shift in frequency and wavelength. Motion perpendicular to that line does not affect the received wavelength and frequency.

Question 39

Examples of the Doppler effect

Answer 39

An ambulance siren has a higher pitch (frequency) as the ambulance approaches and then drops to a lower pitch (frequency) as the ambulance moves away. * Police speed guns bounce pulses of radio waves from moving cars. The size of the Doppler shift on the reflected waves is used to calculate the speed of the car. * Pulses of ultrasound can be reflected from blood cells and the Doppler shift on the reflected waves is used to measure and record the speed of blood flow. * Astronomers can use the Doppler shift of known spectral lines from stars and galaxies to calculate their direction of motion and speed relative to the Earth.

Question 40

frequency and wavelength range of visible light

Answer 40

4.3 × 10^14 Hz to 7.5 × 10^14 Hz 400 - 700 Nm

Question 41

frequency and wavelength range of sound audible to huma ear in air

Answer 41

20 - 20,000 Hz in air 1.7 x 10^-2 - 17 M wavelength