National 5 Waves

Question 1

What do all waves wave transfer?

Answer 1

All wave transfer Energy

Question 2

Explain the difference between a transverse and longitudinal wave.

Answer 2

In a transverse wave the particles of the medium oscillate at right angles to the direction of energy transfer.

In a longitudinal wave the particles of the medium medium oscillate parallel to the direction of energy transfer.

Question 3

Give and example of a longitudinal and a transverse and a wave.

Answer 3

Longitudinal: Sound

Transverse: Water, Light, Radio Waves, All Electromagnetic Waves

Question 4

Define Frequency

Answer 4

Frequency (f) is the number of waves produced (or passing a point) per second.

It is measured in Hertz (Hz).

Question 5

1.4 Define Wavelength

Answer 5

Wavelength (greek letter lambda λ) is the length of one complete wave, often measured between successive crests or troughs.

It is measured in metres (m).

For example, A in the image.

Question 6

Define the Period of a wave.

Answer 6

The Period (T) of a wave is the time taken to produce for one whole wave.
(One whole crest and one whole trough).

It is measured in seconds (s).

Question 7

Define the Amplitude of a wave.

Answer 7

Amplitude is the height of a wave from the line of zero disturbance to the top of a crest, or from the line of zero disturbance to the bottom of a trough.

It is measured in metres (m).

For example, D in the attached image.

Question 8

Define the Speed (v) of a wave.

Answer 8

The Speed (v) is the distance travelled per unit of time by a wavefront.

It is measured in metres per second (ms^-1).

Question 9

Define a Crest

Answer 9

A Crest is the highest point of a wave.

For example, B in the attached image.

Question 10

Define a Trough

Answer 10

A Trough is the lowest point of a wave.

For example, C in the attached image.

Question 11

Which feature of a wave is measure of its energy?

A Wavelength
B Frequency
C Speed
D Amplitude
E Period

Answer 11

D Amplitude

The greater the amplitude of a wave, the greater its energy.

Question 12

f = 1/T

(Define symbols and units)

Answer 12

f - frequency (Hz)

T - period (s)

Question 13

Example

A bat emits ultrasounds with a period of 23 µs. Calculate the frequency of the ultrasound.

Answer 13

T = 23 µs = 23 x 10<sup>-6</sup> s
f = ?

f = 1/T
f = 1/23 x 10<sup>-6</sup>
f = 4.35 x 10<sup>4</sup> Hz

Question 14

f = N/t

(Define symbols and units)

Answer 14

f - frequency (Hz)

N - Number of waves (no units)

t - time (s)

Question 15

Example

A siren produces 26,400 waves per minute.

Calculate their frequency.

Answer 15

N = 26,400
t = 1 min = 60 s
f = ?

f = N/t
f = 26,400/60
f = 440 Hz

Question 16

v = f λ

(Define symbols and units)

Answer 16

v - speed (ms^-1)

f - frequency (Hz)

λ - wavelength (m)

Question 17

Example

Some water waves have a wavelength of 2 m and a frequency of 6 Hz.

Calculate their speed.

Answer 17

v = ?
f = 6 Hz
λ = 2 m

v = fλ
v = 6 x 2
v = 12 ms<sup>-1</sup>

Question 18

d = vt

(Define symbols and units)

Answer 18

d - distance (m)

v - speed (ms^-1)

t - time (s)

Question 19

Example

A cannon is 170 m away.

The sound from the cannon firing takes 0.5 s to reach you.

Use this to calculate a value for the speed of sound in air.

Answer 19

v = ?
t = 0.5 s
d = 170 m

d = vt
170 = v x 0.5
v x 0.5 = 170
v = 170/0.7
v = 340 ms^-1

Question 20

Example

An ultrasound scanner detects an echo from a baby’s head after 0.12 ms.

How far away is the baby’s head?

(Take the speed of sound in tissue to be 1500 ms^-1

Answer 20

Don’t forget that in an echo problem the echo has to travel there and back.

v = 1,500 ms<sup>-1</sup>
t = ½ x 0.12 ms = ½ x 0.12 x 10<sup>-3</sup> = 6 x 10<sup>-5</sup> s (half for there only)
d = ?

d = vt
d = 1,500 x 6 x 10^-5
d = 0.09 m
The baby’s head is 0.09 m away.

Question 21

What is meant by diffraction?

Answer 21

Diffraction is when a wave bends round an obstacle or through a gap.

Question 22

What are the practical limitations of diffraction?

Answer 22

If an object is small compared to the wavelength, the wave will bend round the object and it cannot be detected.

If a gap is small compared to the wavelength, significant diffraction occurs leading to semicircular wavefronts.

Question 23

State the effect of wavelength on diffraction.

Answer 23

Longer wavelengths diffract more than shorter wavelengths.

Question 24

What is meant by the electromagnetic spectrum?

Answer 24

The electromagnetic spectrum is a family of waves which all travel at the speed of light and can travel through a vacuum (no medium required).

They all have different wavelengths and frequencies. All are invisible except visible light.

Question 25

List the members of the electromagnetic spectrum in order of increasing wavelength.

Answer 25

Gamma, X-Rays, Ultraviolet, Visible, Infrared, Microwaves, Radio Waves

Question 26

Give one typical source, detector and application for Gamma Rays

Answer 26

Source: Nuclear Decay, Cosmic Rays, Stars

Detector: Geiger-Muller Tube, Photographic Film

Applications: Treating Cancer (Radiotherapy), Tracers to diagnose illness

Question 27

Give one typical source, detector and application for X-Rays

Answer 27

Source: Man-made electronic sources, Stars

Detector: Photographic Film, Transistor arrays.

Applications: Diagnosing broken bones

Question 28

Give one typical source, detector and application for Ultraviolet

Answer 28

Source: Ultra-Hot objects, Electrical discharges/sparks, Stars

Detector: Diode-probe receiver, Photographic Film, Chemical Flourescence

Applications: Dental Curing (Setting fillings in teeth), Tanning Beds (don’t use them kids!).

Question 29

Give one typical source, detector and application for Visible Light

Answer 29

Source: Very-Hot objects (lamps), Electrical discharges/sparks, Stars

Detector: Photographic Film, Photodiode, Charge-Coupled Device (CCD), Human Retina

Applications: Seeing

Question 30

Give one typical source, detector and application for Infrared

Answer 30

Source: Hot objects, Stars

Detector: Thermometer, Photodiode, thermochromic film.

Applications: Optical fibre communication, Remote controls, “Night” vision

Question 31

Give one typical source, detector and application for Microwaves

Answer 31

Source: Electrical circuits, Stars

Detector: Diode Probe Receiver

Applications: Telephone Communications, Cooking.

Question 32

Give one typical source, detector and application for Radio Waves

Answer 32

Source: Electrical circuits, Stars

Detector: Aerial

Applications: Mobile Phone signals, Television signals

Question 33

Comment on the speed of Electromagnetic Waves

Answer 33

All Electromagnetic Waves travel at the same speed, which is the speed of light (3 x 10⁸ ms^-1).

Question 34

Example

Microwaves have a frequency of 9.4 GHz. Calculate their wavelength.

Answer 34

v = 3 x 10<sup>8</sup> ms<sup>-1</sup> (Since EM Wave)
f = 9.4 GHz = 9.4 x 10<sup>9</sup> Hz
λ = ?

v = fλ
3 x 10⁸ = 9.4 x 10⁹ x λ
9.4 x 10⁹ x λ = 3 x 10⁸
λ = 3 x 10⁸/9.4 x 10⁹
λ = 0.032 m

Question 35

Example

Radio Waves takes 1.28 seconds to travel from the moon to the earth. How far away is the moon?

Answer 35

v = 3 x 10<sup>8</sup> ms<sup>-1</sup> (since EM Wave)
t = 1.28 s
d = ?

d = vt
= 3 x 10⁸ x 1.28
d = 3.84 x 10⁸ m

Question 36

Example

The sun is 1.5 x 10¹¹ m away.

How long does it take light to travel from the sun to the Earth?

Answer 36

v = 3 x 10<sup>8</sup> ms<sup>-1</sup> (since EM Wave)
d = 1.5 x 10<sup>11</sup> m
t = ?

d = vt
1.5 x 10¹¹ = 3 x 10⁸ x t
3 x 10⁸ x t = 1.5 x 10¹¹
t = 1.5 x 10¹¹/3 x 10⁸
t = 500 s

Question 37

Define Refraction

Answer 37

Refraction is the change in speed (or wavelength) that occurs when a wave travels from one medium to another.

Note - the direction need NOT change upon refraction and need not be mentioned in its definition.

Question 38

Identify correctly the angle of incidence, angle of refraction and normal in ray diagrams showing refraction.

Answer 38

The normal is an imaginary line at right angle to the surface where a ray strikes. (The dashed grey vertical line in the image)

The angle of incidence is the angle between the incident ray and the normal. (P in the image)

The angle of refraction is the angle between the refracted ray and the normal. (Q in the image)

Remember
In optics, we always measure angles between the ray and the normal.