National 5 Waves

Question 1

1.1 What does a wave transfer?

Answer 1

A wave transfers Energy

Question 2

1.2 Explain the difference between a transverse and longitudinal wave.

Answer 2

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

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

Question 3

1.3 Classify examples of transverse and longitudinal waves.

Answer 3

Longitudinal: Sound

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

Question 4

1.4 Define Frequency

Answer 4

Frequency (f) is the number of waves 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

1.4 Define Period

Answer 6

Period (T) is the time for one complete wave.

It is measured in seconds (s).

Question 7

1.4 Define Amplitude

Answer 7

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

It is measured in metres (m).

For example, D in the attached image.

Question 8

1.4 Define Speed

Answer 8

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

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

Question 9

1.4 Define a Crest

Answer 9

A Crest is the highest point of a wave.

For example, B in the attached image.

Question 10

1.4 Define a Trough

Answer 10

A Trough is the lowest point of a wave.

For example, C in the attached image.

Question 11

1.5 f = 1/T

(Define symbols and units)

Answer 11

f - frequency (Hz)

T - period (s)

Question 12

1.5 Example

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

Answer 12

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 13

1.5 f = N/t

(Define symbols and units)

Answer 13

f - frequency (Hz)

N - Number of waves (no units)

t - time (s)

Question 14

1.5 Example

A siren produces 26,400 waves per minute.

Calculate their frequency.

Answer 14

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

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

Question 15

1.6 v = f λ

(Define symbols and units)

Answer 15

v - speed (ms^-1)

f - frequency (Hz)

λ - wavelength (m)

Question 16

1.6 Example

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

Calculate their speed.

Answer 16

v = ?
f = 6 Hz
λ = 2 m

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

Question 17

1.6 d = vt

(Define symbols and units)

Answer 17

d - distance (m)

v - speed (ms^-1)

t - time (s)

Question 18

1.6 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 18

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 19

1.6 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 19

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 20

1.7 What is meant by diffraction?

Answer 20

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

Question 21

1.7 What are the practical limitations of diffraction?

Answer 21

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 22

1.8 State the effect of wavelength on diffraction.

Answer 22

Longer wavelengths diffract more than shorter wavelengths.

Question 23

2.1 What is meant by the electromagnetic spectrum?

Answer 23

The electromagnetic spectrum is a family of waves with similar properties.

They can all travel through a vacuum.

Question 24

2.1 List the electromagnetic spectrum in order of increasing wavelength.

Answer 24

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

Question 25

2.2 Give one typical source, detector and application for Gamma Rays

Answer 25

Source: Nuclear Decay, Cosmic Rays, Stars

Detector: Geiger-Muller Tube, Photographic Film

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

Question 26

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

Answer 26

Source: Man-made sources, Stars

Detector: Photographic Film

Applications: Diagnosing broken bones

Question 27

2.2 Give one typical source, detector and application for Ultraviolet

Answer 27

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

Detector: Photographic Film, Chemical Flourescence

Applications: Tanning Beds

Question 28

2.2 Give one typical source, detector and application for Visible Light

Answer 28

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

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

Applications: Seeing

Question 29

2.2 Give one typical source, detector and application for Infrared

Answer 29

Source: Hot objects, Stars

Detector: Thermometer, Photodiode, thermochromic film.

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

Question 30

2.2 Give one typical source, detector and application for Microwaves

Answer 30

Source: Electrical circuits, Stars

Detector: Diode Probe Receiver

Applications: Cooking, Mobile Phone signals

Question 31

2.2 Give one typical source, detector and application for Radio Waves

Answer 31

Source: Electrical circuits, Stars

Detector: Aerial

Applications: Mobile Phone signals, Television signals

Question 32

2.3 Comment on the speed of Electromagnetic Waves

Answer 32

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

Question 33

2.3 Example

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

Answer 33

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 34

2.3 Example

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

Answer 34

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 35

2.3 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 35

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 36

3.1 & 3.2 What is meant by refraction

Answer 36

Refraction is the change in speed that occurs when a wave changes from one medium to another. Wavelength also changes and usually direction (except when the angle of incidence is zero).

Question 37

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

Answer 37

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 ray striking the surface and the normal. (P in the image)

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