Basics, relationships and diffraction

Question 1

Why are waves important?

Answer 1

Because they can carry energy from one place to another

Question 2

What is a wave

Answer 2

a method of transferring energy through a medium without the medium moving from its rest position

Question 3

Difference between mechanical and electromagnetic wave

Answer 3

Mechanical waves need a medium to travel in, while electromagnetic waves do not. This is because the creation of the wave is due to the physical movement of the medium.

Once the mechanical wave has passed through, the medium is left in the same position that it was in before the wave arrived.

Question 4

Can electromagnetic waves travel in a medium

Answer 4

Yes but it is not needed.

We know this because all the different types of electromagnetic waves reach Earth from the Sun, and space is a vacuum.

Question 5

What are the electromagnetic waves

Answer 5

Radiowaves
Microwaves
Infrared
Visible light
UV light
X-ray
Gamma

Question 6

How is a wave generated

Answer 6

The medium must be disturbed in some way.

Eg when you speak, air is passed over your vocal cords, making them vibrate and the vibrating vocal cords give repeated pushes to the air molecules surrounding them. This is the disturbance that creates the sound wave.

Question 7

What is the amplitude

Answer 7

The maximum distance the medium goes out, in any direction, from rest position. Half the distance between crest and trough.

Question 8

What is the wavelength

Answer 8

Distance from one point of the wave to the next equivalent point.

Question 9

Relationship between energy, amplitude and wavelength

Answer 9

The more energy used to generate wave, the greater the amplitude. The greater the rate (shakes per second), the smaller the wavelength.

Question 10

How is amplitude experienced

Answer 10

Sound: louder the sound, the more energy, so bigger amplitude.
Light: more energy, brighter the light so greater the amplitude.

Question 11

What is the local effect of a mechanical wave

Answer 11

make each point in the medium complete one swing (out and back followed immediately by out and back in the opposite direction) ie from rest position.

Question 12

Transverse vs longitudinal

Answer 12

Transverse: when local movement of the medium is at right angles to the direction in which the wave is travelling in
Longitudinal: if local movement is parallel to the direction in which the wave is travelling

Question 13

What is frequency

Answer 13

The frequency of a transverse wave is the number of crests of the medium that pass a point every second. Number of waves produced per second.
The frequency of a longitudinal wave is the number of compressions of the medium that pass a point every second.

Question 14

What is frequency measured in?

Answer 14

Hertz
Hz

Symbol: f

Question 15

What is the period

Answer 15

The time taken for one complete wave. Time for two adjacent crests or compressions to pass a point.

Question 16

What is period measured in

Answer 16

second

s

Question 17

What is the relationship between frequency and time

Answer 17

Inverse. As time period increases, the frequency decreases. Vice versa

Question 18

Equation for frequency in terms of time period

Answer 18

f = 1/T

or T = 1/f

Question 19

How is frequency experienced

Answer 19

Sound: pitch. Higher frequency = higher pitch
Light: colour

Question 20

Does frequency change when it goes from one medium to another?

Answer 20

NO IT DOESN’T.

eg if red light goes from air to glass, it stays red.
eg if a sound goes from cold air to hot air it stays at that pitch.

Question 21

What speed do electromagnetic waves travel at

Answer 21

3 x 10^8 ms^-1

Question 22

Wavelengths of the electromagnetic waves

Answer 22

Radio waves = longest

Gamma = shortest

Question 23

Relationship between wavelength and frequency

Answer 23

Inverse. As wavelength increases, frequency decreases.

Question 24

Frequencies and wavelengths of colours

Answer 24

Red: longest wavelength, lowest frequency
Violet: shortest wavelength, highest frequency

Question 25

Does speed change in one medium

Answer 25

No. It only changes when the wave changes mediums

Question 26

Eg of not obvious medium changes

Answer 26

Hot air to cold air

Deep water to shallow water

Question 27

Do electromagnetic waves change speed when changing mediums

Answer 27

YES. They travel fastest through a vacuum.

Question 28

Equation for velocity, frequency and wavelength

Answer 28

v = f λ

Question 29

How does a reflected wave differ from the incident wave

Answer 29

If a wave strikes a surface, it will be reflected. The reflected wave is still travelling in the same medium as the incident wave, so it will have the SAME frequency and speed (and therefore wavelength). Only the DIRECTION has changed.

Question 30

What happens if the wave strikes the boundary between the two mediums?

Answer 30

The wave is able to travel through the boundary from one medium into the other, and so a wave will both be reflected back into the first medium and be TRANSMITTED into the second medium. Because FREQUENCY DOES NOT CHANGE, the second wave will have the same frequency as the incident wave. But its speed, and therefore wavelength will change (due to a proportional relationship between v = fλ)

The direction of the incident wave often changes when it is transmitted into a different medium, but not always.

The AMPLITUDES of the reflected and transmitted wave will be less than the amplitude of the incident wave, because the energy that the incident wave was carrying has been shared between the reflected and transmitted waves.

Question 31

What is diffraction

Answer 31

The bending of a wave around a barrier

Question 32

What does the amount of bending depend on

Answer 32

The wavelength of the wave - the longer the wavelength, the more it bends around the edge of a barrier

Question 33

Why do shadows have relatively sharp edges

Answer 33

Because light waves have very short wavelengths, so they hardly bend at all when they pass the edge of a barrier.

Question 34

What happens when waves pass through a gap in the barrier?

Answer 34

Diffraction occurs at both edges of the gap, so the wave passing through the gap tends to be circular in shape.

Question 35

Relationship between gap of barrier and wavelength

Answer 35

Bending increases when width of the gap is about the same as the wavelength of the wave

The longer the wave compared to the obstruction, the greater the diffraction is.

Question 36

An image produced by a plane mirror is:

Answer 36

• upright
• same size
• virtual
• same distance behind as the real object is in front of the mirror