Waves + Electromagnetic spectrum

What are the differences between longitudinal and transverse waves?

• In transverse waves, the oscillations/vibrations are perpendicular to the direction of energy transfer/propagation. In longitudinal waves, the direction the oscillations are parallel to the direction of energy transfer/propagation.

• Transverse waves can travel through a vacuum but longitudinal need a medium so they can't travel through a vacuum (since there are no particles). Transverse waves travel fastest in a vacuum while longitudinal waves travel fastest in a solid.

Additional: longitudinal waves have constant density and pressure while transverse waves change pressure and density.

They transfer energy and information but not matter (the particles in the medium — e.g ripples are waves but they don't carry water from one place to another)

What is the structure of longitudinal and transverse waves?

Longitudinal waves are made of compressions and rarefactions, whereas transverse waves are 's' shaped and have peaks and troughs.

Tell me examples of longitudinal and transverse waves.

Longitudinal: sound, earthquakes
Transverse: EM (visible light, x-rays, radio waves, etc), ripples on surface of water, waves on a string

What do waves transfer?

They transfer energy and information but not matter.
They are described as oscillations about a fixed point. For example:

• Ripples cause particles of water to oscillate up and down but they do not travel from place to place with the wave.
• Sound waves cause particles of air to vibrate back and forth.

Amplitude (A) + units

It is the distance from equilibrium position to the maximum (crest) or minimum (trough) displacement.
Metres.

It represents the energy carried by a wave, so a wave with high amplitude carries a lot of energy (loud).

Wavelength (λ) + units

Distance from a point on one wave to the same point on the next wave.
Metres

Wavefront

An imaginary surface representing points of waves that are at the same point in their cycle.

Frequency (f) + units

Number of complete waves that pass a specific point in a second.
Hertz (Hz): the unit Hz is equivalent to "per second" so 5Hz = 5 waves per second.

High frequency = lots of energy

Time Period (T) + units

The time it takes for one complete wave (= one wavelength) to pass a given point.
Seconds.

What is the speed of light in a vacuum and ≈ in air? What is the speed of sound in air?

3.00 x 10^8 m/s
330 m/s

What is the wave equation?

Wave speed (m/s) = frequency (Hz) x wavelength (m)

v = f × λ

What is the equation of frequency and time period?

Frequency (Hz) = 1/time period (s)

f = 1/T

A musician plays an "A" (f = 440 Zz). How long are the waves?

v = f × λ
v = 330 m/s f = 440 m/s

λ = 330/440
= 0.75 m

Oboe plays "A" which is 440Hz. What is the period of each wave?

f = 1/T
440 = 1/T
T = 1/440
= 0.00227s

The visible spectrum is only a small part of the

Electromagnetic spectrum

What is the order of the electromagnetic spectrum?

Radio waves, microwaves, infrared, visible light, ultraviolet, x-rays, gamma rays

What is the relationship between wavelength and frequency?

If the frequency increases the wavelength decreases (but not in refraction of light).
Inversely proportional.
This is explained by the Wave equation a.k.a v=fλ

The EM goes from _____ wavelength and frequency to ______ wavelength and frequency.

Longest and lowest
Shortest and highest

What are the colours of the visible spectrum? How can you separate white light into these colours?

Red, orange, yellow, green, blue, indigo and violet.
Using a prism.

What are the uses of radio waves?

Broadcasting and communications

Radio waves have wavelengths longer than 10cm.

• Long wave radio (1-10km) transmitted long way bc long wavelengths are bent around curved surface of earth

• Short wave radio (10-100m) can because are reflected from the atmosphere (ionosphere) — TV and FM radio broadcasting

What are the uses of microwaves?

Wavelength around 1-10cm

Cooking

• Microwaves are absorbed by water molecules in food

Satellite transmissions

What are the uses of infrared?

Heaters

• Radiate IR to keep warm

Night vision equipment

• Equipment turns it into an electrical signal which is displayed on screen as a picture

What are the uses of visible light?

Optical fibres

• Total internal reflection happens lots of times

Photography

What are the uses of ultraviolet?

Fluorescent lamps

• UV radiation absorbed and then visible light is emitted

• These lamps use UV rays to emit visible light.

• Safe bc nearly all UV is absorbed by a phosphor coating which emits visible light instead

Also (if asks for 2 uses) can say sterilising equipment because it kills bacteria

What are the uses of x-rays?

Observing the internal structure of objects and materials and medical applications.

What are the uses of gamma rays? (2)

Option 1:

• Sterilising food and medical equipment;

• Gamma kills bacteria;

Option 2

• Treating cancer/mutates cancer cells;

• Radiotherapy/focused gamma rays

Why does the dangers of EM waves increase along with the frequency?

Because as frequency increases, so does the energy. Beyond the visible part of the EM spectrum, the energy becomes large enough to IONISE atoms. As a result, ionisation damages cells leading to cell mutations like cancer.
The shorter the wavelength the more ionising the radiation.

EM waves can be harmful. What are the 3 types of EM waves in this sense?

1) Some EM radiation mostly passes through soft tissue without being absorbed — like radio waves

2) Other types I’d radiation are absorbed and cause heating of the cells — like microwaves

3) Some radiations can cause mutation in cells causing cancer — like gamma rays

What are the dangers of excessive exposure to microwaves? Protective measures?

Internal heating of body tissue.
Placing a shield between the source and the person (metal wall).

What are the dangers of excessive exposure to infrared? Protective measures?

Skin burns
Limiting exposure time, wearing protective clothing with insulating materials such as gloves to reduce amount of IR reaching the skin

What are the dangers of excessive exposure to ultraviolet? Protective measures?

Damage to surface cells and blindness.

Can ionise (affect DNA and cause cancer) atoms — básicamente waves have enough energy to kick electrons out of atoms to create ions.

Sunscreen, keeping in the shade.

What are the dangers of excessive exposure to gamma rays? Protective measures?

Can cause cell mutation and lead to cancer or tissue damage because of the ionising of atoms.

Gamma rays are more ionising than UV because they carry much more energy and they can penetrate further into body.

Sources should be kept in lead-lined boxes/use lead screens, lead aprons, limiting time of exposure.

State the properties that are the same for all electromagnetic waves.

They are all transverse.
They all can travel through a vacuum.
They all travel at the same speed (at the speed of light (3 x 10^8)) in a vacuum.
They all transfer energy without transfering matter.

What are the differences of the EM?

They have different wavelengths and frequencies.

Red light has a λ = 700nm. Find the frequency

v = f × λ
1nm = 1 x 10^-9

3 x 10^8
f = ———————
700 x 10^-9

All waves in the EM travel at the speed of light (3 x 10^8)

What is the Doppler effect?

The apparent change in the observed frequency and wavelength of a wave when its source is moving relative to an observer.
For example: the frequency of the sound waves emitted by an ambulance siren are high pitched (high frequency) as it approaches you and low pitched as it goes away.

Explain why the Doppler Effect occurs.

To an observer when object is moving towards them:
The waves appear to get squashed together because the wavelength appears to get shorter (and the frequency higher — inversely proportional)

To an observer when object is moving away from them:
The waves appear to get stretched apart because the wavelength appears to get longer (frequency lower)

Range of human hearing

20-20,000 Hz. Higher frequencies are classified as ultrasound because they are inaudible.