Topic 6 Waves

Properties of a wave

• Waves transfer energy and information without transferring matter. The particles oscillate about a fixed point.

• They can be reflected or refracted

Transverse Waves

• Direction of vibration/oscillation is perpendicular to the direction of the propagation of the wave

• have crests(peaks) and troughs(lowest point)

Example: light waves, any EM wave

Longitudinal waves

• Direction of vibration/oscillation is parallel to the direction of the propagation of the wave.

• consists of compressions (particles pushed together) and rarefractions (particles moved apart)

Example: sound waves

Amplitude

The distance from the equilibrium position to the maximum displacement.

The greater the amplitude, the greater the volume of a sound (soundwaves).

Wavefront

A line joining points on a wave at the same point in their wave cycle at a given time.

Frequency

The number waves that pass a single point per second./Number of vibrations per second.

The greater the frequency, the higher the pitch of a sound (soundwaves).

Time period

Time taken for one complete wave to pass a fixed point./ Time taken for one complete oscillation.

The greater the time period, the smaller the frequency.

Wavelength

The distance between a point on a wave and the same point on the next wave.

The shorter the wavelength, the higher the frequency, hence the higher the pitch.

Wave speed formula

Wave speed = frequency x wave length

Wave speed = the distance travelled by the wave per second

distance travelled per second= (number of cycles per second)×(distance per cycle)

Frequency formula

frequency = 1/Time period

(number of wave cycles that pass in 1 second = 1 second / time taken for 1 wave cycle to pass)

Time period = 1 / frequency

(time taken for 1 wave cycle to pass = 1 second / number of wave cycles that pass in 1 second)

If time period increases, frequency decreases。

Doppler Effect

When a wave source is moving relative to an observer, there will be a change in the observed frequency and wave length. This is because the wavefronts get more compressed or spaced apart.

Doppler effect when a wave moves towards the observer.

When a wave moves towards you:

• the observed frequency increases

• the observed wavelength decreases

• e.g. the sound will appear more high-pitched

Doppler effect when a wave moves away from the observer.

When the wave move away from you:

• the observed frquency decreases

• the observed wavelength increases

• e.g. the sound will appear low-pitched

Reflection of waves + law of reflection

• All waves can be reflected when they travel from a medium of low optical density(such as air) to a medium of high optical density(such as glass）

• Law of reflection: angle of incidence = angle of reflection

• Frequency, wavelength and speed are all unchanged

• (Definition: the change in direction of a wavefront at an interface between two different media so that the wavefront returns into the medium from which it originated)

(3.9) Definition of Refraction

= A change in direction of a wave because it changes speed as it passes from one medium to another with a different optical density. (the ability of a material to transmit light through it)

• All waves can be refracted.

• Frequency stays the same, but the wave speed changes, so the wavelength also changes.

Refraction when a wave enters a denser medium

When a wave enters a denser medium, its speed decreases, so it bends towards the normal.

Refraction when a wave enters a less dense medium

When a wave enters a less dense medium, its speed increases, so it bends away from the normal.

Electromagnetic spectrum

in order of increasing frequency and decreasing wavelength: Rabbits Made In Very Unusual eXpensive Gardens

• Radio waves

• Microwaves

• Infrared (IV)

• Visible light

• Ultra Violet (UV)

• X-rays

• Gamma rays

All electromagnetic waves travel at the same speed in a vacuum, and approx. the same speed in air.

They are all transverse waves.

(Electromagnetic waves can travel in a vacuum because they do not need a medium. They are made of changing electric and magnetic fields.)

Uses and hazards of radio waves

• Used in radio and TV communications

• Reason: They have long wavelengths, so they can travel long distances and are reflected by a layer of atmosphere called the ionosphere.

• No hazards, generally safe

Uses and hazards of microwaves

• Uses: cooking and satellite transmissions

• They have shorter wavelengths， greater frequency than radio waves, so they can penetrate more. →They can penetrate into food, causing it to heat up.
  →They can pass through the ionosphere and reach the satellites.

• Hazard: They can cause internal heating of body tissues because they have a high frequency and can penetrate into human bodies.

Tip: Keep a safe distance from the microwave

Uses and hazards of Infrared.

• Uses: heaters and night vision
  Infrared waves occur in heat radiation. Objects radiate heat, and sensors can detect the IR radiation in the dark.

• Hazard: can cause skin burns if exposure is too high

• wear protective clothing

Uses and hazards of visible light

• Uses: photography and fibre-optic cables

• Hazards: very bright lights can damage eyes

• Avoid staring, wear protective eyewear such as sunglasses

Uses and hazards of Ultra Violet

• Uses: fluorescent lamps

• Certain surfaces of objects absorb UV light and re-emit it as visible light, causing them to glow. This is known as fluorescent.

• Hazard: UV can damage skin cells and cause blindness in the eye when too much exposure(due to high frequency of the wave).

• Avoid prolonged exposure to the sun

• wear sunscreen, hats, long clothing

Uses and Hazards of X-rays

• Uses: observing the internal structure of objects or materials, including medical imaging and security check. This is because they have very short wavelengths and high frequencies, so they can penetrate materials easily. X-rays are absorbed by the bones.

• Hazard: mutations due to high frequency

• Stand behind barriers, only expose necessary body parts for medical imaging

Uses and hazards of gamma rays

• Used to sterilise food and medical equipment due to its very high frequency, very short wavelength and high energy

• Can damage the DNA in our cells and cause mutations, causing cancer

• Can also help cure Cancer by killing the Cancer cells

• maintain distance

• shielding with dense materials

(3.9) Refraction of light

• When light enters a medium of higher optical density, the angle of incidence is greater than the angle of refraction.

• When light enters a medium of lower optical density, the angle of incidence is smaller than the angle of refraction.

• Optical density : air < water < glass

3.18 relationship between refractive index(n), angle of incidence(i), angle of reflection (r )

n = sin i / sin r

3.21 Critical angle

• At a certain angle called the critical angle, the light ray will travel along the boundary between the two media.

• Total internal reflection occurs when the angle of incidence becomes greater than the critical angle.

• All of the light will reflected back to the medium where it originated.

• The reflection will occur inside the more optically dense medium.

• This happens when the light travels from a medium of high optical density to a medium of low optical density. (e.g. glass to air)

3.22 Relationship(formula) between refractive index (n) and critical angle ( c )

n = 1/sin c

3.20 Role of total internal reflection in transmitting information along optical fibres and in prisms

• Optical fibres are thin rods of glass (surrounded by cadding/layer) that uses total internal reflection to transfer information by light, even when bent.

• Light waves are reflected over and over again inside an optical fibre as the angle of incidence is greater than the critical angle of the glass.

• These are used in communications (fibre-optic cables) and in medicine (endoscope, an inside body flexible camera, used for 胃镜 ）(endo = inside, scope = ‘looking for’)

• Endoscope uses a bundle of optical fibres to create an image.

Refractive index practical