Waves

Transverse wave

oscillations are perpendicular to direction of energy propagation

Longitudinal wave

oscillations are parallel to the direction of energy propagation

Example of a transverse wave

Light

Example of a longitudinal wave

Sound

Amplitude

Max displacement of particles from equilibrium (middle) position

Unit for amplitude

Metres

Wavefront

The front of wave / same point on each wave

What is frequency

number of waves passing any point each second / number of vibrations per second of source

Frequency unit

Hertz

A hertz is

s^-1

Wavelength

Distance between two successive points in phase (e.g. two peaks)

Wavelength unit

Metres

Period

Time taken for one complete vibration of the source/particles

Period unit

Seconds

All waves can be…

Reflected and refracted

Waves transfer…

Energy and information without transferring matter

Doppler effect

Change in observed frequency/wavelength of a wave when its source is moving relative to an observer

When does Doppler effect happen

When source of wave moves relative to observer

What happens when a source of waves moves relative to an observer

Waves at front of source are bunched together; waves at back are spread out

What do you observe if a source of waves is moving away from you

Increased wavelength / decreased frequency

Order of EM spectrum

Radio micro IR visible UV X-ray gamma

EM wave with highest wavelength / lowest frequency

Radio

EM wave with highest frequency / lowest wavelength

Gamma

Use of radio waves

Broadcasting, communications

Use of microwaves

Cooking, satellite transmissions

Risk of microwaves

Internal heating of body tissue

Protective measure for microwaves

Shielding on microwave ovens to prevent microwaves reaching user

Use of infrared

Heaters, night vision equipment

Risk of infrared

Skin burns

Protective measure against IR

Wear sunscreen to reduce IR reaching skin

Use of visible light

Optical fibres, photography

Risk of visible light

Damage eyes

Use of UV light

Fluorescent lamps, detecting fake cash

Risk of UV

Blindness, damage to surface cells

Protective measure against UV

Wear sunscreen

Use of X rays

Medical imaging

Risk of x rays

Small chance of cancer after accumulated exposure

Use of gamma rays

Sterilising food and medical equipment

Risk of gamma rays

Cancer, mutation

Protective measure against gamma rays

Wear film badge, keep source in lead-lined box

What do film badges do

Show when wearer is overexposed to radiation

What is the law of reflection

i = r

What is refraction

When wave passes boundary between two different media and undergoes a change in direction

What does change in direction (refraction) depend on

Difference in density between media

When would light be refracted towards the normal

Less → more dense

When light slows down it bends…

Towards the normal

When would light be refracted away from the normal

More → less dense

What parts of the wave change when refracted

Speed and wavelength (not frequency)

What is the normal

Line perpendicular to the boundary

Why should you use a set square in ray box practical

Draw normal correctly

Why should you use sharpened pencil in ray box practical

Accurately mark in the middle of beam

What do you need to measure angles accurately

Perpendicular normal; sharp pencil; high res protractor

Safety for ray box practical

Don’t look directly into light, don’t touch box if too hot, keep liquids away

Relationship between refractive index, angle of incidence and angle of refraction (Snell’s law)

N = sin i/ sin r

What is n in snells law

Refractive index

How do u know sin i is on top of sin r

I is before r in alphabet

How can you check the refractive index is right

Always larger than 1

What kind of materials have a higher refractive index

More optically dense materials e.g. diamond

What kind of materials have a lower refractive index

Less optically dense materials e.g. glass

Total internal reflection

Light reflected instead of refracted when moving from denser to less dense medium

2 conditions required for TIR

i>c; incidence material denser than second material

Optical fibres are used in

Communications, endoscopes

What is an endoscope used for

Examine inside the body e.g. stomach

What are prisms used in

Binoculars, telescopes, cameras, periscopes

Reflection by right angled prisms

Critical angle

Angle of incidence where angle of refraction is 90 (along the boundary)

Relationship between refractive index and critical angle

sin c = 1/n

How would you find refractive index given the critical angle

N = 1/sin c