OCR A-Level Physics A: Electrons, waves and Photons

Intensity units

W/m^2

Intensity equation

I = P/A (Power / area)

Intensity and distance relationship

AS a wave travels out from a source, the radiant power spreads out, which reduces the intensity.

From a point source of wave, the energy and power speeds out evenly in all directions (over the surface of a sphere)

The total radiant power P at a distance r from the source is spread out an area equal to the surface area of a sphere (4 x pi x r^2), which changes the formula to I = P / (4 pi r^2)

What is the intensity equation in relation to a sphere?

I = P / (4 pi r^2), where r is the sphere radius

What is the relationship between intensity and amplitude?

Intensity is directly proportional to amplitude squared

What is the wave-particle duality?

Light behaves both as a wave and a particle

Can photons behave as particles while having a wavelength?

Yes, photons can have a wavelength and still behave as a particle.

What did Einstein realize about photons?

Photons are massless but behave as particles and can have momentum.

What is the formula for photon momentum?

p = h / λ

What does 'p' represent in the context of photons?

Photon momentum.

How is energy related to momentum for photons?

E = pc, where E is energy and p is momentum.

What is the equation for energy in terms of frequency?

E = hf, where E is energy and f is frequency.

Frequency range of X-rays

3x10^16 --> 7.5x10^20

Wavelength of radiowaves

0.1 m

Wavelength of microwaves

0.1m --> 1x10^-3m

Wavelength of infra-red radiation

1x10^-3 --> 7x10^-7

Wavelength of visible light

4x10^-7m --> 7x10^-7

Wavelength of UV light

4x10^-7 --> 1x10^-8

Wavelength of X-rays

1x10^-8 --> 4x10^-13

Wavelength of gamma waves

1x10^-10 --> 1x10^-16

Frequency of radiowaves

~ 3x10^9 HZ

Frequency of microwaves

3x10^9 Hz --> 3x10^11 Hz

Frequency of infra-red radiation

3x10^11 Hz --> 4.3x10^14 Hz

Frequency of visible light

7.5x10^14 Hz --> 4.3x10^14 Hz

Frequency of UV light

7.5x10^14 Hz --> 3x10^16 Hz

Frequency of gamma waves

3x10^18 Hz --> 3x10^24 Hz

What happens to the energy of the wave as frequency increases?

As frequency increases, the energy of each wave increases

Uses of radio waves

Communications - radio and TV
GPS systems

Uses of microwaves

Heating food
Communication - wifi, phones and satellites

Uses of infra-red radiation

Remote controls
Fibre-optic cables
Thermal imaing (medical)
Can be used to heat objects
Motion sensors (beam broken)
Night vision

Uses of visible light

Seeing things!!

Used to help cameras record images

Uses of ultraviolet waves

Checking notes / driver's liscences for forgery
Sunbeds
Fluorescent bulbs

Uses of X-rays

Medical images
Airport / other security scanners

Uses of gamma waves

Cancer treatment (radiotherapy)
Medical / other tracers
Sterilisation of medical instruments / food

Emitters of radiowaves

AC current producer with antenna produces a 'carrier wave'

Emitters of microwaves

The sun
Microwaves (the things used to cook food)
Phones and other technology

Emitters of infra-red radiation

LED
Any thermal emitter

Emitters of visible light

The sun
Stars
Fire
Light bulbs
Anything hot

Emitters of UV light

An object at 0K
Some LEDs
Lasers
Stars

Emitters of X-rays

X-ray tube

Emitters of Gamma waves

Unstable nuclei
Supernovae
radioactive chemical reacitons

Detectors of radio waves

Antenna attached to AC inducer with the same frequency as the wave

Detectors of microwaves

RADAR
Sensors found in cars / automatic doors

Detectors of infra-red radiation

Various electrical components
Thermal / heat sensors - eg thermometers

Detectors of visible light

Retina in eyes
Photoelectric cells in cameras / solar panels
LDRs

Detectors of UV light

Photo-diode array sensors
Variable-wavelength detectors (VWDs)

Detectors of X-rays

Photographic film
GM tube

Detectors of gamma waves

GM tube
Gold-leaf electroscope

Dangers of radiowaves

None - radiowaves don't have enough energy to harm our bodies as they pass through us. There are always radiowaves passing through us (background radiation), so we don't notice a difference.

Dangers of microwaves

Heating body parts (if oscillating at the resonant frequancy), but microwaves are all around us, so the chance of this is very low.

Dangers of infra-red radiation

Heating of tissues and organs
Danger to the eyes, as there isn't enough blood flow in the eyes to tranfer the heat elsewhere.
Don't shine lasers in eyes

Dangers of visible light

Eye danger:
From bright lights
The sun
Welding without a visor
Flash photography

Dangers of UV

Sunburn
Skin cancer
(both due to damage to DNA)

Dangers of X-rays

Cell damage and cancer risk - ionising power can split DNA strands, causing genetic mutations

There is a limit to the number of X-rays allowed per year due to this

Dangers of gamma waves

Highly ionising, so can cause cancer and radiation sickness, as well as genetic mutations
Can cause cancer to ANY body part, because they can pass through our bodies very well.

How many radians are in a full circle?

2pi

How many radians are in a semicircle?

pi

How many radians are in a quarter circle?

pi/2

How do you convert degrees to radians?

Multiply by 2pi/360.

How do you convert radians to degrees?

Multiply by 360/2pi.

Transverse Wave

Direction of energy transfer perpendicular to the oscillation direction.

Longitudinal Wave

Direction of energy transfer parallel to the oscillation direction.

Wavelength

The distance between two corresponding points on adjacent waves.

Amplitude

Distance from equilibrium to the middle of either a peak or trough.

Frequency

The amount of waves propagated in one second, measured in hertz.

Time Period

Time taken for one full wavelength to pass a point.

Displacement

Distance from the equilibrium position in a particular direction

vector - positive or negative.

Wave Speed

Distance the wave travels per unit time.

Wave Speed Equation

Wave speed = frequency x wavelength.

Frequency Equation

Frequency = 1 / Time period.

Refractive index

Ratio of speeds in the two media

Movement into a more optically dense medium

Slows down, refracted ray closer to the normal

Movement into a less optically dense medium

Speeds up, refracted ray further away from the normal

Relative optical density

Can deduce from refractive index

Snell's Law

N = Speed of light in incident medium / Speed of light in refractive medium = sin(I) / sin(r)

When to use Snell's Law

Only use Snell's Law when it goes less to more.

What is the formula for refractive index?

N = 1 / sin(c)

What is C in the context of optics?

C is the critical angle.

What does N represent in optics?

N is the refractive index.

What conditions are necessary for total internal reflection?

Wave must travel from a more dense to a less dense medium and the angle of incidence must be greater than the critical angle.

Principle of superposition

The sum of the displacements of each wave is equal to the final displacement of the combined waves, or the resultant.

Coherence

Needs a constant phase difference.

Constructive Interference

Occurs if the displacements are in the same direction.

Destructive Interference

Occurs if the displacements are in the opposite direction.

What do polarizing lenses do to light waves?

They allow light waves to oscillate in only one direction.

What are the possible oscillation directions for transverse waves?

Vertical, horizontal, and anything in between.

How can two polarizing lenses be used together?

They can block all light or vary the amount of light blocked depending on the angle between them.

What is the effect of polarizing lenses on glare?

They protect from glare by blocking light from unwanted directions.

What is the plane of propagation in the context of polarizers?

It refers to the specific direction in which polarizers allow light to pass through.

Maxima

Waves interfere constructively

Minima

Waves interfere destructively

Coherent waves

Waves must have a constant phase difference

Phase difference for maxima

0, 2𝜋, 4𝜋, etc. radians

Phase difference for minima

𝜋, 3𝜋, 5𝜋, etc. radians

Path difference for constructive interference

Even half-wavelengths

Path difference for destructive interference

Odd half-wavelengths

Sound wave maxima

Loud points

Sound wave minima

Quieter points

Path difference measurement

Measured as a distance