Modern Physics Lecture 1 - Waves

Why is the particle model ruled out?

Light arriving at screen has to “know” about both slits

Give two things that phase shift can be caused by

• Difference in path length

• Passage through a crystal

What is the definition of the classsical theory of light?

A wave distrubance in the electric and magnetic fields in space

How do you calculate intensity for a wave?

I = A² (where A = amplitude)

Give 3 important properties of intesity

• It is never negative but can be zero

• The “intensity” of lighr is an average over time

• It is carried by both electric an magnetic fields

What is an advantage of using an interferometer?

It has a very high degree of accuracy

What is a beam splitter?

A beam splitter transmits 50% and reflects 50% of light intensity

Define doping

Adding impurities to change the % of transmission

What did interferometers help discover

• Helped discover gravitational waves

What does reflection from a beam splitter cause and why?

Phase shift - arises because of energy conservations

Desribe a symmetric beam splitter

• Transmission causes no phase shift

• Reflection causes ¼ wavelength phase shift

Describe the detector

• Indicates the intensity(power) of incident light

• Units: Watts

Describe the mirror

• No transmission

• No phase shift on reflection

Give 3 problems with the wave picture

• Black body radiation

• Photoelectric effect

• Compton scatterring

Define black body

An idealised object which absorbs all light incident on it and re-emits it having brought it into equilibrium with matter

Give Stefan’s law

Give Wien’s law

Give the Rayleigh-Jeans law

What is the key assumption in deriving the Rayleigh-Jeans law?

At all wavelengths ligth energy is absorbed and emitted continuously

What is the problem with the Rayleigh-Jeans law?

• Didn’t fit the data

• Diverges for small wavelengths, prediciting infinite intensity for very small wavelengths.

• Known for ultra-violet catastrophe.

Give Planck’s law

What is hbar?

hbar = h/2pi

Give two experimental observations for the photoelectric effect

• When incident light is above a threshold frequency, current flowa, proportional to intensity.

• When incident light is below the threshold frequency, no current flows, irrespective of intensity

What is binding energy

Energy needed for electron to escape from the metal

What is the wavevector?

k = 2pi/lambda

What is compton scattering?

When x-rays are incident on a metal, they scatter from the free electrons in the metal

What is Thomson scattering?

A theory for how electromagnetic waves should scatter from charged particles

What are the key elements of Thomson scattering?

• Incoming wave induces the charged particle to oscillate at the same frequenc f as the wave.

• Oscillating particle emits light at the same frequency f in all directions

Why did high energy x-ray scattering experients not match Thomson’s predictions:

• Back-scattered light was weaker than Thomson predicted

• Scattered light experienced an angle-dependant wavelength wavelnegth shift

What did compoton model scattering as?

An elastic collision between electron and photon

Give compton scattering formula:

When is the shift only measurable?

When photon wavelength is not enormous relative to a picometre

What happens when a very very weak beam is projected at a screen?

Detection is no longer continuous

“clicks” at random intervals, on average once per second

Why does measurement have a profound effect on the sustem,?

• When not observed the photon behaves like a wave

• When measured, the photon behaves like a particle