Physics Unit 4 - Light (Waves and Particles)

if waves move from faster medium to slower medium

- they bend towards the normal (angle of incidence is greater then angle of refraction)
- wavelengths decrease (in size) because velocity decreases

if waves move from slower medium to faster medium

- they bend away from the normal (angle of incidence is less then angle of refraction)
- wavelengths increase bc velocity increases

Refraction

The bending of a wave as it passes at an angle from one medium to another. Frequency not changed because frequency depends on the source of the waves.

Total Internal Reflection

the complete reflection of a light ray back into its original medium

Dispersion of light

- when light passes from one material to another and light waves slow down, wavelength shortens
- waves bunch up and wavelengths of each colour change by different amounts
- means that each colour travels at a slightly different speed in the new medium and therefore each colour is refracted by a slightly different amount
- allows for separation of white light into its diff freq
- red light refracts the least, violet refracts the most

Diffraction is significant when

size of the opening or obstacle is similar to or smaller then the wavelength of the wave. If the wavelength is much smaller then the gap or obstacle, degree of diffraction is less.

Diffraction

The bending of a wave as it moves around an obstacle or passes through a narrow opening

Diffraction Grating

a plate of glass or metal ruled with very close parallel lines, producing a spectrum by diffraction and interference of light.

Polarisation

The phenomenon in which oscillations of a transverse wave are limited to only one plane. Requires transverse wave model.

Unpolarised Light

light consisting of transverse waves vibrating in all conceivable random directions

Vertical Polariser

only allows vertically aligned light through

Horizontal Polariser

only allow horizontal component of aligned light through

Young's double slit experiment

shows the constructive and destructive interference of waves that occur as light passes through parallel slits, resulting in minima (dark fringes) and maxima (bright fringes) of intensity.

Path Difference

- difference in the distances two waves must travel from their sources to a given point
- constructive interference: when path difference is integer multiple of wavelengths (n___, 1*__)
- destructive interference: when path difference is 1/2, 3/2, 5/2 etc

the shorter the wavelength

the greater the penetrating power

Forward Voltage

Negatively charged photoelectrons will be accelerated or helped across the gap because the anode is postitive, creating a photocurrent.

Reverse Voltage

electrons are repelled by the negatively charged anode, slowing them down. As anode voltage is increased, photoelectrons are repelled more and more until photocurrent drops to zero.

Threshold Frequency

minimum light frequency necessary to eject an electron from a given metal and hence observe photoelectric effect. If frequency is greater then threshold frequency, photoelectrons are collected and photocurrent registered. If frequency is less then threshold frequency, no photoelectrons are detected

Relationship between Frequency and Intensity (Photoelectric Effect)

for light that has frequency greater then threshold frequency, rate at which photoelectrons are produced varies in proportion with intensity (brightness) of incident light

Relationship between Intensity of Light and Photocurrent (Photoelectric Effect)

As intensity of light increases, photocurrent increases

Stopping Voltage

The minimum magnitude of negative voltage needed to reduce the photoelectric current to zero, meaning that the most energetic photoelectrons are prevented from reaching the detector. Unique to every metal

Varying Frequencies, Same Intensity (Photoelectric Effect)

if light source has different frequencies but the same intensity, they produce the same maximum current but higher frequency light has higher stopping voltage.

Why Photoelectric Effect Disproves Wave Nature of Light

- if light were purely a wave, frequency of light should be irrelevant to whether or not photoelectrons are ejected. It would be expected that even low frequency light would transfer enough energy to emit photoelectrons given enough time since waves are a continuous transfer of energy.
- wave model also predicts that there should be a time delay between light striking the metal and photoelectron being emitted (since energy builds up over time), but this is not observed
- in the experiment all intensities of light with the same frequency had the same stopping voltage. Wave nature of light suggested that higher amplitudes would lead to a higher stopping voltage (not observed)
- in the experiment as frequency of light increased, stopping voltage increased proportionately. wave model suggested that frequency of light would not affect stopping voltage.

Work Function

minimum amount of energy required to eject a photoelectron from the suface of a metal. Characteristic of the element since each metal has different strength of bonding slightly.

- How to think about work function: if the work function of lead is 4.14eV, that means 4.14eV is required to release one electron from the surface of a piece of lead.

Ways to excite atoms of an element

Applying energy in one of the following ways:
- heating to high temperatures
- passing an electric current through element
- bombarding element with electrons
- photo excitation by light

Requirements for emission spectra

- exactly enough energy from photons when the photon energy is exactly equal to the difference in energies between an occupied orbit and higher orbit (if light or heat is used)
- not greater then ionisation energy
- incident light that does not carry enough energy to lift an electron up a shell or has more then the exact amount of energy will pass straight through rather then being absorbed by the atom

Absorption from electron excitation

- bombaring electrons can bombard the electrons of an atom and promote electrons to a higher energy level, and any excess energy is energy that the electron possesses as it leaves the atom.

e.g: if Mercury was bombarded by 5.7eV electrons, it can promote some electrons from n=2 to n=4 since energy difference is 5.5eV. The bombarding electrons leave the atom with 0.2eV

De Broglie Wavelength in everyday scenario

since wavelength of electrons (using De Broglie's equation) is much larger then wavelength of a tennis ball, significant diffraction can be observed since the wavelength of an electron is significantly larger then its actual size. It can be passed through a gap that is similar to its wavelength and it is able to pass through. But with a tennis ball, since its wavelength is so small compared to its size, no experiement could show it diffracting since the wavelength has to be similar size to the gap, but the tennis ball can't fit in the gap.

Solving Ultraviolet Catastrophe

- classical physics supported the idea that energy was continuous. For classical physics model to work, black bodies would have to emit radiation at infinite frequencies. To maintain thermal equilibrium, it makes sense that radiation is emitted as photons (discrete packets of energy) of varying frequencies.
- it is more probable for an object to lose energy by emitting a large number of lower energy quanta rather then one large high energy quantum (that corresponds to ultraviolet radiation)

Classical Physics Model of Black Body

- classically, thermal radiation is emitted by accelerating charges near the surface of a material.
- charges have distribution of acceleration that leads to range of thermal energies
- gives the average energy per oscillating charge proportional to temperature.
- hence as frequency became larger model showed intensity of light to approach infinity.