Physics Lecture 2: Nature of Light

Study Notes

Light travels in a vacuum at a velocity of c = 3 × 10^8 m/s
When light travels through matter, its speed is less than c and is given by n, the index of refraction of the substance, which depends on both the composition of the substance and the color of light

Light can be described as a sinusoidal wave, with period T, frequency ν, and wavelength λ, related by the equation: λν = c
As light moves from one medium to another, its wavelength changes as the speed changes, while the frequency remains the same

Each particle of light, or "photon", has an energy E given by: E = hν, where h is Planck's constant (6.63 x 10^-34 Js = 4.14 x 10^-15 eVs)

The internal energy of an atom is the sum of the energies of each electron
Isolated atoms have specific discrete internal energies (energy levels or states)
An atom can change from one energy level to another by emitting or absorbing a photon with an energy equal to the energy difference between the two levels

X-rays are a type of electromagnetic spectrum with wavelengths between 10^-8 and 10^-12 meters
X-rays are not visible to the human eye
X-rays have energies expressed in electron-volts (eV)
X-rays with longer wavelengths are called soft X-rays, while those with shorter wavelengths are called hard X-rays
X-rays are made up of particles of light or "photons"
X-rays travel in a straight line at the speed of light
Each photon has an energy E given by: E = hν

Diagnostic radiology uses lower-energy X-rays for imaging, with no tissue damage
Radiotherapy uses higher-energy X-rays to treat tumors, inducing the formation of free radicals that cause genetic material damage
Diagnostic X-ray range: 20-150 keV

Photons in a vacuum travel in a straight line
When photons travel through matter, they can be:
- Transmitted (passing through)
- Scattered
- Absorbed
The loss of photons due to scattering and absorption is called attenuation
Beer-Lambert law states that the number of photons transmitted decreases with distance

Ionizing radiation has enough energy to remove tightly bound electrons from atoms or molecules
Non-ionizing radiation does not have enough energy to remove tightly bound electrons
Examples of non-ionizing radiation: microwaves, radio waves, infrared
Examples of ionizing radiation: high-energy photons, charged particles

X-ray imaging is based on the differential absorption of X-rays by various tissues
Components of X-ray imaging:
- X-ray tube (produces X-rays)
- Body (X-rays interact with the body)
- Image receptors (X-rays interact with film or detectors)

Components of the X-ray tube:
- Filament (heated to produce electrons)
- Anode (target for electrons to produce X-rays)
- Rotating anode (to prevent overheating)
Anode material: tungsten-rhenium alloy
Filament material: tungsten wire
Focusing cup: negatively charged to focus electron beam onto anode

Focal spot: the volume of target within which electrons are absorbed and X-rays are produced
Anode heel effect: X-ray beam has higher intensity at the cathode end than at the anode end

Dual-focus tubes: contain two cathode filaments of different lengths, producing different focal spot sizes
Cathode assembly: consists of filament, focusing cup, and anode

Filament current: controls the number of electrons released from the filament
Tube current: the flow of electrons from the filament to the anode across the X-ray tube

Space-charge limited tubes: tube current is limited by the accumulation of electrons around the filament
Emission-limited tubes: tube current is limited by the rate at which electrons are released from the filament