Modern Physics Test Review – Key
LIGHT
A photon is one “quanta” of light.
Violet light is the highest frequency of visible light. Red light is the lowest.
All light travels at a constant speed.
Energy and frequency are directly related. A higher frequency means more energy.
As wavelength decreases, frequency increases.
The ratio between energy and frequency is Planck’s Constant.
Light has behaviors both as a particle and as a wave.
Reflection and Refraction are properties that both particles and waves exhibit.
Waves can exhibit interference, diffraction, and polarization, whereas particles do not.
The Photoelectric effect supports the particle model of light, but not the wave model.
If two photons of light have the same frequency, then they must have the same amount of energy.
The EM frequency scale near the range of visible light includes Infrared, ROY G BIV, and Ultraviolet, in order of increasing
frequencies (low to high).
PHOTOELECTRIC EFFECT
In the photoelectric effect, a metal will absorb a high frequency photon, and release an electron. Low frequency light
will have no effect.
Only certain materials will undergo the photoelectric effect
The photoelectric effect requires high frequency light.
Bright light only increases the number of electrons emitted.
If light is low frequency, no electrons will be emitted regardless of how bright it is. High frequency light is required.
The wave nature of light can’t explain the photoelectric effect. The effect works because of the particle nature of light.
All light has the same speed.
The higher the frequency of light, the more energy it has.
Electrons have more energy if they absorb photons of higher frequencies.
MASS- ENERGY EQUIVALENCE
E=mc2 means that Energy is equal to mass times the speed of light squared. The speed of light is 3x108 m/s. It never
changes.
Energy = mass * (3x108) * (3x108) = 1 * (9x1016) = 9x1016 J
3.2x10-8 = mass * (3x108)2 mass = 3.2x10-8/9x1016 = 3.6x10-25 kg
2.7x1017 = mass * (3x108)2 mass = 2.7x1017/9x1016 = 3 kg
RELATIVITY
As your speed increases, time slows down and distances get shorter. (relative to someone standing still)
FISSION AND FUSION
In Fission, a large unstable particle splits into two smaller particles, several neutrons, and energy.
In Fusion, two small particles combine to produce a larger particle as well as energy.
The three products of a fission reaction are daughter products, neutrons, and energy.
The neutron will lodge itself in an atomic nucleolus which causes the unstable particle to split.
Control rods absorb excess neutrons. By doing so, they prevent an uncontrolled fission reaction.
The energy released in fission or fusion reactions is initially stored within the atoms in the form of mass.
A controlled fission reaction is called “cold” fission. “Cold” fusion would be a controlled fusion reaction, but we do not
have the technology for it. We use fission in nuclear power plants.
Mass is lost when a uranium atom and neutron split into daughter products.
Mass is lost when hydrogen atoms fuse into helium.
Mass is lost when an atomic bomb explodes (the energy for the blast comes from the lost mass)
In a nuclear reactor, fission causes a loss of mass in fuel cells as energy is released.
RADIATION
Alpha particles are the largest and slowest of the radioactive particles. They can be stopped by a sheet of paper.
Beta particles will penetrate paper but can be stopped by foil.
Gamma radiation is composed of high frequency photons. It takes thick sheets of dense material (like lead) to stop
them.
Targeted radiation therapy can be used to kill cancerous cells.
A nuclear medicine scan will use radioactive tracers to see inside the body.
ABSORPTION, EMISSION, CONTINOUS SPECTRA
Absorption spectra are the wavelengths that cool gases will absorb light.
Emission spectra are the frequencies emitted by gases that have been excited.
Continuous spectra is the unbroken spectra emitted by objects that glow. (ROY G BIV)