Frequency
Waves are created by vibrating sources and carry energy from one point to another.
Thefrequency of a wave is to the frequency of the source which
generated it. Thefrequency does when a wave passes from on
medium into another.
of a wave is the timetaken to complete one wave cycleof movement.
Interference
Where two or more waves cross, the effect on a particle will be the sum of the separatewave effects. One wave with the effect of the other.
For normal unrelated wave sources the effects will be complicated and difficult to predict.
If the sources are coherent, however, it is possible to generate stable wave patterns orinterference patterns.
Coherent waves are waves have equal and are generated in constant with each other or, if not in , they are out of by a constant amount.
Constructive Interference
The troughs and peaks of one wave coincide with the troughs and peaks of theother. This produces a combined wave which has a amplitude.
Destructive Interference
The peaks of one wave coincide with the troughs of the other. The waves ‘cancel’producing a combined wave of amplitude.
Condition for a maxima (constructive interference)
Path difference =
Condition for a minima (destructive interference)
Path difference =
White light spectra
Diffraction gratings produce a seriesof continuous spectra.
The 0th order spectra is a band of lightwhere all wavelengths interfere .
Prism
A prism produces a continuousspectra. The colours are refractedthrough the greatest angle.
Wavelength and colour.
The wavelengths of light in the visible spectrum range from nm at the red end to nm atthe blue end. Green light is around nm, the half-way point in the visible spectrum.
Refraction of Light
The of light depends on the medium through which it is travelling. When light passesfrom one medium to another, the in speed causes the light to change .
This change of direction at the boundary between two media is called .
sin
sin = constant =
Critical Angle and Total Internal Reflection
When monochromatic light passes from an optically dense medium to an optically lessdense medium light is directed from the normal. As the angle of incidence increases the angle of refraction will eventually reach O . Any increase in the angle of incidencepast this point will cause the loss of the refracted ray and .
The angle of incidence which gives rise to an angle of refraction equal to 90 Oiscalled the .
Irradiance.
The irradiance of radiation falling on a surface is the radiant energy per second on a squaremetre of the surface.
Irradiance ( I ) is measured in ( )
The Inverse Square Law
The irradiance of radiation from a point source varies as the of thedistance from the source.
Photoelectric Effect.
When certain metal surfaces are exposed to ultra-violet light, are ejected fromthe surface. This effect is termed the photoelectric effect.
For a particular metal surface the effect can only take place for light with a frequencygreater than a frequency . Below this frequency the photoelectric effect will not take place even for light with very high . Above this thresholdfrequency, the effect takes place, instantly and at all intensities.
The photoelectric effect can be explained if light is regarded as a stream of particles: . Each photon carries a package of which depends on the ofthe light. The electron absorbs this package of energy. If the package carries enoughenergy, the electron can , if not, the electron cannot escape.
Line spectrum.
Emission Spectra.
When electric current is passed through a gas, the gas , emitting light. When thelight is viewed through a spectroscope, the resulting spectrum is composed of .
The light is emitted as a series of single represented by each line in thespectrum.
Each element has its own characteristic line spectrum which can be used to identify thepresence of that in a gas.
Each line represents an emitted of light. The spectrum represents the range ofphotons which can be by a single free atom of the element; the atoms in a gasmoving freely around.
Energy levels.
Electrons in a free atom occupy energylevels.
An electron from a higher level to a lower a photon of light with an equal tothe difference between the two levels
Line spectra represent all the different possibleways an can fall back down the levels.
Absorption Spectra
When white light is passed through a gas then the resulting continuous spectra has aseries of lines corresponding exactly to the spectra of the gas.
This is due to the absorption of ant their re-emission in directions away fromthe beam.
LASERS
If a large number of atoms are raised to the same state, a photon of lightpassing through them will rapidly collect a large number of identical asit stimulates the of photons from the atoms it passes.
Conductors, Insulators and Semiconductors.
Conductors. Metallic conductors contain many which are to move.
Insulators. All the electrons in an insulator are tied into and are not normallyfree to move. Given enough energy, the electrons will become free and theinsulator will conduct. Insulators will start to conduct at high temperatures orat high voltages.
Semiconductors. Pure semiconductors (Intrinsic semiconductors ) are insulators atvery low temperatures. As they warm up, thermal energy frees electrons frombonds leaving in the bonding electrons. Bonding electrons can movethrough these holes to form a . As the electrons move one way, thehole moves in the opposite direction like a charge.
The electric current is formed from the negative current and thepositive current.
p and n type semiconductors.
Adding a small amount of other atoms to pure semiconductor material can increase theelectrical of the semiconductor. This process is termed .
To produce n-typesemiconductor, pentavalentatoms like phosphorus areadded to the semiconductor.
This provides extra free for the conductionband.
p-type is produced by addingtrivalent atoms like aluminium.
These create extra inthe valence band.
Light Emitting Diode ( LED )
A light emitting diode emits light when itis biased. A series resistor isalways included in the circuit to thecurrent flowing through the LED.
The emission of light is caused byelectrons combining with holes at thep –n .
The electrons in the conduction band loseenergy when they combine with holes inthe lower valence band.
The Photodiode.
A photodiode is a solid - state device used to detect light. Photons of light entering a p – njunction cause the of an electron from a hole.
Geiger and Marsden Experiments 1909 - 1911
Geiger and Marsden, two research assistants of Ernest Rutherford, were directed by
Rutherford to set up at experiment to observe how particles were scattered by thinmetal foils.
particles from radioactive Radon gas were directed at the foils. The particles, emergingfrom the other side were detected with a screen coated with zinc sulphide. This emits atiny flash of light when struck by an particle. The flashes of light were observed using amicroscope. The number of flashes at different angles q, were noted.
Results (1) Most of the particles passed through with very littledeviation.
(2) Some of the particles were scattered through large angles. Of theseapproximately 1 / were scattered through angles greater than90O .
Rutherford explained the results by proposing a new model of the atom with almost all ofthe and all of the positive charge concentrated in a tiny . The rest of the atomconsisted of moving around the nucleus.
Radioactivity
Radioactive atoms contain unstable nuclei. An unstable nucleus will decay; emit a radioactiveparticle to become more stable. The three types of radioactive emission are .
particles are essentially helium nuclei. They consist of two protons and two neutrons.
particles are high energy electrons from the nucleus. When a b particle is emitted, oneof the neutrons changes to a proton. The atomic number increases by one. The atomicmass is unaffected.
rays are bursts of high frequency electromagnetic radiation emitted by a nucleusto lose excess energy. rays do not change the atomic number or atomic mass of a nuclide.
Spontaneous Nuclear Fission The nucleus simply breaks up in a process similar to radioactive decay. This is a natural, uncontrolled, event.
Induced Nuclear Fission The nucleus which undergoes spontaneous nuclear fission can beinduced to undergo fission if it captures a . When a neutron enters the nucleus, it becomesunstable and breaks in two.
Fusion
Nuclear fusion takes place when two light nuclei are forced to combine and form a nucleus. As with fission, the binding energy is increased and the resulting mass defectappears as energy.