EMR, Light
What are some uses of X-rays?
- Medical images as dense substances attenuate x rays by absorbing them - able to ionise and destroy body tissue - high frequency and energy
If electrons are incident on a target metal, what four different things could happen to the electrons IN THE TARGET METAL?
1. Electrons could bounce of elastically with all of the energy from the incoming electron 2. Electrons could get part of the energy be excited to a higher energy state 3. Electrons could get part of the energy and be scattered with this energy 4. Excited electrons could drop down between levels to replace others
Specify the beginning of the process of creating fluorescence in a fluorescent tube.
1. A high voltage across the ends accelerates free electrons 2. The tube contains Hg vapour 2. The high energy electrons collide with the Hg vapour etc etc
Explain the two ways that you can get electrons up into higher energy states.
1. Bombard with photons - when you bombard with photons you need to bombard with exactly the correct energy and the electrons jump the specific energy level for that energy input 2. Bombard with electrons - collisions will give the electrons in lower states enough energy to go up to higher ones
Explain the defining characteristics of the Bohr Model.
1. Electrons orbit the nucleus without emitting radiation in discrete energy levels 2. Transmission between stationary states produces / absorbs radiation 3. The angular momentum of the electrons is quantised
Explain the two elements that make up an X-ray spectrum
1. Bremsstrahlung - this is the characteristic curve plotting intensity against wavelength when an electron is rapidly decelerated (producing high energy photons and thus the continuous spectrum) 2. Characteristic peaks - the peaks in the spectrum show where incoming electrons have struck an atom with enough energy to remove an electron from an inner orbittal to release an X ray.
State three properties of the electromagnetic wave.
1. Can travel through a vacuum 2. Can move at c=3x10^8ms-1 3. Consists of perpendicular, oscillating electric and magnetic fields
Which two experiments proves light's capacity to exhibit it's dual nature as a particle and a wave?
1. (Wave properties) - Young's Double Slit - when monochromatic light is passed through a double slit a pattern of light and dark fringes will occur. The spread of light and the alternate light / dark regions indicates interference. 2. (Particle properties) - Photoelectric effect - when light of a high enough frequency hits a metal plate it will eject an electron. The kinetic energy of the ejected electrons only depended upon the frequency of the incident light. This could only be explained by considering light as being made up of discrete energy bundles.
Explain the two ways that you can produce an emission spectrum
1. Heat up a gas - molecules collide into each other and get enough energy to jump energy levels, re-emitting energy as it falls back 2. Bombard a gas with electrons - i.e. in fluorescent tubes. High voltages will mean that there are always free electrons that accelerate rapidly (because of the high potential differences involved). These electrons then smash into the atom with enough force that it ionises that atom, releases an electron, and the process repeats. 3. From high energy photons being bombarded as well
Describe the process of fluorescence.
1. High energy photons such as UV (or X rays) are shone onto a substance. 2. The electrons jump to high energy levels (absorbs the transferred energy) 3. Because these higher energy levels are only metastable the electrons begin to fall back down in stages 4. As they fall down they emit EM radiation with at least one stage corresponding to the visible light spectrum
Give examples of the differences between line, band and continuous emission spectra
1. Line - atoms of single elements used; involves discrete energy level transitions 2. Band - liquids or solutions or excited molecules (energy levels are very close and overlap) 3. Continuous - hot solids are used; energy levels spread out in many overlapping bands
State some special properties of X-rays
1. Not deflected by electric or magnetic fields 2. Produced by the rapid deceleration of charged particles such as electrons 3. Are absorbed by the atmosphere 4. Causes (some) minerals to fluoresce 5. Affect photographic emulsions like light 6. X rays can be scattered by particles
Define the differences between a synchotron, the Hadron collider and a mass spectrometer
1. Synchotron produces radiation by accelerating charged particles with an oscillating magnetic fields. It is particularly useful as it can be used to synchronised to produce any frequency of EMR wave, by producing photons which are all in phase. Therefore can create EMR specific for experiments 2. Particle smasher which simulates the conditions of the Big Bang to analyse subatomic particles 3. Used for chemical analysis via mass calibrations of deflections
Explain how / why we get a continuous spectrum from white light.
1. White light is made up of light of different wavelengths 2. These different wavelengths will have different refracted wavelengths 3. The different wavelengths (colours) will each need to travel difference distances to reflect back in phase 4. Therefore when some colours of light undergo constructive interference others will undergo destructive interference, leading to different colours along the surface
Why are radio waves preferred over visible light waves?
1. visible light covers only a very narrow band of the em spectrum unlike radio waves 2. radio waves can diffract around objects as large wavelengths 3. even low energy processes can produce radio waves 4. enable detection of more distant or ancient objects as radiation from these objects has redshifted into the radio wave spectrum
Why do we use a very high potential difference in an X-ray tube?
A high potential difference establishes a strong electric field between the cathode and the target anode. This applies a force which accelerates the electron to a very high speed toward the target anode
Briefly explain how a line absorption spectrum is formed.
An absorption spectrum occurs when a continuous emission spectrum of light passes through cold gas atoms and atoms in the gas absorb characteristic frequencies thus giving rise to dark lines in the spectrum
Explain why the classical Rutherford of the model predicts that atoms should collapse.
As the electrons orbit the nucleus, they are accelerating (centripetal acceleration) Accelerating charged particles emit electromagnetic energy This would mean that the kinetic energy of the electron would decrease, hence also its speed As the speed of the electron decreases, its orbital radius will decrease and it would eventually spiral into the nucleus
Explain Compton scattering and what is showed.
Compton scattering is the name given to Compton's experiment of hitting X-rays against a graphite target and observing how they behaved. He found that some rays were scattered and emerged with LESS energy than they initially possessed and a longer wavelength. It was later found that the energy lost had been transferred in the collision to an electron. Because the collision was inelastic it prompted the conclusion that light could act as particles called 'photons' as they had exhibited momentum in the experiment.
Explain how blueshift on a spectrum works?
Incoming light will appear to be the same, but spectrum will show signature lines to be shifted towards the blue side of the spectrum
What is the white powder in a fluorescent tube for?
It is there for absorb UV light which has been emitted and then re-emit visible light
Explain why we see a continuous emission spectrum.
Particles are diffracted so that they are moving at all different speeds and collisions mean they will undergo acceleration. Accelerating charged particles produce electromagnetic radiation with apparently continuous values of wavelength (producing the rainbow - in reality the wavelengths are discrete)
Explain why the spectral lines for each element are unique to and characteristic of that element.
Spectral lines are produced when electrons drop to lower energy levels and emit photons of energy equal to the energy level difference. Each element has a unique set of energy levels determined by its nuclear charge and hence number of electrons. Hence the set of photons given off by a certain element will be unique and give a characteristic set of spectral lines
Outline the major differences between synchotron radiation and X rays.
Synchotron radiation is more intense, is tunable, is supplied in a collimated beam, travels in short pulses, is highly polarised, is tunable and over a broad spectral range
Explain the origins of the discovery of the element Helium
The absorption lines (Fraunhofer lines) could all be accounted for by known elements except for a set of 'unknown' lines which must be from a new (unknown) element... this element was named helium prior to its detection of Earth
Explain why there is not a corresponding absorption line for all of the emission lines.
The cold atom can only absorb photons that have the exact same energy as the difference between energy levels and the ground state. Atoms in excited states, however, can decay through one jump or through a number of cascading steps.
What is the photoelectric effect and explain the major observations from the experiment.
The photoelectric effect is the ejection of electrons from a metal surface when illuminated by energetic electromagnetic radiation. 1. Work function discovered - the minimum energy needed to remove an electron from the surface; giving you the maximum kinetic energy 2. If brightness/intensity of the source increased, the Ek max didn't change 3. There was a threshold (minimum) frequency that was required to just eject electrons (Ekmax = 0)
What is the difference between a laser and a torch?
The waves from the laser are collimated from a coherent source i.e. all in phase. This is not the case for the torch.
How are absorption spectra produced?
They are produced when a white light is shone onto a material (usually through a prism) which is able to diffract the light. The dark lines of an emission spectra can be matched up with the lines of absorption spectra to identify an element.
How do the high speed electrons in an X-ray tube produce X-rays after they have been ejected from the cathode?
They are rapidly decelerated on collision with target anode resulting in "braking radiation". Energy is then released as high quanta photons of x-ray frequency/wavelength with considerable heat
What are the dark lines on any spectrum?
They aren't spots were light energy have been absorbed, they are where light has been absorbed and then re-emitted but in a scattered and random direction
What did Young's double slit experiment show?
When a monochromatic light is passed through a double slit, a pattern of light and dark fringes will occur. The spread of light and the alternate light/dark regions indicates interference
Explain black body radiation
When objects are in thermal contact (share energy) they reach thermal equilibrium. Every object at a finite temperature is therefore radiating energy. The black body radiation graphs the intensity of this radiation against the wavelength and taught us that hotter objects emit more radiation, and peak at a smaller wavelength.
