hello and welcome to today's lesson on redshift which is part of the astrophysics topic in aqa a level physics so in today's lesson we're going to look at the method of using redshift to understand astronomical distances so if we've been successful and learned in today's lesson we should be able to understand the doppler effect detail examples of redshift found in astrophysics and understand and apply the formulae for redshift from observations so we're going to be looking at the following part of the aqa a-level physics specification 3.9.3.1 the doppler effect so all electromagnetic waves have a frequency and wavelength this allows for physicists to measure these quantities now the motion of either an observer or an emitter changes the wavelength and frequency of the wave detected now this is called the doppler effect and is shown in everyday life by a siren on a passing vehicle now the doppler effect is an observational effect it only changes the observed properties of the wave not its actual properties so the doppler effect is caused by the relative motion between the observer and the emitter of radiation now this causes the observed frequency and wavelength of the wave to be altered now what we can see is in the previous example that the motion of either the observer or the emitter changes the wavelength and frequency of the wave detected so consider a motorbike moving towards you emitting a sound now as you move closer the space between the emitter and the observer decreases this pushes the wavelength together so it means the frequency gets bigger yet the wavelength gets smaller now if there's no change between the emitter on or the observer then there's no difference in the wavelength or the frequency so there is no doppler effect and if you move further away then the space between the emitter and the observer increases this spreads the wave out the frequency gets lower so the wavelength gets bigger so the doppler effect is when the wavelength and frequency of a wave is altered by the relative movement of either their source or the observer when the source or observer is receding the wavelength is longer than it should be and the frequency is lower when the source or observer is moving towards the other the wavelength is shorter than it should be and the frequency is higher so now any wave experiences this phenomena of the doppler effect this includes visible light now humans experience the wavelength and frequency of visible wet light waves as color now a long wavelength visible light is interpreted as red a short wavelength visible light is interpreted as blue so you can see the comparison of color and wavelength of visible light in the following image red gives you a long wavelength and blue gives you a short wavelength so now if we consider the previous example in terms of light as as what an object is moving towards you the space between the emitter and the observer decreases this compresses the wave together this means the frequency gets bigger the wavelength gets smaller the lighter p is bluer now if there's no change between the observer or emitter there's no difference in wavelength or frequency there is no change of color to in of light in the perception of the observer if you move further apart then the space between the emitter and the observer increases this spreads the wave out the frequency gets lower the wavelength gets bigger the light appears redder to the perception of the observer so redshift is the doppler effect for light as light is a wave it also experiences the doppler effect if either an observer or an emission source are in relative motion if the emitter and observer are moving further apart the wavelength is longer and the frequency is shorter so it is redder to humans if two sources are moving closer together the wavelength is shorter and the frequency is longer so it appears bluer to humans so this means when we observe distant objects if an object is moving towards an observer it will appear bluer than the object observer expects if an object is moving away from the observer it appears redder than the observer expects so it's a very important idea to understand now we can see the following idea in this particular example it can be determined that if a star is red shifting or blue shifting by comparing the spectral lines of a mission with a known source and seeing if they are different to the emission spectral lines of the distant object this occurs as the absorption spectra of an element is the same regardless of the motion location or temperature in the universe so normally we use light produced in a stationary earth based experiment in the lab or light produced in the sun as a comparison for distant galaxies now we can use lab based samples of projected composition of stellar objects to observe the effect of redshift or blueshift but this is only an estimation now redshift can be easily determined by looking to see if there's a change in reference points in the absorption spectra of distant galaxies compared to the absorption spectra of a stationary sample of matter in the lab now the most common reference line to look is the bomber lines the visible absorption lines of hydrogen as hydrogen is the most common element in the universe so if you can look in here you can see that hydrogen has their bomber lines you can compare to see what it would look like in a lab sample compared to observed supernova spectrum now we can calculate the amount of redshift the galaxy exhibits zed and then use to calculate how fast it's moving away or towards us with the equation z equals the change in wavelength over wavelength equals the change in frequency over frequency equals the speed of the object divided by the speed of light now this only works when v is a lot less than c so the speed of the galaxy is a lot less than the speed of light now you are given these equations in your examination but you've got to be able to use these equations correctly and you've got to be able to equate the different equations of redshift now z is a quantitative measure of the redshift effect it has no units it's just a ratio it comes considered to be the absolute change in wavelength but only the approximate change in frequency due to relativistic effects now z is positive this means red shift is occurring if z is negative it means the object is blue shifting now z can also be a measure of time since the light takes time to reach us so z equals zero indicates present time now the highest red chip recorded is due to the cosmic microwave background where z equals 1089 and the highest z value for a galaxy is 11.1 now we can look at a question uh using the equations in the following example in a laboratory sample the hydrogen alpha spectral line is at a wavelength of 656.258 nanometers in the spectrum from a nearby star this line is observed at a wavelength of 656.315 nanometers how fast is this star moving and in which direction well firstly the wavelength of the star is longer than should be so the star is moving away so you work out your change in wavelength then you say the change in wavelength over wavelength equals b over c you then rearrange it and make v the subject by saying v equals c times by the change in lambda over lambda now as this change in lambda over lambda is a ratio there's no need to change the nanometers into meters as the units will cancel through we then work out our answer is one three seven zero zero meters per second or 13.7 kilometers per second now there are two examples of redshift detected in astrophysics there's cosmological redshift and stellar redshift now cosmological redshift is redshift detected on distant galaxies while stellar redshift is redshift detected on nearby stars now cosmological redshift is detected on all galaxies while stellar redshift is detected on certain stars now the cosmological redshift is proof of the expansion of the universe and further proof of the hot big bang model while stellar redshift allows us to determine the properties about the stars so we'll look at cosmic redshift in the next lesson but we're going to look at stellar redshift now so as well as measuring the redshift of objects from relic moving relative to us we can use redshift to detect rotational motion so for example consider the rotation of a star which we call stellar redshift now when we observe red and blue shift with stars it's due to its due to rotational motion now the most common cause of this are binary stars so this is when two stars orbit around a common center of mass as a pair so a binary star system is a star system with two stars rotating around a common center of mass so for example series a and serious b now we denote the larger and brightest stars a and the smaller ending star is b however how can we make sure that it's a true and binary system and not a phenomena caused by optical inaccuracy we've got to use redshift is when the two stars orbit above one another one will be moving towards us and the other will be moving away from us now the star that's observed to be moving towards the us is going to be blue shifting whilst the star observatory red shifting is moving away from us so in our line of sight one star at one point will pass in front of the other causing an eclipse now a simplified light curve from an eclipse and binary system is shown below now the apparent magnitude scale increases going upwards so the dips in the graph correspond to the light dimming so in this example the two stars have different surface temperatures so you can see that when stefan's law now when the apparent magnitude is 3.30 the system is at its brightest as both stars can be seen now the deeper dips are caused by the coolest star passing in front of the hottest star so the apparent magnitude is 3.60 and the shallower dips are 3.45 when the hottest star passes in front of the cooler now if the light from one star is analyzed in more detail the shift in wavelength of one particular spectral line can be measured as the star follows a circular orbit now the peak at the graph occurs when the star is receding from our point of view at its maximum velocity so the two stars are next to each other the eclipse occurs when the stars move the right angles our line of sight now you can see in this particular diagram that the orbital period of a star is eight days as it repeats itself after eight days now the doppler equation can then be used to calculate the maximum recession velocity which equates to the orbital speed of the star with the following equation as shown with the orbital speed and time period the diameter of the orbit can then be calculated using the circular motion equation where we know circumference of a radius is equal to orbital speed times by orbital period so therefore you can use that to then work out the diameter of the actual orbit now as well as having a binary system you can observe the rotational motion of one star now as the part of the star rotates towards us we will observe that as blue shifting whilst as it rotates away from us we will observe it as red shifting that's a very important idea and that's how we can determine properties regarding stellar rotation of that star about its axis so what have we learned in today's lesson that change in f over f equals v over c z equals delta lambda over lambda but it also equals minus v over c but this only works for v being a lot smaller than c and it's in the optical and radio frequencies and should be able to do calculations on binary stars viewed in the plane of orbit so if we've been successful and learnt in today's lesson we should be able to understand the doppler effect detail examples of red shield found in astrophysics and then understand and apply the formulae for redshift from observations so hope you've enjoyed today's lesson on redshift which is part of the astrophysics topic in aqa a level physics thank you very much for watching and have a 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