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  1. #1 Photon frequency/wavelength - 
    Forum Senior bill alsept's Avatar
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    How do you find the frequency of a photon when you don't have the wavelength or the energy. Where do you start?

    For example:

    E=hv ??

    How do you find v?

    c/wavelength=v ??

    How do you find wavelength?

    c/v=wavelength ??

    How do you find v?

    Etc, etc

    Isn't it an electromagnetic oscillation that is being measured in Hz? How is that done?
    Thanks


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  3. #2  
    Moderator Moderator Markus Hanke's Avatar
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    How do you find the frequency of a photon when you don't have the wavelength or the energy. Where do you start?
    If you know neither energy nor wave length, then you can't determine the frequency. You need more information.

    Isn't it an electromagnetic oscillation that is being measured in Hz? How is that done?
    Through interference or refraction. The patterns obtained directly depend on the wave length.


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  4. #3  
    Forum Senior bill alsept's Avatar
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    Quote Originally Posted by Markus Hanke View Post
    How do you find the frequency of a photon when you don't have the wavelength or the energy. Where do you start?
    If you know neither energy nor wave length, then you can't determine the frequency. You need more information.

    Isn't it an electromagnetic oscillation that is being measured in Hz? How is that done?
    Through interference or refraction. The patterns obtained directly depend on the wave length.
    So what is actually oscillating?

    How does interference or refraction measure this oscillation?
    Last edited by bill alsept; February 18th, 2014 at 02:21 AM.
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  5. #4  
    Moderator Moderator Markus Hanke's Avatar
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    Quote Originally Posted by bill alsept View Post
    So what is actually oscillating?
    The electric and magnetic aspects of the electromagnetic field :



    How does interference or refraction measure this oscillation?
    It doesn't measure the oscillation directly, just the wave length - once you measure the spacing between the interference zones in a pattern, you can immediately calculate the wave length. It is in principle possible to measure the oscillation directly with a device called an EMF meter.
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  6. #5  
    Forum Cosmic Wizard
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    This was the simplest explanation I could find about how to measure the wavelength of light with a diffraction grating.
    It might not be very elegant but it does present the idea.

    Measuring the wavelength of light

    Class Practical

    A simple and elegant experiment to measure the wavelength of light using a fine diffraction grating.
    Apparatus and materials

    Fine diffraction grating (about 300 lines/mm)
    Metre rules, 2
    Lamp in holder, 12V 36W
    Green filter
    Power supply, low voltage, variable, able to supply 6A

    Health & Safety and Technical notes


    Read our standard health & safety guidance [1]
    Each student pair will need fine diffraction grating, and 2 metre rules.

    Procedure


    Set up the 12-V 36-W line-filament lamp high at one end of the laboratory, so that students can see it clearly. Place a green filter in front of the lamp. If necessary, increase the applied voltage to 14 or 15 V so there is enough light.

    Students follow these instructions.

    a Hold a metre rule straight out in front of you towards the lamp, with the near end of the rule at your face. Hold the diffraction grating against the near end of the metre rule and look at the lamp through it.

    b Ask your partner to place another metre rule, at 90 to your metre rule at its far end (see the sketch).

    c Your partner should hold a pencil vertically above their metre rule and move it along until you see it in the green region of your bright spectrum. Note the distance, x, along your partner's ruler from the pencil to the far end of your ruler.

    d When you have made your observation, record it and change places with your partner so that he or she can take their turn.

    e Divide your measurement x by the length of your ruler, 100 cm. This gives you tan A, where A is the angle between the line of direct white light and the light to the green in the spectrum marked by the pencil. From tan A, use your calculator to find the angle A, and from this find sin A.

    f Use the formula d sin A = wavelength to calculate the wavelength of green light. You will need the value of d, spacing, i.e. distance from one ruling to the next. If the grating has 300 lines / mm then the spacing is 1 / 300,000m.
    Teaching notes


    You must judge whether this experiment is appropriate for your students. If it is likely to represent a burden of strange geometry and unsure measurements, omit it. If they are able to cope, this experiment has the potential to give them a sense of delight and insight. It is a real achievement to make such a small measurement using fairly crude equipment.
    For a large class, set up a lamp at each end of the room, so that half the class can work facing one way, with half facing the other way, each pair as far as possible from their lamp.


    Students may need help with the trigonometry and/or with the calculation of wavelength. To avoid the use of tanA, students could do a scale drawing and find sinA from x / t, as in this diagram.
    Then wavelength = sinA x d, where d is the grating spacing.


    If you supply the value of d, explain where it came from, and make it clear that a mechanical counting during manufacture can supply it.

    If suitable microscopes are available, students could use them to look at their piece of grating and at the graduations on a finely divided ruler. Although students may not be able to measure the grating space, they will certainly see that a direct measurement could be made.

    This experiment was safety-checked in February 2006










    Source URL: Measuring the wavelength of light | Nuffield Foundation
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  7. #6  
    Brassica oleracea Strange's Avatar
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    Quote Originally Posted by bill alsept View Post
    How do you find the frequency of a photon when you don't have the wavelength or the energy. Where do you start?

    For example:

    E=hv ??

    How do you find v?

    c/wavelength=v ??

    How do you find wavelength?

    c/v=wavelength ??

    How do you find v?

    Etc, etc

    Isn't it an electromagnetic oscillation that is being measured in Hz? How is that done?
    Thanks
    This doesn't really answer your question, but it is pretty cool: Measuring the speed of light with chocolate and a microwave oven – Morning Coffee Physics

    Note that there are also ways of measuring frequency (or period) directly; for example, using a tuned filter. So, as with all "facts" in science, the results are confirmed by multiple different lines of evidence.
    Without wishing to overstate my case, everything in the observable universe definitely has its origins in Northamptonshire -- Alan Moore
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  8. #7  
    Forum Isotope
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    Quote Originally Posted by bill alsept View Post
    How do you find the frequency of a photon when you don't have the wavelength or the energy. Where do you start?

    For example:

    E=hv ??

    How do you find v?

    c/wavelength=v ??

    How do you find wavelength?

    c/v=wavelength ??

    How do you find v?

    Etc, etc

    Isn't it an electromagnetic oscillation that is being measured in Hz? How is that done?
    Thanks

    If in free space, you know that the velocity is c, so at least you know the product of wavelength and frequency. Measurement of one then gives you the other. As others have said, there are many ways to perform the relevant measurement. Here are a few:

    Frequency: Your radio does this all the time, The numbers on the dial are the frequency of the incoming radio signal (e.g., 1000 on an AM radio really means 1000 kilohertz, or 1 megahertz). There are things called tuned filters (the electromagnetic analogs of a high-quality bell) that respond strongly only to a narrow range of frequencies. This property of being selectively responsive can be used to measure frequency. The filters in a typical AM or FM radio are able to discriminate between two frequencies that are about 1% apart.

    (Much) higher precision is possible if one uses a known reference signal and compares that to the unknown. Devices known (somewhat ungrammatically) as frequency counters are routinely capable of measurements in the sub-ppm range for commercially available instruments, and quite a few more orders of magnitude of precision in laboratory-grade setups. The current record, I believe, is held by NIST's F2 time standard, with an accuracy of about one part in 10^16, corresponding to an uncertainty of less than one minute out of the age of the universe. Such accuracy is possible by using the energy transitions within atoms themselves.

    Wavelength: You can use interference phenomena to make wavelength measurements, because the distance between peaks or nulls is a function of wavelength. Diffraction gratings are a popular choice at optical frequencies, and electrical transmission lines do nicely at microwave and lower frequencies. The melted-chocolate experiment that Strange mentioned makes use of interference effects that are measurable within an ordinary home microwave oven, where the wavelengths are of the order of 12cm. Wavelength-based measurements are much less accurate than are ones based on the energy transitions within an atom. In metrology, one typically seeks to convert the measurement problem into one that relies primarily on a measurement of time or frequency whenever possible.

    Once you have frequency or wavelength, you're basically done.
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