The electromagnetic waves of different frequency are classified in different categorizes and used in spectroscopic analysis of materials. I read that their energy is calculated by E= hf or using frequency. I want to know that all the EM waves have the same amplitude or different and is it possible to produce EM of different amplitude such as radio waves of high amplitude.
Dear Veerendra Singh ,
E = h f is valid for single photons (quanta) of an electromagnetic wave. So, a photon of a given frequency has invariably a certain energy.
Macroscopic waves usually have such a high density of photons that a virtually arbitrary amplitude is possible (upper limit given by technological restrictions, of course). For example, the antenna of a transmitter with frequency = 100 MHz and output power = 100 W releases about 1.5*1027 photons per second.
According to the principle of energy conservation, the power density of the radiation decreases as 1/r2, r being the distance from the source of radiation. Since power density S = E x H, with E being the electric component and H the magnetic component of the radiation field, the amplitude of E and H decreases as 1/r.
So, the answer to your question is "yes", as long as we are not dealing with extremely weak radiation.
We mostly talk about frequency of EM ways, why not amplitude ? Is it fixed or not have any importance ?
How could the amplitude be fixed? If you are not blind then you are familiar with light. Can you imagine a world in which each source of light seems to be equally bright, independent of the power of the source and of your distance to the source?
For example, microwave ovens for food warming have a knob with positions marked "400 W", "600 W", "800 W". Each power value corresponds to different amplitudes but the common user is more familiar with watts instead of volt/meter or ampere/meter, and recipes say things like "heat for 5 minutes at 600 W". Furthermore, while the output power of the magnetron is specified by a single number, the amplitude of the wave depends additionally on the geometry of the oven chamber and varies with position.
So, users of microwave devices might talk rarely about amplitude but the developers of these devices certainly do. (If they didn't, microwave ovens would come without rotating plates, for example.)
Joerg Fricke
thanks
Can a high amplitude microwave carry as much as energy as a low amplitude X ray ?
Veerendra Singh
No, radiation with a high amplitude always has a higher energy density than radiation with a low amplitude. As mentioned in my first answer, the power density is S = E x H, so S is independent of the frequency. However, according to E = h f (here, E denotes energy), the energy of a single photon is much higher in the X ray domain than in the microwave domain; accordingly, the number of photons per volume in radiation of a certain power density is much smaller in the X ray domain than in the microwave domain.
Example: Microwave frequency = 3 Ghz, X ray frequency = 3 EHz, S = 1 W/m2 in both cases, consequently the amplitude of E is identical for both frequencies, and so is the amplitude of H. At both frequencies, a volume of 1 m3 contains an energy of S / c = 3.34 * 10-9 Ws; here, c denotes the speed of light.
3 GHz: energy per photon = 1.99 * 10-24 Ws => 1.68 * 1015 photons/m3.
3 EHz: energy per photon = 1.99 * 10-15 Ws => 1.68 * 106 photons/m3.
Quite another matter is the absorption of energy, and the effects of the absorbed energy when radiation meets matter. In most cases, the primary effects are caused by single photons; only in rare cases, the combined energy of two or more photons causes an effect.
The energy of an X ray photon is enough to ionize atoms resp. molecules and to break chemical bonds in molecules but the energy of a microwave photon just suffice to accelerate charge carriers and thereby to heat materials without changing them otherwise. This doesn't change if we increase the energy of microwave radiation; any chemical altering would be a secondary effect caused by increased temperature.
Let us forget definitions, theories, formulas and symbols. Is there any specific device to find amplitude of an electromagnetic wave? No. Then: why should we use a concept of amplitude of an electromagnetic wave? Just to say something is in theory part; and no other purpose.
Thanks Prof. Moorthy. Purpose is that if we hit a rock with 1 kg hammer ten times then it may not break but a single hit of 10 kg hammer can break it. In first case frequency is high and in second case frequency was low but intesity was high .
Yes Prof. Veerendra Singh. We used use the term "amplitude" keeping "intensity" in our mind. E=hf may be "considered for theoretical derivation purposes" as the amount of (proportional to) energy that can be released by a single light ray (wave) or an electromagnetic ray (wave) with frequency f when it collides on a matter for a second (a unit of time), where h refers to the Planck's constant. Fortunately, the researchers worked during the period 1870-1930 were aware of interpretations for theoretical formulas. So, science survives even today.
I like to add the followings to my earlier comment. The amplitude is measured in length, although I have mentioned about intensity in my earlier comment. It should be observed that units in both sides should be verified, when validity of a physics formula is under verification.
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