# Why do different forms of electromagnetic waves have different sizes?

I have just watched a Kurzgesagt video about light, in the video it was stated that gamma rays are smaller than hydrogen atoms, and that Radio waves can be as large as asteroids, but I don't understand the link... I Know that gamma rays have higher frequency than radio waves (and vice versa for wavelength) but does this make the size of the whole wave larger, why can't radio waves detect small microbes as well as gamma rays? I would agree with this only if you prove to me that the vertical distance in a radio wave is larger than the vertical distance of a gamma rays. (vertical distance between maximum and minimum points in the wave) If you feel lost just respond to this question, why can't electromagnetic waves detect objects which are smaller than their wavelength (try proving either by formulas or by physics through an analogous experiment and thanks in advance)

• Why are you looking at frequencies, instead of wavelengths that you are invited to contrast? What is this "vertical" stuff? What is the distance of consecutive wavefronts you find in the direction of propagation? – Cosmas Zachos Feb 3 at 19:03

Electromagnetic waves have no "vertical distance" ascribed to them, the way water waves do; this analogy doesn't work with with EM radiation. The peaks in the EM wave are where the intensity of the electric or magnetic field is at a maximum, depending on how you measure the field.

The basic idea here is that all types of EM radiation possess both frequency and wavelength, with high frequencies corresponding to short wavelengths and low frequencies corresponding to long wavelengths. The equation is

(frequency) = (speed of light)/(wavelength)

so, for example, an EM wave with a frequency of 30 megahertz has a wavelength of about 10 meters.

In general, an EM wave has little effect on a material object if its wavelength is longer than the physical dimensions of that object, which is why you cannot use a light microscope to look at individual atoms or molecules and why a metal wall 30 meters across acts like a mirror for a 30 megahertz radio signal.

• My problem here is understanding how can wavelength be related to this, imagine an electromagnetic wave coming towards you, If you increase the wavelength of the wave its energy will decrease so both fields will have lower energy and you will think it became smaller, this means that radio waves should be the ones able to detect very small particles – user597368 Feb 3 at 19:14
• @user597368 When you write "If you increase the wavelength of the wave its energy will decrease so both fields will have lower energy..." you have made an implicit assumption that you are holding the photon rate constant (so that you can apply the rule that photon energy is proportional to frequency), but this is actually an extremely difficult situation to arrange. Part of the problem here is mixing the classical and quantum description of light without being clear about when and where you are using each one. Not that you started that: it is ubiquitous in pop-sci treatments. </rant> – dmckee Feb 3 at 20:28

If the vertical distance is the wave amplitude, it is not the thing to compare because the amplitude determines the wave intensity rather than the wavelength.

A wavelength is some characteristic of the EMF, but it is not very speaking either. For example, one photon of a (nearly) certain frequency contains many-many wavelengths.

Small objects may emit and absorb long waves, remember the heat radiation/absorption.

• This doesn't exactly answer my question sir, let me phrase it this way: Why can't electromagnetic waves detects objects smaller than their wavelength? – user597368 Feb 3 at 19:44
• It is not true. For example, a Hydrogen atom may absorb a wavlength about 21 cm. A small, but complicated object, may have many very different energy levels including close levels leading to an effective interaction with long waves. – Vladimir Kalitvianski Feb 3 at 19:50
• There is, of course, no "vertical distance" in an electromagnetic wave at all. Just a difference in field vectors. – dmckee Feb 3 at 20:23
• @Vladimir Kalitvianski so it's not entirely true, alright thanks – user597368 Feb 3 at 20:47