# How do we know the wavelength and frequency of electromagnetic wave? [closed]

From my understanding, the frequency and wavelength of a mechanical wave can be observed by the motion of the particles (i.e., as the particles oscillate from their state of equilibrium, the wave propagates through the medium).

But, with electromagnetic waves, how do we measure its frequency and wavelength? If an electromagnetic wave travels across a vacuum, isn't it pretty much invisible to the naked eye?

• "If an electromagnetic wave travels across a vacumm, isn't it pretty much invisible to the naked eye" Not unless it is in the visible spectrum. Are you asking about a specific scenario (like how would one test the frequency of EM waves in a microwave oven)? Or are you wondering more generally, like how do we think about and classify EM waves based on frequency? Jun 8 at 17:41
• Welcome randomHighSchooler! You may want to take a look at this related question: How do you measure wavelength/frequency of light
– Amit
Jun 8 at 17:44
• Here's one way to do it: web.physics.ucsb.edu/~phys128/experiments/interferometry/… Jun 8 at 18:00
• It is not invisible to the naked eye because after it travels through the vacuum it shakes the matter in the eye. Jun 9 at 1:56
• Does this answer your question? How do you measure wavelength/frequency of light Jun 9 at 4:14

Wavelength in the optical can be measured fairly easily with interference experiments, e.g. with a Michelson interferometer. We count the number of minima and maxima as we change the optical path length. The frequency for a given wavelength then follows indirectly from the formula $$f=c/\lambda$$.