First: I've found these questions (Here and Here) about this subject, but i don't think any of them will help me in this case since it was about measure it when you have a beam of light.

I'm looking for ways of determining the wavelength of a LED lamp, whose light disperses in all directions, and not in a "focused" way that can be pointed somewhere (to use other experimental procedures).

Is there a way of measuring it? I tought of measuring the frequency, and then determine the wavelength from it, but i also don't know how to measure frequency.

Question: How to measure wavelength or frequency of a light source that can't be pointed somewhere?

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    $\begingroup$ You can certainly collimate an LED with a lens. Or you can use slits to select a fairly narrow cone of light. Or, just be far enough away and it will start approximating a plane wave. Then use the appropriate technique. $\endgroup$
    – Jon Custer
    Mar 2, 2017 at 14:39
  • $\begingroup$ You need a spectrometer of some kind. A lot depends on what you have available and how comfortable you are with DIY experiments, and how accurate you want the result to be. One can get a rough idea very inexpensively using a replica diffraction grating and a tape measure. But it will be hard to give a detailed answer without knowing more. You cannot measure frequency. It's much too high. $\endgroup$
    – garyp
    Mar 2, 2017 at 14:41

1 Answer 1


There are several ways. I would say the spectroscope is the easiest. They are about 7 dollars. Here's a photo of one. I took a pic of what you'll see inside too.enter image description hereenter image description here The numbers represent hundreds of nanometers, so a color that is under a 5 would have a wavelength of 500 nm. The photo doesn't quit represent the actual image. If you point the scope at your LED you can read its wavelength. Once you have the wavelength you can then calculate the frequency.


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