Radio wave receivers are designed to resonate at a particular frequency. If you look at the response of a resonant device as a function of frequency you get something like (this image is from the Wikipedia article):



This is a rather busy plot, but the point to take away is that the response of the resonant system is greatest when the frequency matches the resonant frequency. The more resonant the system is (the higher its Q factor) the more sharply peaked the response is.

So if you want your radio to pick up just 98.4MHz (the frequency of the radio station I'm listening to at the moment) you tune your receiver to resonate at 98.4MHz. It will still pick up other radio frequencies as well, but because of the resonance it's far more sensitive to the resonant frequency than to the other frequencies.

How are electromagnetic waves differentiated?
I am planning to serve images on a CDN using a CNAME: images.mysite.com. I am doing this because CDNname.mysite.com makes ugly URL's and MAY be bad for SEO (debatable).

Will Google crawl the subdomain (only storing images and nothing on the main domain will link to it).

If so, would a simple robots.txt be suitable? Is this even possible seeing that the subdomain is ONLY serving images, JS, CSS i.e. not HTML?

Thanks
