I stumbled on this yesterday, as my wife and I were working on a project involving infrared light. This is very simple, will take about a minute, and if you are even mildly into science, you might find it interesting.

Find a remote control. Any remote will do, as long as it is an infrared remote (almost all remotes are).

Aim the remote at your iPhone, press and hold one of the buttons (the volume button, for example) and take a picture with the rear-facing camera (the camera that comes out the back of the iPhone).

Here’s a picture I took:

As you can see from the photo, there’s nothing special here. All you can see is the tail end of the remote.

Now repeat the experiment, but this time use the front facing camera (the one you’d use to take a selfie). Remember to hold down a button on the remote while you take the picture.

Here’s my image:

Note the red dot in this second picture. That’s the infrared beam being generated by the remote. Turns out, the rear-facing camera has an infrared filter built in. The front facing camera does not have this filter.

Not certain, but I believe this filter has been in place on the rear-facing camera since at least the days of the iPhone 4.