OK, I admit that this is a short and hopefully sweet Instructable, but it stumped me until just now so, I thought I'd share.
Back in the old days, you could use the camera in your PDA or cell phone to "see" the infrared light that was coming out of a TV remote controller or similar device. I actually did just this when I worked on the MythBusters show when we did the "Beating Police Radar and Lidar" story...you saw my hand holding my old Palm Zire 72 and through the camera's viewfinder, you could see that the infrared LEDs on the license plate frame were actually working and producing light. The studio cameras couldn't see the IR light because they have IR filters. But the cheap camera in the Palm could see the IR light quite well.
Since the iPhone 4 came out, I've been bugged that I was unable to see IR light anymore, and I was very disappointed to learn that the iPhone 4 added an infrared filter to its camera. Although it makes photography look much better, it was no longer useful to me as an IR troubleshooting tool. That explained why I couldn't see IR sources anymore, and I had to borrow my 11 year old son's hand-me-down iPhone 3GS to diagnose questionable IR sources.
I considered buying a cheap VGA resolution keychain digital camera with a viewfinder, but then that would be one more thing to carry around with me.
Today I was trying to use my TV-Be-Gone (thanks, Mitch) in the Delta Skyclub at the Washington DC airport to turn off a TV with loud chattering new people on it. My TV-Be-Gone wasn't working to turn off the TV, so I decided to try to see if it was working or not. I fired up my iPhone 4 and opened the camera app, pointed the camera at the TV-Be-Gone's IR LED, and pressed the button on the TV-Be-Gone. I didn't see any light from the IR LED in the iPhone's viewfinder.
Then it occurred to me to try the forward-facing FaceTim camera. I clicked the camera switch button on the iPhone screen and pointed the FaceTime camera at th still-blinking TV-Be-Gone IR LED, and sure enough I was able to see the light coming out of the IR LED!
The following steps will reiterate the above steps, and will show you how to see infrared light with your standard iPhone 4, and possibly other smartphones and tablets too.
Step 1: Try Using the Rear-Facing Camera to See Light From an Infrared LED
On your iphone, start the Camera app, and point the camera at the LED on your favorite TV remote control.
As you look at the iPhone screen, press some buttons on the remote.
Although the remote is probably putting out a bright infrared light beam, you cannot see it with your eye because your eye is not sensitive to light in the frequency of infrared (around 940nm for a remote control).
Your iPhone's main camera cannot see infrared light, because Apple added a filter over the lens that blocks out infrared light, so the infrared light cannot be seen on the screen.