I think I have a better understanding. So since R = V/I , because Voltage is at 5V, increasing the resistance will only decrease I. That makes sense and explains the LED.
Ok, but in our context V = 5v - 3.5v = 1.5vdc, not 5volts.
To clarify the resistor was connected to the output of a Infrared Receiver, while the LED was on Pin 13 on the board.
Well then that resistor has no bearing on the led at all! The infrared reciever outputs a digital signal to an arduino input pin, and an input pin requires almost no current at all so the resistor doesn't hurt anything but technically it should not even be used there at all. Wire directly from the receiver signal output to the desired arduino digital input pin.
You mentioned for a green LED the Vf is 3.5V. Is it possible to measure the Vf of my IR receiver or other devices, perhaps using a multimeter ?
Your IR receiver is not an LED, it's an 3 terminal IC that includes a IR photo detector and electronics. It requires no external resistors.
So I'm worried about how you have wired your real LED, the green one, not the 3 terminal IR receiver IC. How is it wired please, both wires go where? and no series resistor used with it? That would be bad bad for your arduino output pin, making it cry in pain and may soon parish.
PS: And Yes you do need to get a multimeter, soon, now, if you plan on building stuff with an Arduino. A multimeter is to a electronic hobbyist what a text editor is to a programmer, you can't do proper work without one.
Lefty