In a point-to-point optical link the basic way to modulate the optical signal with information is IM - Intensity Modulation - and the way to demodulate is the DD - Direct Detection. The typical optical sources employed are semiconductor devices, and the method to detect optical signals is also semiconductor devices. In both cases, the optoelectronic devices have a linear relation between the optical power and the electrical signal. For a student usually more accustomed to the electronic field, the relation between the power and electric current is a quadratic relation. Moreover, the decibel, or "dB", unit employed to describe SNR, channel losses, and so on, it is a power relation. The way to measure the power it is the same in both electric and optical fields. In both cases it is necessary to measure the current thought a resistor, but the relation between current and power is different in each case. In this paper we present a way to show to the student the linear relation between optical power and electric current in optical sources and detectors. We will use the mathematical relation P=P (I) to represent, by software, the characteristic function of the optical source. By applying an analogical or a digital function to the current we will describe how is made the IM. Only an additional parameter will be studied: the dependence of the characteristic with temperature. In a first exercise the student will analyzed briefly how is detected light on a photodiode, studying the linearity of the generated current I=I(P) with respect to the optical power. No reference to optical wavelength or noise will be made.