Mobile phones, computer networks, remote controls, satellite links all involve communications technologies which make use of electromagnetic radiation as a means of carrying information. This booklet looks at how these ideas can be illustrated with two specially developed resources: the SEP Optical Transmission Set and the SEP Analogue/Digital Transmission Set. The first of these uses visible light to demonstrate some basic principles of sending signals using electromagnetic radiation, such as transmission and detection, attenuation, noise and the use of optical fibres. The second set uses infrared radiation to demonstrate how carrier waves can be used for analogue and digital signals. Both sets can be used to model the way that radio waves are used to transmit information about audio signals.