The Principle of Feedback is one of Norman´s three usability principles. The other two are the Principle of Visability and the Principle of Affordance.
The principle of feedback is something that most appliances designers should be aware of. Feedback allows user to observe if his or hers actions have been accomplished. As with most usability principles, when used correctly, the user wont even notice that the principle is in use. And if used incorrectly or not at all, will needlesly aggravate the user.
Take for example my TV channel provider.
As you can see, there is no difference. One would think if wouldn’t be too taxing to make the light turn green when the device was turned on, and red when turned off. This means I have no way of knowing if this device is turned on or not, when the TV is turned off.
In general , it seems that the designers of this appliance have never heard of the principle of feedback. The remote control that controls the device does not emanate a small light when button is successfully pressed as most remote controls do. The device can also control the volume of the TV, but unlike every single other such controls I´ve ever seen, there is no visible indication on the TV screen of the volume level when using the control. Volume level can of course be heard, but that is a poor indicator. With no visible feedback of volume level I as a user feel that I have less control over the device, which is aggravating.