Another advantage of the track ball over the conventional mouse is that the buttons can be pushed after removing the hand from the ball. This prevents the cursor from changing position while a button is being pressed. People with unsteady hands (such as common with elderly people, or being awake too long on coffee), or hands with very long fingers, have difficulty using a conventional mouse, but have found easy success using a track ball.
A light pen does not use an internal light as part of the position-detecting signal. If a light source is used, it is merely to help locate the position of the pen tip on the face of the glass envelope of a CRT. A light pen is used with a CRT. It has a sensor that detects the intensity of light received at its tip. The CRT knows where the electron beam is pointing at any instant. When the electron beam points to the light pen position, the light pen detects the emitted light. When you push the button on a light pen, the light pen transmits the signal from the pen to the CRT. The CRT reports the position of the electron beam at the instant it receives the signal from the light pen.