This is not a homework question but for a project. We are using a laser diode, one that you would normally find in a laser pointer to transfer information to a photo-transistor for a free space optical communications system. Now I know the power output is around 5mW, but over a distance will we lose power as the light travels towards the photo-transistor? If so, how do you calculate the power at a certain distance, say 25 m or so? Also, could someone also tell me the formula for calculating the spread of the laser light over a distance. This is so we know how much light is missing the photo-transistor and so we can correct this by using lenses to focus the light onto the photo-transistor? Thanks in advance.