Exposure Control Lesson 1 - The Basics
The #1 problem people have with photographs is exposure. What is exposure? Exposure the amount of light allowed to fall onto the development plane (whether it be old-fashioned film or a charge-couple-device ie. digital).
Cameras control exposure by a combination of two different mechanisms. The first is by shutter speed. The faster the shutter speed the less light there is and of course the opposite holds the more time the shutter is open the more light the greater the exposure. The second mechanism is by aperture that is how much the lens is opened up.
The combination of the aperture and shutter speed gives the total exposure. A common term associated with exposure is stop which is a relative measure of light. One stop is twice or half the light. To increase exposure by one stop you can double your shutter speed or increase the aperture in such a way to double the light admitted. Let us say the radius of your lens is opened to 1" (for instance) and you opened it up to 2" note your exposure increases by four (the old pi-r-squared formula from your geometry lessons).
Next lesson the f-stop.