what would be the best sensor to use

I am very new to Arduino with good experience in c programming, after going through a lot of reading i decided to do my first simple project.
the project is a simple cash box that has 3 different slots for each different coin, when the coin is put in the slot and goes inside the box the sensor should detect it and count it as (1) then the program will add (1 * denomination) to the slot counter, an LCD should display the slot total inside the box.
my question is what would be the sensor to use to detect the coin traveling this very short distance from the slot to the bottom of the box.

thanks for any feed back

I would use a slot photo interruptor:

There is an IR LED on one side and a phototransistor on the other side. When an object passes through the slot it blocks the light from the LED from hitting the phototransistor.

You can get them mounted on various different styles of modules that simplify the wiring for you:

Thank you very much, I didnt even know that there is such a sensor, I will give it a try.