Question on feasability

I am wanting to use arduino and am new to programming... Here is what i want to accomplish

Basically this is what I want to do

Have a device sit and wait for something to come in its view ( it will be between 30-60 CM from the device). Once it is in view send a static alarm to another arduino and wait for the something to leave its view once it does send another alarm to the other arduino until a reset button is pushed.

What type of sensors should I use? Where do I begin?

I have a few arduino r3's and sensor kits. I have done some basic programming to get the device to spit out centimeters to serial ( the example codes out there).

Hi,

wait for something to come in its view ( it will be between 30-60 CM from the device).

You need to define that the "something" or "somethings" is..

Is it a warm-blooded animal?

How big or small?

How fast does it move?

Tell us what you want to accomplish...

Sorry, should have been more specific!

It is a piece of metal that looks like a hook about 4 inches in width

It looks like the attached image , however there can be 2-3 inches between the bar and the hook.

screenshot.png