When it comes to detecting light and movement with microcontrollers, one has quite a bit of options, after explaining each one, I will compare them for you at the bottom of this pots
LDR Light Dependent Resistor
This is the simplest one, and it only costs a few cents
Phototransistor
A photo transistor is in my oppinion a great tool to use with microcontrollers, very fast response time (Slightly slower than a photodiode), and can be used with the digital pins, the reason is that it works exactly like a transistor, but instead of enabling the current by providing voltage to the base, the current is provided from light !
So, if you were to connect the Emitter to ground, and the Collector to the digital pin, they would get connected when light is detected, that is it
Photo interrupter
A photo interrupter detects the presence, absence, or movement of an object by sensing when a light beam between an emitter and a receiver is blocked.
This is basically something you would find in your inkjet printer ! or in a factory (Conveyor belt), or most commonly, on en elevator (Door opens when an object is in the way)
The transmitter and receiver work together either side by side or opposing each other, when they are side to side, a mirror reflects the light onto the receiver
If you are trying to detect a red light or a light of the wavelength close to that of the interrupter, odds are that you can use the receiver on its own without the LED (Transmitter), simply do not power it up !
photodiode
PIR (Movement) sensors
PIR Sensors such as the HC-SR501 can detect movement, but they requier more than 1 component, the sensor itself, the plastic cover for the sensor (Pir Lens Cover creates a FRESNEL LENS), and an IC that amplifies the signal and analyzes it !