1. Light comes in through the lens and is reflected into the viewfinder
SLR cameras use a mirror and pentaprism to reflect the light captured through the camera's lens into the viewfinder. That means that the image you see is exactly the same as the one your camera is taken.
2. Capturing the image
When you click the button, the mirror flips up and exposes the camera's sensor to the light captured by the lens.
3. Ready for your next shot!
As soon as the image has been captured, the mirror flips back into place, ready for your next shot. Simple!