What is stethoscope?

What is a stethoscope? Name the principle on which a stethoscope works.

Doctors use stethoscope to hear the sounds produced during each cardiac cycle. The second sound is heard when

How does a stethoscope help a doctor to hear the sound of a patient's heart-beat ?

Which two heart sounds are heard through the stethoscope when placed on the chest? When are these sounds produced respectively?