An endoscope is employed by a physician to view the internal parts of a body organ. It is based on the principle of:
Total internal reflection
An endoscope is based on the principle of total internal reflection.
The endoscope is made of two or three main optical cables. From one or two of the cables light goes down into the patient's body and another one carries reflected light (the image of the patient's body) back up to the physician's eyepiece.
And, in this way, the physician can see the internal organs of the body through the endoscope.