How can a doctor see blood vessels on the lining of the eye when he shines a light through the eye?

When a doctor shines a light into the eye, the light illuminates the retina and the underlying blood vessels, allowing them to be seen. The retina is highly vascularized, and the contrast between the light and the darker surrounding tissues makes the blood vessels more visible. Additionally, the use of specialized tools like a fundus camera or an ophthalmoscope enhances the view of these structures, enabling the doctor to assess the health of the eye and detect any abnormalities.