A crack in a glass vessel often shines, like a mirror, in sunlight. Which of the following phenomenon of light is responsible for this?

total internal reflection

Reason — A crack in a glass vessel contains a thin layer of air. When light travelling inside the glass (denser medium) strikes this air layer (rarer medium) at an angle of incidence greater than the critical angle for the glass-air interface, the entire light is reflected back into the glass. Because 100% of the light is returned, the crack appears bright and shines like a mirror.