question_answer

Why does an excessively thin film appear black in reflected light?        

Answer:

                For an excessively thin film \[(t<<\lambda ),\] the factor \[2\mu t\cos r\]is negligibly small. The effective path difference between any two successive rays in reflected f system is \[\lambda /2\]. This is the condition for minimum intensity and hence the film will appear dark. But the film appears bright in transmitted light.