Optics Series Lecture, Lecture – XIV, XV, XVI.
“Color of thin films, Newton’s rings, Lloyd’s mirror and Phase changes during reflection”
These lectures were delivered on 16th February, 21st February and on 17th March. The lecture sessions were of 1 and 1/2 hours. The lectures were delivered to both Physics honors as well as Physics elective students on different days.
We have previously discussed what is interference and what is wave-front splitting and amplitude splitting interference. We have also discussed in much details two wave-front splitting interference viz. Young’s double slit interference (Lecture – IX) and Fresnel’s bi-prism (Lecture – XI). Today we will discuss one more wave-front splitting interference namely Lloyd’s mirror interference before moving onto the amplitude splitting interference of the Newton’s Rings. Also we will discuss two interesting and related concepts; i. Phase change on reflection and ii. Color of thin films.
Interference in Lloyd’s Mirror.
The Lloyd’s mirror is a set-up for wave-front splitting interference. Here two rays one of which undergoes reflection from a reflecting surface like a mirror meet up at the point of observation and subsequently interfere. The interference pattern that is produced in a Lloyd’s mirror interferometer is complimentary to the pattern produced in a Young’s interferometer. That is because of an additional phase change of ± π during reflection.