When two or more waves superimpose over each other, resultant intensity is modified. The modification in the distribution of intensity in the region of superposition is called as interference. Interference fringes are alternately bright and dark patches of light obtained in the region of superposition.
There is no loss of energy in interference phenomenon, only redistribution of energy takes place. The energy absent at dark places is actually present in bright regions.
When a plano-convex surface is placed on a glass plate, an air film of gradually increasing thickness is formed. The thickness of the air film is zero at the point of contact and gradually increases outwards from the point of contact.
When the air film is illuminated by monochromatic light normally, alternate bright and dark concentric circular rings are formed with dark spot at the centre. When viewed with white light, the fringes are coloured shown in the wrapper of the text book. Fig 5. A monochromatic source of light S is kept at the focus of a condensing lens L 1.
The parallel beam of light emerging from L 1 falls on the glass plate G kept at 45 o. The glass plate reflects a part of the incident light vertically downwards, normally on the thin air film, enclosed by the plano convex lens L and plane glass plate P.
The reflected beam from the air film is viewed with a microscope. Alternate bright and dark circular rings with dark spot as centre is seen. If t is the thickness of the air film at a point on the film, the refracted wavelet from the lens has to travel a distance t into the film and after reflection from the top surface of the glass plate, has to travel the same distance back to reach the point again.
A ray AB incident normally on the system gets partially reflected at the bottom curved surface of the lens Ray 1 and part of the transmitted ray is partially reflected Ray 2 from the top surface of the plane glass plate. The rays 1 and 2 are derived from the same incident ray by division of amplitude and therefore are coherent. Ray 2 undergoes a phase change of p upon reflection since it is reflected from air-to-glass boundary. Cite this Simulator:. Aim: To revise the concept of interference of light waves in general and thin-film interference in particular.
To determine the wavelength of the given source. Thin film interference: A film is said to be thin when its thickness is about the order of one wavelength of visible light which is taken to be nm. In the above figure the rays r 12 and t 21 interfere and results in a constructive or destructive interference depending on their path differences, given as, constructive interference destructive interference Where, refractive index of the medium 2 and the order of interference.
Consequently, the interfering waves at the centre are opposite in phase and interfere destructively. Class Attendance. Important Questions. MST-I Result. Exam Schedule. MST-I Nov Amita Mourya. Engineering Physics by Dr. Amita Maurya, Peoples University, Bhopal.
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