OPTICS and WAVES--concepts and problems for JEE mains-24 feb-14

Concept 1: जब भी लाइट  का reflection ऑप्टिकली denser माध्यम  से होता है  तब फेज अचानक pi से change  हो जाता है  यानि कि अचानक आधी  wavelength के बराबर  path में change आ जाता  किन्तु यदि रिफलेक्सन  rarer medium से हो तब अचानक कोई फेज नहीं बदलता है !

Concept 2-: When light moves in a medium other than air, while considering its interference with some other wave travelling in air, the actual path of the wave travelling the other medium is  not directly taken  but it is modified  to a value  which is  the product of the refractive index of the medium and the actual path.

Example- 

Let light ray falls normally on a plane slab of thickness t  and refractive index 1.5. The slab is placed in the air. The light is partly reflected and remaining part is refracted and goes to other face where it gain is partly reflected and comes back and finally interfere with the wave which was reflected at the first face only. Wavelength of the light is  y in air. What is the phase difference between the waves interfering after reflections  from the tow faces ?

Under what conduction destructive interference  takes place between the reflected lights ? 

ANS :  (2p / l) ( 3 t ) - p   will the net phase difference between the interfering waves 

So   we get 3t- l/2 = (odd) l/2 ; Placing different values of  odd we can get different  possible value of  t  for a given wavelength l.