Sentence:  The photochemical reaction cells consisted of 10 mm. i.d. Pyrex tubing , 5.5 cm. long , <WSD>diffraction</WSD> effects being minimized by the fact that the light passed through only liquid glass interfaces and not gas glass interfaces . 
Meaning of the word marked with <WSD> tags:  when light passes sharp edges or goes through narrow slits the rays are deflected and produce fringes of light and dark bands