The science of light . 33, ABrepresents a ray of lightincident on the thin film,magnified greatly in thediagram, at B. Part ofit is reflected along BCand the other part re-fracted along BD. At Dpart of the ray is reflected along DE, and the otherpart is transmitted along DG. Of the ray DE partis reflected, and the other part is refracted alongEF. The two parallel rays BC and EF interferewhen they are brought to a focus at the same placeby any lens such as that in the eye. For the reflectedray at B has only to travel from B to H in the airin order to reach the wave-front EH, whilst the rayBDE h
The science of light . 33, ABrepresents a ray of lightincident on the thin film,magnified greatly in thediagram, at B. Part ofit is reflected along BCand the other part re-fracted along BD. At Dpart of the ray is reflected along DE, and the otherpart is transmitted along DG. Of the ray DE partis reflected, and the other part is refracted alongEF. The two parallel rays BC and EF interferewhen they are brought to a focus at the same placeby any lens such as that in the eye. For the reflectedray at B has only to travel from B to H in the airin order to reach the wave-front EH, whilst the rayBDE has to travel the distance BD and DE in the filmin order to reach the same position. There is thereforea path difference between the two rays which is greaterthe greater the thickness of the film and which alsoincreases with the obliquity of the ray. We shalltherefore get reinforcement or darkness according asthe path difference is a whole number of wave-lengths oran odd number of half wave-lengths. Since the different. 52 THE SCIENCE OF LIGHT colours in the spectrum are merely waves of differentlengths the different colours will be reinforced withdifferent thicknesses of film and with different obliquities,and so the well-known coloured effects of soap filmsand other thin films are produced. Newtons Eings.—Newton investigated one of thebest-known examples of the interference in thin films ;and it is ironical that his name should be specially con-nected with what is so essentially a wave placed a convex lens of very slight curvature on apiece of plate-glass and viewed it by reflected coloured circular rings were seen immediatelysurrounding the point of contact between the lens andthe plate and gradually shadingoff into uniform illuminationfurther away from the monochromatic light, asodium flame, be used, alter-nately dark and bright ringsare observed which becomenarrower and closer the furtherFig. 34. they are from the centre, but wh
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Keywords: ., bookcentury1900, bookdecade1910, booksubjectlight, bookyear1910
