A) 0.30mm
B) 0.40mm
C) 0.53mm
D) 450 urn
Correct Answer: A
Solution :
Key Idea: When light passes from one medium to another, its wavelength changes. In Youngs double slit experiment if distance between coherent sources is d, and distance between screen and source is D, then fringe width W is given by \[W=\frac{D\,\,\lambda }{d}\] ?. (i) where \[\lambda \]is wavelength of monochromatic light source. Also, from Huygenss principle \[\mu =\frac{\lambda }{{{\lambda }_{w}}}=\] refractive index In water \[W=\frac{D\,{{\lambda }_{w}}}{d}\] .... (ii) Dividing Eq. (i) by Eq. (ii), we get \[\frac{W}{W}=\frac{\lambda }{{{\lambda }_{w}}}\] \[\Rightarrow \] \[W=\frac{{{\lambda }_{w}}}{\lambda }W\] \[=\frac{W}{\mu }\] Given, \[W=0.4\,mm,\,\,\mu =4/3\], we have \[W=\frac{0.4}{4/3}=0.3\,\,mm\]
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