It is proved that when v<<c , then speed of the light measured in the laboratory frame is , u = c/n + v -v/n^2 .
Given ,
The motion of a transparent medium influences the speed of light .
The water moves with speed v in a horizontal pipe .
Assume that the light travels in the same direction as the water moves .
The speed of the light with respect to the water is c/n
Where n = 1.33 is the refractive index of water .
Let us assume ,
u' be the speed of light in water , in the frame moving with the water .
u' is related to the refractive index of water ,n as :
u'=c/n
where , c is the speed of light .
let , u be the speed of light in water in the lab frame .
Now , u and u' are related as : u = (u'+ v )/(1+ u'v/c^2)
Here v is the speed of water in the horizontal pipe .
we know the value of u' , so by substituting the value , we will get ,
u= (c/n+ v)/(1+cv/nc^2)
u= c/n(1+ nv/c)/(1+v/nc)
(b) We have , v<<c
v/c<<1 .
so , (1+v/nc )^-1 = (1-v/nc)
Now substituting this , we will get ,
u = c/n(1+nv/c) (1-v/nc)
u≈c/n(1+ nv/c-v/cn)
u≈c/n + v - v/n^2
Hence , it is proved that when v<<c , then speed of the light measured in the laboratory frame is , u = c/n + v -v/n^2 .
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Question: The motion of a transparent medium influences the speed of light. This effect was first observed by Fizeau in1851. Consider a light beam in water. The water moves with speed v in a horizontal pipe. Assume the light travels in the same direction as the water moves. The speed of light with respect to the water is c / n , where n=1.33 is the index of refraction of water.(a) Use the velocity transformation equation to show that the speed of the light measured in the laboratory frame isu = c/n (1 + nv/c / 1+ v/nc) . (b) show that for v<<c , the expression from part (a) becomes , to a good approximation , u ≈ c/n + v - v/n^2 .
