from sum no  21 hcv

a narrow silt s transmitting light of wavelenght  is placed a distance d above the mirror as hown . thr light coming directly from the silt and that coming after reflection interfere at  a screen  E1 at a distance d from the silt

At what distance from the silt does the first maximum

i hv used the concept that after reflection we wil assum the source at a distance d behind the mirror so the distance between the silts is 2d so we can calculate by using phase change=lambda/2 and youngs formula   

is my concept right

sum no 29

in a youngs double silt experiment th seperation between the silts is 2 MM the wavelength of light is 600nm and the distance of the screen from the silts =2 m if the intensity at the central maximum is 0.20 w/m^2 what will be the intensity at  a pt .5 cm away from the centre alonm the widths

i hv solved the sum sing the simple concept of phase change by delta x 

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and intensity=41(1+cosphase cahnge)

pls solve fast and help me

and if i am wrong suggest alternative solutions