An oil film (n = 1.48) of thickness 290 nm floating on water is illuminated with white light at normal incidence. what is the wavelength of the dominant color in the reflected light?

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the answer is the first electromagnet (a). this is because it has more coils around the ferromagnetic material. increasing the number of coils increases the surface area in which the metal interacts with the current in the coil. this enables more atoms in the metal to become aligned hence becoming more magnetized.