As described in the problem introduction, a criterion for a dark band to appear at point p is that the phase difference between light arriving at point p from the top of the slit and light arriving at point p from the bottom of the slit equal 2π. what length of path difference d will give a phase difference of 2π

the device is called “ rocket engine”

show the wavelengths of a wave. show the crests of a wave.

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.