where in the second part we have included the induced dipole moment (in MKS units) of a spherical dielectric particle: p = α α E ( r , t ) = 4 π π n 1 2 ϵ ϵ 0 a 3 ( m 2 − − 1 ) / ( m 2 + 2 ) E ( r , t ) , where a is the particle radius, n 0 is the index of refraction of the particle and m = n 0 / n 1 is the relative refractive index between the particle and the medium. The square of the magnitude of the electric field is equal to the intensity of the beam as a function of position. Therefore, the result indicates that the force on the dielectric particle, when treated as a point dipole, is proportional to the gradient along the intensity of the beam. In other words, the gradient force described here tends to attract the particle to the region of highest intensity. In reality, the scattering force of the light works against the gradient force in the axial direction of the trap, resulting in an equilibrium position that is displaced slightly downstream of the intensity maximum. Under the Rayleigh approximation, we can also write the scattering force as