# Why didn't huygens use hemispherical secondary wavelets instead of spherical secondary wavelets?

According to huygen's principle, every point on a wavefront maybe considered the source of secondary wavelets that spread out in all directions with a speed equal to the speed of propagation of the wave.

Why did he mention that the wavelets spread out in all directions? We don't get a new wave front by constructing a surface tangent to the 'Spheres', rather we get a new wave front by constructing a surface tangent to the 'Hemispheres'.

There is a (much later) refinement of Huygens's principle that modulates the amplitudes of the wavelets by a factor of $(1+\text{cos} \ \theta)$ in which $\theta$ is the angle between the propagation direction of the wavelet and the propagation direction of the wave at the point from which the wavelet originates. This suppresses the parts of the wavelet more and more the further 'backwards' they are going.