Huygens' belief was that every wave is made up of smaller wavelets, which are basically circles. I was wondering, is the radius of these wavelets equal to the length of the wavelength of its wave? I have another related question: If a wave is passing through a slit, what is the maximum length the slit can have so that the waves-fronts coming out of the slit are complete circles? If the radius of a wavelet is equal to the wavelength of its wave, then doesn't that mean that the maximum the slit can be is twice the wavelength? It think it is twice the wavelength because if the middle of a slit is the center of a wavelet, then the circumference of the wavelet will cover the slit openings so long as the slit is smaller than the diameter, which will be twice the wavelength if the radius of the wavelet is the wavelength. Sorry if the question is confusing. Here is a picture to help. The slit here is twice the wavelength.