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According to Huygens' principle:
Every point on a wave-front may be considered a source of secondary spherical wavelets which spread out in the forward direction at the speed of light. The new wave-front is the tangential surface to all of these secondary wavelets.
Can you explain what actually is meant by a secondary wavelet. Is it just a copy of wavefront pasted at every point of the original wavefront?
Since your question is about secondary wavelets, let's turn away from light for a second. Let's look at water waves instead. If you drop a stone in a pond, spherical waves spread out from the point where the stone hits the water (though we only see the 'circular' waves at the surface). The waves can be thought of as transmitting the disturbance that the stone caused. Hence, whenever there is a "disturbance" at a point, spherical waves will spread out from that point. Huygens postulated that since a wavefront is a line (more accurately a surface) made of 'disturbances' in phase with each other, each point on that wavefront can be thought of as being the center of its own spherical wave, just as the stone's disturbance is the center of a spherical wave. A secondary wavelet is therefore a wave caused by a disturbance that is itself a result of another disturbance. Ie, a disturbance causes a wave (primary) and each point on that wave is a disturbance which causes its own wave (secondary wavelets)
Re. {slightly edited}
https://www.colorado.edu/physics/phys1230/phys1230_fa01/topic14.html
"Huygens’ principle (Christaan Huygens, 1629-1695, published about 1690) describes how a wavefront moves in space. According to this principle, we imagine that each point on the wavefront acts as a point source that emits spherical wavelets. These wavelets travel with the velocity of light in the medium. At any later time, the total wavefront is the envelope that encloses all of these wavelets. That is, the tangent line (or surface) that joins the front surface of each one of them."
The key words here are 'we imagine'. Huygens' Principle as used now is intended to be a model of wave propagation. It is not now used to describe the actual physics ( for that we need strings, springs, air, water, etc. ....). However Huygens originally did intend it to describe the physics--see Treatise on Light https://www.gutenberg.org/files/14725/14725-h/14725-h.htm
To answer your question: A secondary wavelet means an imaginary point source at every point of the original wavefront emitting a spherical wave of the same form (or shape in time) as the original wave.
