@Ali Suppose the distance from the ceiling to the centre of the pulley is h, the length of the string is 𝓁 and the length of the spring is s, then 𝓁 + s = 2h.
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and why charged case, in the beginning, what if the instantaneous voltage of AC and the voltage of capacitor are same, then no current should flow, right?
Except in very simple cases the Huygens constructions aren't very useful, and they aren't used much by working physicists.
For example trying to explain how light propagates through a lens using the Huygens' construction would be very difficult. I don't think I have ever seen it done.
You just need to know Snell's law, and also that the angle of reflection is equal to the angle of incidence. (Both of these can be proved from a Huygens' construction though it's tedious).
And the wavefront is perpendicular to the light ray.
So for example if you look at the convex mirror the angle of incidence changes at different points on the mirror, so the angle of reflection changes. That's why the reflected light rays spread out at different angles.
But if you look at the ray B it strikes the mirror at an angle θ to the normal. We can work out θ using basic geometry, though it's a little tedious, but we probably don't need to know its exact value. It's enough that we can see from the diagram that θ increases as we move away from the centre.
A spherical wave is also a collection of rays, though this time the rays come from a single point and they radiate out from that point instead of being parallel. Do you want me to draw this?
Electrons can collide and bounce off each other because they are charged and their negative charges repel. But photons are uncharged so they do not interact with each other at all.
The Huygens' construction is important here. We assume the slit width is very small, and then it behaves like a point source so the light rays travel radially away from it.
Like that.
A "source" is anything that emits light rays i.e. if we follow the light rays back to see where they came from, then the point where all the light rays intersect is the source.
All the red rays I've drawn come from the slit, so the slit is their "source".
Quick question: It is known that $F=ma$ only holds for constant mass, otherwise one would have to use $$F=\dot p=\frac{\mathrm dm}{\mathrm dt}+ma$$ Is this any different to using $$F=m(t)a$$ where $m(t)$ describes mass as a function of time? And in both cases, is there anything to pay attention to when the force $F$ depends on the mass and thus time for non-constant mass (say gravity, for example)?
