https://padlet.com/weirdguitar/t0w664j098dg42sf Please leave any comments or questions that you might have about each lecture. These can be open questions about the course material or questions specifically for me. Please feel free to answer each other's questions if you would like to as this is intended to be a highly interactive resource! en-us 2021-09-22 14:21:39 UTC 2026-01-18 15:51:40 UTC hello@padlet.com weirdguitar https://padlet.com/weirdguitar/t0w664j098dg42sf/wish/1760194661 Alex: A few people have asked why we didn't solve for sin(wt) = 0 to find the positions of the nodes. The reason for this is that, in this case, we're interested to find the points x for which the displacement is zero for all time. Thus, these are the values of x that make the displacement zero, regardless of the time t (in which case we can consider sin(wt) to be something that's possible non-zero). In this case this requires us to solve for x such that sin(kx) = 0 (referring to the example that we worked through during lecture 2)

]]> 2021-09-22 14:21:39 UTC https://padlet.com/weirdguitar/t0w664j098dg42sf/wish/1760194661 weirdguitar https://padlet.com/weirdguitar/t0w664j098dg42sf/wish/1760194665 How come 2 pulses with amplitudes A and -A travelling in opposite directions on a string (assuming no energy loss)  pass through each other rather than just cancelling out (which would also be possible by momentum conservation)? ]]> 2021-09-22 14:21:39 UTC https://padlet.com/weirdguitar/t0w664j098dg42sf/wish/1760194665 weirdguitar https://padlet.com/weirdguitar/t0w664j098dg42sf/wish/1760194676 To derive the standing wave function we need to write down the wave function for an incoming wave and its reflection. The derivation only works if the reflected wave has undergone a phase shift of pi. Why does the reflected wave change phase? Thank you!]]> 2021-09-22 14:21:39 UTC https://padlet.com/weirdguitar/t0w664j098dg42sf/wish/1760194676 weirdguitar https://padlet.com/weirdguitar/t0w664j098dg42sf/wish/1760194680 I followed the derivation guide but it doesn't seem to follow all the way through to the end like in the book (I may have not understood well). Either way, from 15.18 to 15.19, how does the LHS become the second partial derivative as x approaches zero? Thanks!]]> 2021-09-22 14:21:39 UTC https://padlet.com/weirdguitar/t0w664j098dg42sf/wish/1760194680 weirdguitar https://padlet.com/weirdguitar/t0w664j098dg42sf/wish/1760194683 Hi there! Thanks for your question :)

I originally typed an answer to this then realised it was rubbish so I just recorded myself instead, I hope this helps :)]]> 2021-09-22 14:21:39 UTC https://padlet.com/weirdguitar/t0w664j098dg42sf/wish/1760194683 weirdguitar https://padlet.com/weirdguitar/t0w664j098dg42sf/wish/1760194689 Are these two derivations included in the first waves and optics lecture examinable?]]> 2021-09-22 14:21:39 UTC https://padlet.com/weirdguitar/t0w664j098dg42sf/wish/1760194689 weirdguitar https://padlet.com/weirdguitar/t0w664j098dg42sf/wish/1760194695 As light's wavelength is in the order of hundreds of nanometres, wouldn't a tiny error in the distance between A and each mirror make a huge difference, as even 250nm might cause the phase difference to be an entire wavelength?

In that case, how can we assume that both distances are exactly equal? Was the original experiment done so that they expected a phase difference, and then checked, say, 3 months later to see if the phase difference was changed?]]> 2021-09-22 14:21:39 UTC https://padlet.com/weirdguitar/t0w664j098dg42sf/wish/1760194695 weirdguitar https://padlet.com/weirdguitar/t0w664j098dg42sf/wish/1760194699 Is the single slit diffraction 'envelope' over the interference patterns for more than one slit an approximation? My reasoning is: shouldn't the diffraction pattern for each slit have minima/ maxima in slightly different places as the slits themselves are in slightly different places?]]> 2021-09-22 14:21:39 UTC https://padlet.com/weirdguitar/t0w664j098dg42sf/wish/1760194699 weirdguitar https://padlet.com/weirdguitar/t0w664j098dg42sf/wish/1760194700 All slits are a common distance apart doesn't imply that the angle between each slit uses m(lambda)=dsin(theta). So, is the need for the angle to be the same between all phasor arrows a consequence of a small angle approximation? Thank you!]]> 2021-09-22 14:21:39 UTC https://padlet.com/weirdguitar/t0w664j098dg42sf/wish/1760194700 weirdguitar https://padlet.com/weirdguitar/t0w664j098dg42sf/wish/1760194703 In slide 9 where the compressions and rarefactions are overlayed with the sinusoidal wave, I was confused about the wave drawn as I would have thought the crests and troughs match the compression and rarefaction. I wanted to understand the shape of the curve. Thanks

]]> 2021-09-22 14:21:39 UTC https://padlet.com/weirdguitar/t0w664j098dg42sf/wish/1760194703 weirdguitar https://padlet.com/weirdguitar/t0w664j098dg42sf/wish/1760194707 What is the difference between v and v subscript y?]]> 2021-09-22 14:21:39 UTC https://padlet.com/weirdguitar/t0w664j098dg42sf/wish/1760194707 weirdguitar https://padlet.com/weirdguitar/t0w664j098dg42sf/wish/1760194708 I am confused as to how standing waves can form in completely open and half-open pipes. What are the incoming sound waves reflecting off? I understand that the incoming and reflected sound waves interfere  to produce the standing wave, but how is the incoming wave reflected if there is an 'open end', isn't there no boundary to reflect off? ]]> 2021-09-22 14:21:39 UTC https://padlet.com/weirdguitar/t0w664j098dg42sf/wish/1760194708 weirdguitar https://padlet.com/weirdguitar/t0w664j098dg42sf/wish/1760194715 When deriving the full intensity pattern for the single slit in video 3 around 3 minutes the phasor arrows of the different segments are compared, the arrow at top has phase 0 and as we go down the arrows rotate anticlockwise, why is it anticlockwise and not clockwise?  The ray at the top has the longest path hence his phase should be the 'most advanced', shouldn't it? And the other arrows should lag behind the top arrow and be slightly rotated in the clockwise direction, where is my mistake?]]> 2021-09-22 14:21:39 UTC https://padlet.com/weirdguitar/t0w664j098dg42sf/wish/1760194715 weirdguitar https://padlet.com/weirdguitar/t0w664j098dg42sf/wish/1760194718 Does the light which is allowed through the polarizing filter get diffracted like how light is diffracted by a diffraction grating? The series of parallel wires in a polarizing filter seems very similar to a diffraction grating for me.]]> 2021-09-22 14:21:39 UTC https://padlet.com/weirdguitar/t0w664j098dg42sf/wish/1760194718 weirdguitar https://padlet.com/weirdguitar/t0w664j098dg42sf/wish/1760194726 Hi. The Object-Image Relations for spherical reflecting/refracting surfaces both relied on the paraxial approximations. However, if the light rays under consideration were at quite a large angle with respect to the Optic Axis, would it be possible to derive a (most likely fairly nasty) relationship between s and s' ?]]> 2021-09-22 14:21:39 UTC https://padlet.com/weirdguitar/t0w664j098dg42sf/wish/1760194726 weirdguitar https://padlet.com/weirdguitar/t0w664j098dg42sf/wish/1760194728 Hi, I can't for the life of me get my head around the refraction derivation and how to get to the final equation (na/s + nb/s' = nb - na/R). Is there any chance you could go through it?]]> 2021-09-22 14:21:39 UTC https://padlet.com/weirdguitar/t0w664j098dg42sf/wish/1760194728 weirdguitar https://padlet.com/weirdguitar/t0w664j098dg42sf/wish/1760194730 How I can create a convex/concave lens on this app?]]> 2021-09-22 14:21:39 UTC https://padlet.com/weirdguitar/t0w664j098dg42sf/wish/1760194730 weirdguitar https://padlet.com/weirdguitar/t0w664j098dg42sf/wish/1760194734 Were there any changes that had to be made to Huygen's theory, as didn't he propose it with light as a longitudinal wave rather than transverse? ]]> 2021-09-22 14:21:39 UTC https://padlet.com/weirdguitar/t0w664j098dg42sf/wish/1760194734 weirdguitar https://padlet.com/weirdguitar/t0w664j098dg42sf/wish/1760194737 I understand Huygens principle and how it can help us pick up how waves propagate, but not exactly how I'd use it in a question. In what way would we be expected to apply the principle, if not just as a way to understand waves?]]> 2021-09-22 14:21:39 UTC https://padlet.com/weirdguitar/t0w664j098dg42sf/wish/1760194737 weirdguitar https://padlet.com/weirdguitar/t0w664j098dg42sf/wish/1760194740 2021-09-22 14:21:39 UTC https://padlet.com/weirdguitar/t0w664j098dg42sf/wish/1760194740 weirdguitar https://padlet.com/weirdguitar/t0w664j098dg42sf/wish/1760194745 I'm having difficulty fully understanding how the following change can be applied generally:
Vs->Vs-VL
I understand how this change works when the source and listener are moving in the same direction, but am struggling to make sense of the sign on VL when the source and listener move in opposite directions, wouldn't Vs-> Vs+VL in this case?

]]> 2021-09-22 14:21:39 UTC https://padlet.com/weirdguitar/t0w664j098dg42sf/wish/1760194745 weirdguitar https://padlet.com/weirdguitar/t0w664j098dg42sf/wish/1760194758 I do not understand why the image is erect, should it not be like lecture slide 17 and be flipped?]]> 2021-09-22 14:21:39 UTC https://padlet.com/weirdguitar/t0w664j098dg42sf/wish/1760194758 weirdguitar https://padlet.com/weirdguitar/t0w664j098dg42sf/wish/1760194761 Why is s' negative? shouldn't it be positive since it's on the same side as the reflecting ray.

]]> 2021-09-22 14:21:39 UTC https://padlet.com/weirdguitar/t0w664j098dg42sf/wish/1760194761 weirdguitar https://padlet.com/weirdguitar/t0w664j098dg42sf/wish/1760194763 Hey, I don't really understand the derivation of the lens maker's equation, mainly, why the image formed by the first surface of a lens serves as the object for the second surface? I don't see how this is acts as an object from the diagram ]]> 2021-09-22 14:21:39 UTC https://padlet.com/weirdguitar/t0w664j098dg42sf/wish/1760194763 weirdguitar https://padlet.com/weirdguitar/t0w664j098dg42sf/wish/1760194764 With the example considering the image of an extended object in a concave mirror, why is there only one s' if P' and Q' are at different distances from V?]]> 2021-09-22 14:21:39 UTC https://padlet.com/weirdguitar/t0w664j098dg42sf/wish/1760194764 weirdguitar https://padlet.com/weirdguitar/t0w664j098dg42sf/wish/1760194767 In the interpret section of the derivation guide you have asked the question "3. What if the radii of curvature of the lens are both the same, does this
mean that the focal length must be infinite?". I have tried to reason an answer to this question by looking at the equation and I think that if the radii are the same, the focal length is infinite, but this feels wrong intuitively. What is the correct answer to this question and could you explain why?]]> 2021-09-22 14:21:39 UTC https://padlet.com/weirdguitar/t0w664j098dg42sf/wish/1760194767 weirdguitar https://padlet.com/weirdguitar/t0w664j098dg42sf/wish/1760194769 Is there a reason other than for simplification that another small angle approximation is used to equate the expressions for sin x and tan x instead of just using the pythagorean identity to equate these expressions? Is there a reason I'm missing that would invalidate this?

(by x I really mean theta, it's just easier to write x)

Thank you!]]> 2021-09-22 14:21:39 UTC https://padlet.com/weirdguitar/t0w664j098dg42sf/wish/1760194769 weirdguitar https://padlet.com/weirdguitar/t0w664j098dg42sf/wish/1760194772 In part c of the worked examples on phase difference (the 2nd video) the question specifies that the second wave is at zero and rising. I thought this would mean pi/2 is not correct, but 3pi/2 is? I drew the phasors that just confirmed what I thought. I got the general answer to be 3pi/2 + 2kpi. Please could you explain how the other values work too? I would understand if it was negative pi/2. Thank you, Liz ]]> 2021-09-22 14:21:39 UTC https://padlet.com/weirdguitar/t0w664j098dg42sf/wish/1760194772 weirdguitar https://padlet.com/weirdguitar/t0w664j098dg42sf/wish/1760194779 At around the 11 minute mark, the slide shows that I=4Ep^2 and we know that  Ep=2Ecos(phi/2)
Therefore wouldn't this mean that for intensity we would get I=4(2Ecos(phi/2)^2=16E^2cos^2(phi/2) and not just 4 as the coefficient in place of 16.
Thank you!]]> 2021-09-22 14:21:39 UTC https://padlet.com/weirdguitar/t0w664j098dg42sf/wish/1760194779 weirdguitar https://padlet.com/weirdguitar/t0w664j098dg42sf/wish/1760194783 Hello, could you clarify what physically Q and Q' are? I understand which objects they are the images of but don't understand how you would 'see' Q or how the images would appear to a viewer. Also I appreciate the cat content. Thank you :)]]> 2021-09-22 14:21:39 UTC https://padlet.com/weirdguitar/t0w664j098dg42sf/wish/1760194783