Exploring Light Diffraction: Insights from Feynman's Books

[Lucw]

Hello

Here is the shadow of the edge of a screen. This picture is in one of Mr Feynman's books. (Http://www.feynmanlectures.caltech.edu/I_30.html)
Of course, there is light interference in the edge of the shadow. It's normal. It's diffraction. Mr Feynman explains how to geometrically build this curve.
But I also note that "light" enters the geometric shadow of the screen. (at point Q). And itcan not go in a straight line ...
When we study diffraction, we usually look at where the light is going, not coming from.
If we look in the direction of the light source, just hidden by the edge of the screen. Will we see the souce of light in another direction than it is since the light is deviated?
I think this experience has never been realized.

Thank you for your opinion.

Have a good day

Lucw

 

[sophiecentaur]

This sort of thing happens all the time.
Where does the light come from when you see yourself in a mirror? Exactly the same issue but more familiar.

 

[Lucw]

Hello Sophiecentaur

Thank you for your reply.

Ok ok. But here, it looks a lot like the gravitational deviation; the photographs taken by Eddington in 1919.
It seems to me (it's my opinion ...) that if we replaced the sun with a huge cardboard disk of the same diameter, Eddington would have had exactly the same effect on his photos.
But as when we study diffraction, we do not look where the light comes from, it will take a little longer to put that into question.
There is also another diffraction experiment. When we look at the shadow of a disc (at a slight oblique angle), the center of the shadow has a bright point ...

Have a good Saturday

Lucw

 

[sophiecentaur]

But as when we study diffraction, we do not look where the light comes from, it will take a little longer to put that into question.

After my rather facile answer, I went and had another think. You mention "diffraction" and I think that's the basis of the real answer. You see the image in a mirror when the aperture of the mirror is wide enough (i.e. a regular mirror) to form a recognisable image from every point on the object.
Forgetting that for a minute and think of the diffraction by a narrow slit. The pattern of the light that gets through will be the well known sinx/x pattern on a 'screen' which has a width depending on the ratio of λ to d. But if you put your eye in place of the screen, you won't 'see' that pattern but you will see a different pattern [Edit: light level] from any fixed position. That pattern is the diffraction limited pattern of the aperture of your eye. Your pupil d will be different from (probably wider than) the slot d so the sinx/x type pattern you will see will be finer than the pattern on the screen. So I would say you will see light coming from a narrow range of angles. So I think you need to apply diffraction ideas in the appropriate direction, depending what you need to know.
Back to the mirror; you have a very wide aperture so the diffraction fringes will be very narrow and the apparent direction of any point object will be what a simple geometric ray trace will give you. Your brain can only work on the information about the direction of arrival of light at your eye so the best you can do is to appreciate the virtual image out front. But our brains are pretty good at dealing with this stuff. We can shave and apply makeup accurately and also drive a car on the basis of reflected images. Imagine the extra coding that a computer prog needs, to deal with that.

 

[Lucw]

Sophiecentaur.

Thank you for your reply.

I like very much your "So I think you need to apply diffraction ideas in the appropriate direction, depending what you need to know"...

Mr Einstein claimed that the light that passed near the sun was deflected by its mass. It is very good.
But it seems to me that the light passing near the edge of a screen is also deflected in its shadow, towards its geometrical shadow ...
And the light that passes through a narrow slit is also scattered at the exit, in the shadow to the left and to the right.
And not to risk contradicting Mr. Einstein, we do not think about it. It depends on what you want to know ...

I come back to this other experience. The shadow of a disc shows a bright spot in the center. It's a Fresnel experience.

If we look towards the light source, hidden by the disc, we must see an Einstein ring around the disc.
But you see, no one has ever experienced. It puts too much into question.
Just a question of time ...

Have a good day.

Lucw

 

[Dale]

that if we replaced the sun with a huge cardboard disk of the same diameter, Eddington would have had exactly the same effect on his photos.

I don’t think this is correct. Do you have a peer reviewed reference which supports this? I would be surprised to learn a) that diffraction can form a coherent image and b) that the sun could cause significant diffraction at optical wavelengths despite its corona and c) that either of those effects were capable of explaining the Eddington observations or other subsequent observations.

 

[sophiecentaur]

And not to risk contradicting Mr. Einstein,

Which bit of Mr Einstein would be be contradicting? We are seldom concerned with gravitational effects in the lab.

When we study diffraction, we usually look at where the light is going, not coming from.

That doesn't apply to astronomers who are particularly interested in where the light that is recorded by their 'apparatus' is coming from when they need to resolve detail. I think you are referring only to the most elementary experiments that students of basic diffraction are involved in.
You seem to be implying that the Arago spot is a gravitational phenomenon when it is predicted by regular diffraction theory without gravity.
How seriously should we be taking your original post?

 

[Lucw]

Eh.

I just speak about experiences that could be done.
And, according to my reading and questionning, they have not be done.
And the picture of the original post comes from Mr Feynman.
Have a look on his page...

Lucw

 

[ZapperZ]

You are forgetting one very important thing. Your "eye" has a lens, and that lens plays a significant role in this.

Here's something that you can try. Do a simple single-slit diffraction experiment, i.e. get the typical diffraction pattern onto a screen. Now, put a lens in between the slit and the screen, Adjust it until you get a focused image on the screen. What image do you see?

You do NOT seen images of the diffraction pattern. Instead, you see image of the SLIT!

I will refer you to a topic called "Fourier optics". What actually is going on is that the diffraction pattern from the single slit is equivalent to the Fourier transform of the SHAPE of the slit. So if one were to do a Fourier transform of the slit shape (simulating the opening with a square function), then one can get the same result as the diffraction pattern.

However, what a lens does after that is that it performs what is essentially an inverse Fourier transform, i.e. it takes the diffraction image and recombines it back to get the image of the slit!

So to answer your question, if you were to look at the slit from an angle, all you see is the image of the slit seen from an angle, because the lens in your eyes will reform the image of the slit seen from that direction!

Zz.

 

[Lucw]

Hello ZapperZ

Well well.
I didn't use my eyes.
I use a camera...
And i do not look to a single slit.
I look to a light gently obscured by the edge of a screen.
My question is: has someone made this experience?
I think not. But i am not sure...

Luc w

 

[jtbell]

I didn't use my eyes.
I use a camera...

Both use a lens, no?

 

[Lucw]

Hello Jtbell.

Oh yes. Both use a lens.
But when a camera "see" a light spot, (a point source), there is a light spot on the photography...

But never mind.
Find a disused tunnel. And make your own experience.

Before to say that the mass deviates the light, try if it is NOT deviated by something with a very low mass.

But all is normal. It is not allowed to say that our knowledge has something wrong...

Have a nice day.
Lucw

 

[ZapperZ]

Hello ZapperZ

Well well.
I didn't use my eyes.
I use a camera...
And i do not look to a single slit.
I look to a light gently obscured by the edge of a screen.
My question is: has someone made this experience?
I think not. But i am not sure...

Luc w

Your "camera" has a lens!

And you are looking at the light that has been diffracted by the single slit!

Again, you don't have to believe me. Try it yourself. After all, how difficult is it to do such a test? We do diffraction experiments in undergraduate intro physics labs all the time! And even if you're not in school, laser sources are trivial to get nowadays.

Zz.

 

[Lucw]

Yes yes ZapperZ.

I make the experience not of the single slit. Just of the edge of a screen.
And approch the edge of the screen slowly.
And it seems (i am cautious) that the direction of light change...
Same that in Eddington experience.
But i am wrong. That is sure.
Have a nice day.
Lucw

 

[ZapperZ]

Yes yes ZapperZ.

I make the experience not of the single slit. Just of the edge of a screen.

Why should this matter? A diffraction is a diffraction. A diffraction pattern from a single slit is slightly different than the diffraction pattern from an edge, but the principle is the same, and BOTH produce patterns in the "shadow" region of the slit/edge.

If you don't know anything about diffraction, and I give you a lens and ask you to predict what you'd see if you put a lens somewhere AFTER the slit/edge, I bet you'll say that you'll get an image of the slit/edge. After all, that is what a lens does, and even young kids are aware of this when they play with magnifying glasses.

Yet, when we "distract you" with the fact that there is actually a diffraction pattern AFTER light passes through the slit/edge, somehow putting a lens after that is NOW a puzzle that "...this experience has never been realized... "

Oh yes it has!

Zz.

 

[sophiecentaur]

My question is: has someone made this experience?

Of course. Every kind of aperture and shape has been examined. An edge is particularly relevant in radio propagation (exactly the same calculations) and tells you what happens to a radio wave when it arrives at a hill or cliff.
Look at this link, which describes edge diffraction and shows how a Cornu Spiral can be used to plot the diffraction.
All this Maths was done waaaaay back before Modern Physics came in.

 

[sophiecentaur]

But all is normal. It is not allowed to say that our knowledge has something wrong...

It's alright to "say" something is not normal but, unless you can demonstrate that present Science has got it wrong then how can you expect anyone to take you seriously? You would at least need a good reference to back up your idea. Science is innocent until proved guilty, you will find.

 

[Dale]

@Lucw please back up your claims with some supporting references. You are just making completely unsubstantiated suggestions. The topic can be discussed, but the discussion must be informed and not promote ignorant supposition.