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Reflection in spherical mirrors

by rishch
Tags: mirrors, reflection, spherical
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Simon Bridge
#19
Nov13-12, 04:55 PM
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Quote Quote by sophiecentaur View Post
Conceptual?
If you've been following - OP has remained insistent that the image in his spoon is in his spoon.
Why any more than just a mirror? You have to drink the soup first
Oh do I have to? Can't I just skip to dessert?
I just picked up a soup spoon and managed to locate a real image inside the concave bowl fairly easily. I used the 'no parallax' method that we were taught at School in the fifties. You can get a real image of a pencil tip (inverted, of course) which will appear to be in exactly the same place as a small pointer - when you get it right, the pointer moves in exactly the same way as the image as you move your head from side to side so they are in the same 'place' (a real place in the space within the bowl).
Unfortunately, the spoon is too small to get a nice looking result - like the optical toy - but it certainly proves the point.
There used to be a toy in the 50's for this? <imagines Sophiecentaur sitting for hours in front of a small mirror wiggling his head back and forth and shouting excitedly: "look mummy, it moves the same way" while his parents muttered worriedly between "yes, well done"'s.> Could have been my childhood.
The old 'shaving mirrors' can be used for the same thing - or even the make-up mirrors on powder compacts will give interesting results.
Ah - forgot about those.

This is taking me back ... we used to have to scrounge all our educational/science equipment when we were little... and we were better off for it! The youth of today are all soft...
sophiecentaur
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Nov13-12, 05:02 PM
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Sounds a bit like a Secret Policeman's Ball sketch.

Indeed the image is in the spoon because it is below the rim of it, for a deep spoon. But it is in front of the surface and it doesn't appear behind the spoon. There are a million diagrams like this one on the net to show what happens.
Simon Bridge
#21
Nov13-12, 06:31 PM
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I wouldn't sketch a policeman's ball, secret or otherwise.
There are a million diagrams like that in the naked internet, but none of them show why so many people's first impression is that the image behind the spoon.

The geometry says it cannot be - but it takes a while before people learn how to trust the math. When the physics says one thing and intuitive experience says something else, people use the experience part to show how all these egg-heads are not so clever really :)

There's three methods now - I hope OP will be try them out. Of course a dessert-spoon will give less reliable results.

I used to show students, after learning all about images etc, a large (2m across) concave mirror. The usual reaction is a scream or a yelp of some kind and some people fall over. This is when they see themselves hanging upside down in the air in front of them. After they recover, they have loads of fun getting their images to poke someone in the eye.

One of these mirrors cracked a while ago, and locates the image in the air in that one without using a trick like discussed above. The crack seems to cue their visual systems to treat the image differently.

Another example I show people is how they think they can see the colour of objects (other than orange) in sodium light. I arrange colored squares and get people to label them with what colour they see and then turn up the lights.

What interesting is to see when (at what white-light level) they see the actual colors (people report that the colors fade-in) and then turn the light down again and see when/if the previous impression reasserts itself.

(I no longer see colors - just different shades of orange - it can take practice to see whats actually there. Terry Pratchett calls this "first sight".)
sophiecentaur
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Nov14-12, 10:17 AM
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Quote Quote by Simon Bridge View Post
I wouldn't sketch a policeman's ball, secret or otherwise.
This was the sketch to which I was referring. It's a Monty Python thing - more UK than US, I believe.
No Physics at all - just reminiscing about the past.
Simon Bridge
#23
Nov14-12, 07:16 PM
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I don't care if there are four policemen or that they come from Yorkshire either...

(Sorry, I should have given that hint earlier.)
Be assured: the "Four Yorkshiremen" sketch is well known everywhere that people do funny walks, keep a look-out for the Spanish inquisition, and fear weaponized sarcasm - which would include the USA. You did correctly pick up on a deliberate Python reference back there - kudos. (Ahhh - nostalgia - it sure ain't what it used to be aye?)

I'm in New Zealand BTW.
People keep thinking I'm from the US for some reason - maybe I should baa more often?


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