tomatosquid's posterous

I get what you're saying in this post, but when I teach this I want to make sure my students understand that the image is the same size as the object, except that it's virtual and *behind* the mirror. You're measuring things *on* the mirror where the image does not exist. This is a great experiment, though, for showing that a "full length" mirror only needs to be half your height!

I tend to agree with Andy here. You are making measurements on the mirror, which is not where the image is located. So technically, you aren't measuring the size of the image, but the size of a perspective projection. The mirror is always *half way* between you and the image, thus that measurement is always half. But this only has to do with perspective, not with images. We could do the same experiment with a window and tree. Hold a window halfway from you and a tree and trace out the tree. Now move backward, this time being careful to keep the window halfway between you and the tree. The tree cut out will always look the same size as the tree, as long as window is placed half way.

OK. So, I had another thought about this. During an eclipse, it would be tempting to say that the moon is the same size as the sun, because the moon *covers* up the sun. But no one would be surprised to find that if we placed the moon and sun side by side that the two wouldn't be the same size. I think, rather the ratio of their size would be same ratio of their distance from the viewer on the earth. It's actually coincidence that right now that ratio is nearly perfect... it hasn't always been and won't be as the moon is receding. Anyway, in my analogy, the moon is like the *mask*, and the sun is like *the image*. Maybe I am crazy, does this make sense?

You are both right using a rather technical understanding of the word image (though I accept, that thinking of the image as being located on the surface of the mirror is no better). If you are teaching people about Physics you will need to be careful with the way you describe the basic problem consistent with technical language of the discipline. I should really refer to the projection of the image on the surface of the mirror.

BTW I am not sure it makes any more sense to think of a virtual image "existing" behind the mirror than to think of an image on the surface of the mirror - but then I am neither a philosopher or a physicist!

You may think this is obvious, but it isn't (even to Physics students).

I first learned about it in a talk by Bertamini whose paper on the topic is here:

http://www.liv.ac.uk/vp/Publications/BertaminiParks2005.pdf

I couldn't believe it and had to try the experiment out for myself in the hotel room.

The post here was stimulated by someone @itsDanBull and @amuchmoreexotic who talked about an artist painting over their reflection on the surface of a mirror. I pointed out that the painting would be half the size of the real thing. They didn't believe it, so I demonstrated.

It was raining while I was on holiday.

Totally agree that the result is surprising and cool, and a bit unexpected. I, for one, had never specifically thought of mirrors this way. For that, I am in your debt as a learner. That said, I wouldn't dimiss the interpretation part completely, because science is fundamentally about explaining phenomena, not just uncovering cool things. Don't get me wrong, I love uncovering cool things, but mostly because they are in need of explanation. I think it's awesome that you took the time to document this carefully with a neat setup. This is a big part of the science I want my student to do-- to be intriqued enough by the world to carefully interact with it and document it. The next part, for me, has to be to sort through possible explanations, and part of that is to sort through how that phenomena is consistent with others experiences we have.

I agree that it is common to think of the image as *on* the mirror--experience with students and research on student thinking supports this. The fact that those psychologist measured the *image size* on the mirror is a testament to how common it is to think of the image as on the mirror. I don't think, however, that it is arbitrary to describe the image as being behind the mirror. Now, I guess You CAN decide to say that the image is on the mirror. There is nothing wrong with that. However, then you are somewhat forced to say that there are an infinitely different images for each unique viewing location, because each person has to look at a different spot *on* the mirror to see the image.

What is true is that each person points to the same location *behind* the mirror. One way of doing this is to have multiple people point to where they see the object, and find where each line-of-pointing intersects. You can use yarn to extend the line of pointing if need be. You will find that they intersect behind the mirror, but they cross the mirror at different points. Now think about it: This is also how we locate regular objects in the world; we point to them. If we couldn't all agree on where to point (as would be the case if you pointed *on* the mirror), life would get confusing fast about where things are located. The point is that it is consistent with our everyday experience of locating objects to say that the image is behind the mirror. This has nothing to do with physics, terminology, or philosophy, rather it is about looking for connections among everyday experience, namely how we find out where things are.

Sorry for the long comments. This has just stirred up lots of thinking on my end. ;)

This post and the link to the post on the Lawson et al paper made me post this: http://psychsciencenotes.blogspot.com/2011/08/mirrors-are-literally-windows-t.... It's an interesting topic, perceptually speaking :)