About this blog

Physics can be difficult to learn, but this blog aims to help you get into physics by connecting your GCSE physics lessons with things you see in the world around you.

Thursday, 13 October 2011

Mirror Mirror On The Wall

We take it for granted that the mirrors in front of us show us a true reflection of the world, but it’s not always the case. Like in the funfair’s hall of mirrors, just a small deviation from a true and flat surface can squish, stretch or otherwise change the appearance of the reflected object. But thankfully in Physics we like to assume that our mirrors are always perfectly flat. That is to say we think about the fact that they might not be, but then to simplify our own lives we’ve decided to assume they are.

As an aside, an assumption is not the same as neglecting to think about certain aspects of your experiment that may have an effect. It’s merely a way of simplifying complicated things, based on deciding to ignore, temporarily, the effect of something. Always state your assumptions, so that you can justify them, or go back and revisit them easily.

But back to mirrors, now we have a perfectly flat one in front of us, what can we see? Our own faces and the objects around us are crystal clear, only in reverse, but how far away are the things we can see? Are they true to the world around you, or just a projection, like a TV screen?

If we imagine looking at a mirror more from the side we can perform a simple experiment to see what happens.

So the real object and its virtual image appear identical, and as far away from the mirror as each other. This is because the angle of incidence (incoming ray) and the angle of reflection (reflected ray) are always the same with a pure and flat surface. The object distance u is therefore equal to the image distance v. There are loads of ways to test this our yourself, so why not have a go?

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