# Concave mirrors

1. Oct 30, 2006

### jnimagine

we used a concave mirror and a candle for an experiment
when the candle wa placed between the vertex and the focus of the mirror (f=12.7cm) a virtual image appeared behind the mirror.
We measure the image distance approximately by pointing a finger towards where the image might be and got 6.0cm (-6.0 when used in mirror equation) and the object was at 6.35cm. However, when I use the mirror equation to calculate the focus, i get a negative answer that is not even close to the actual focus 12.7cm. Do u think this is just due to human errors of not measuring the image distance accurately or is there some other factors that can account for this? OR is the image distance supposed to be almost the same as the object distance in this case?

2. Oct 31, 2006

### andrevdh

Yes, the virtual image did form at -12.7 cm according to the formula (which do work by the way!).

Locating a virtual image by parallax (or by finger as you say) can be tricky. Especially if you do not know what you are actually doing and did not had a bit of practice.

So I would think that you did not succeed too well at pointing at the correct position behind the mirror, that is you had to hold your finger two times futher backwards.