A spider hangs by a strand of silk at an eye level 20cm in front of a
plane mirror.  You are behind the spider, 50cm from the mirror.  What
is the distance between your eye and the image of the spider in the
mirror?

---

Request for Question Clarification by livioflores-ga on 06 Mar 2005 16:31 PST

Hi dear daisy!!

The image of the spider is ON the mirror, then if you are 50cm from
the mirror, the distance between my eye and the image of the spider in
the mirror is 50cm.
The apparent distance is different, it will be 50cm + 20cm = 70 cm .

Does this answer your question?

regards.
livioflores-ga

Isn't the image of the spider in the mirror 20 cm in front of the
mirror and then only 30 cm from your eyes?

Nop, the apparent distance is 20cm INTO the mirror, so you must add
this apparent distance to your eye distance.
To confirm that you can try with the following experiment:
Try to focus a subject through the mirror with a reflex camera
manually and then check the distance indicator.
But again do not forget that the image of the spider is actually ON
the surface of the mirror, so its distance to your eyes is the
distance between your eye and the mirror. There are two different
things but related.

okay, okay, okay!

Here's another way to look at it (pun intended).  Whichever image of
the spider one is viewing at any given moment, is, in fact, on one's
retina.  ;)