# Warm-Up Exercise 29

## Due 12:15 pm, Fri 2 Nov 2012

Physics 123, Fall 2012

Reading assignment: 36.3-36.4

In the previous day's reading, we found a relationship for the
distance of an object and the distance of its image for a reflecting
surface, the "mirror equation". Today we've done the same but for a refracting surface,
the "thin lens equation". The two equations *look *identical. How
exactly are they different? (Don't answer in purely mathematical language.)

Two major differences are:
(1) the signs--for example, the equation demands that an image distance (q) is positive for mirrors when the image is on the same side as the object, but it's positive for lenses when the image is on the opposite side from the object.
(2) how f is calculated--for mirrors, it's simply related to the radius of curvature, but for lenses it relates to the curvature as well as the index of refraction of the lens glass.

When you look straight down at an object under water, will it appear to be
closer to you or farther away than actuality? Why?

It appears to be closer. My best explanation is Fig 36.20 (8th edition)--the light coming from the fish bends away from the perpendicular, making the fish seem closer.

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