\(\def \u#1{\,\mathrm{#1}}\)
\(\def \abs#1{\left|#1\right|}\)
\(\def \ast{*}\)
\(\def \deg{^{\circ}}\)
\(\def \ten#1{\times 10^{#1}}\)
\(\def \redcancel#1{{\color{red}\cancel{#1}}}\)
\(\def \BLUE#1{{\color{blue} #1}}\)
\(\def \RED#1{{\color{red} #1}}\)
\(\def \PURPLE#1{{\color{purple} #1}}\)
\(\def \th#1,#2{#1,\!#2}\)
\(\def \lshift#1#2{\underset{\Leftarrow\atop{#2}}#1}}\)
\(\def \rshift#1#2{\underset{\Rightarrow\atop{#2}}#1}}\)
\(\def \dotspot{{\color{lightgray}{\circ}}}\)
How Things Move
Why Things Move
virtual images
real image
concave
convex
thin spherical devices
axis
focal point
focal length
lens equation
\(\frac1f=\frac1{d_i}+\frac1{d_o}\)
If \(d_i>0\), the image is real.\(\hskip3ex\)
If \(d_i<0\), the image is virtual.