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CED objectives

Images Formed by Mirrors

▶︎  Watch it animatedinteractive step-through · ~3 min · optional ⚙︎  Open the appletMirror Bench · hand the bench a convex focal length without its sign, then read where a negative image distance really puts the image

A plane mirror forms a virtual image as far behind the mirror as the object stands in front, so the object-to-image separation is twice the distance to the glass. For curved mirrors, $1/s_i + 1/s_o = 1/f$ with $f = R/2$, positive for a converging concave mirror and negative for a diverging convex one. A negative image distance is a result to interpret rather than an error: it locates a virtual image behind the mirror. The CED writes $|M| = |h_i/h_o| = |s_i/s_o|$ for the size ratio, and this course carries orientation in the sign of $M$, negative for inverted and positive for upright.

Five errors dominate. Placing a plane mirror's image on the glass, which loses how far the image looks and how it retreats as you step back. Stripping a negative image distance instead of reading it as a virtual image behind the mirror. Entering a positive focal length for a convex mirror because the printed number was positive, which produces a real image that a diverging optic cannot form. Equating virtual with invisible, when both kinds of image are visible to a properly placed eye and only a real one lands on a screen. And reading the sign of the magnification as information about size or about which side the image is on, when it reports orientation alone.

the image is BEHIND the glass, not on it object virtual image 2 m 2 m total 4 m from your eyes
The dashed backward extensions are where the reflected rays appear to come from. No light ever reaches that point, which is exactly what virtual means.
one number, two separate readings M = −2 magnitude 2 SIZE: twice as tall as the object sign − ORIENTATION: inverted the sign does NOT say the image is behind the mirror which side is the job of s(i): negative = behind, virtual CED form: |M| = |h(i)/h(o)| = |s(i)/s(o)|, bars on all three, magnitude only consistency check: for a single optic, real goes with inverted and virtual with upright, so M and s(i) must agree. When they disagree, one of them is wrong
Size, orientation and side are three questions with three different sources. Reading any one of them off the wrong quantity is where this topic goes wrong.

The work

3 ways in · any order
Lesson
Images Formed by Mirrors ›

Places a plane mirror's image behind the glass, reads a negative image distance as a side rather than an error, signs a diverging mirror's focal length, and splits magnification into size and orientation.

Skill check · 10 scenarios
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Diagnostic
10-item topic check ›

Ten items spanning the failure modes of this topic: putting the image on the mirror surface, stripping a negative image distance, using a positive focal length for a convex mirror, equating virtual with invisible, and reading the magnification's sign as size. Take it cold to find which one is yours, or after the lesson to confirm it is not.

Not started · 10 items · ~15 min
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Targeted Practice
Drill a single misconception ›

Pick one of the failure modes you missed and drill it on its own. The round is adaptive: two correct in a row clears it for now and moves you to the next. Two in a row is a checkpoint, not proof: if the error resurfaces later, the misconception comes back.

Take the diagnostic to identify your misconceptions
Diagnostic first