Eleventh Grade · Science · week 27

Reflection and Mirrors

State the law of reflection and apply the mirror equation to find the location, orientation, and size of an image formed by a curved mirror.

The law of reflection governs every mirror, flat or curved: the angle of incidence equals the angle of reflection, both measured from an imaginary line perpendicular to the surface at the point of reflection (the "normal"). A flat (plane) mirror always produces a virtual image (light only appears to come from behind the mirror; it does not actually converge there) that is upright, the same size as the object, and located as far behind the mirror as the object sits in front of it.

Curved mirrors -- concave (caving inward, like the inside of a spoon) and convex (curving outward, like the back of a spoon) -- are described by the mirror equation, 1/do + 1/di = 1/f, where do is the object's distance, di is the image's distance, and f is the focal length (positive for concave, negative for convex, in the standard "real is positive" convention). Magnification, m = -di/do, tells you the rest of the story: a negative m means an inverted image, a positive m means upright; |m| greater than 1 means enlarged, less than 1 means reduced. A real image (di positive) can be projected onto a screen; a virtual image (di negative) cannot.

📋 For the teacher
Teach it (10–15 min):
  • This week's lab: shine a flashlight or laser pointer at a small flat mirror lying on a piece of paper marked with a protractor (or an estimated angle), and verify that the reflected beam leaves at the same angle from the normal as the incoming beam arrived.
  • If a metal spoon is handy, look at your reflection in both its concave (inside) and convex (outside) surfaces -- the concave side flips your image upside down once you are far enough away (past its focal point), while the convex side always shows a small, upright image, a vivid, everyday mirror-equation demonstration.
  • Work through the sign convention out loud, every single time: positive f for concave, negative f for convex; positive di means real, negative means virtual -- the arithmetic is simple, but sign slips are the main source of error this week.
  • Connect the convex mirror's "objects are closer than they appear" warning (printed on passenger-side car mirrors) directly to the reduced, upright image found in try-item 2.
Talk about it:
  • What is the law of reflection, and does it apply the same way to curved mirrors as to flat ones?
  • What is the practical difference between a real image and a virtual image?
  • Why do passenger-side car mirrors use a convex shape rather than a flat one?

Mastery looks like: They apply the mirror equation and magnification formula to find and describe an image for a concave or convex mirror, using the correct sign convention.

Common stumbles: Mixing up the sign of the focal length for concave versus convex mirrors; treating a negative image distance as a mistake rather than correctly recognizing it as a virtual image.

Homework for this lessonType it, write it with a stylus, or print it for pencil & paper.open →
Records attendance + 15 min of Science for this learner.
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