Eleventh Grade · Science · week 28

Refraction and Lenses

Apply Snell's law to find an angle of refraction, and apply the thin lens equation to find the location, orientation, and size of an image formed by a lens.

Refraction is the bending of light as it crosses into a medium where its speed changes -- light slows down entering a denser (higher-index) medium, like going from air into water or glass, and this bends the ray TOWARD the normal (the reverse happens moving into a less dense medium, bending AWAY from the normal). Snell's law makes this precise: n1*sin(theta1) = n2*sin(theta2), where n is each medium's index of refraction (about 1.00 for air, 1.33 for water, 1.5 for typical glass) and theta is measured from the normal.

Lenses use refraction at two curved surfaces to form images, and remarkably reuse the SAME equations as mirrors last week: 1/do + 1/di = 1/f and m = -di/do, with the same sign convention (converging lenses have positive f, diverging lenses have negative f). The key physical difference from a mirror is geometric: a lens's real image forms on the OPPOSITE side from the object (light passes all the way through), while a mirror's real image forms on the SAME side (light bounces back) -- but the numbers work out identically either way.

📋 For the teacher
Teach it (10–15 min):
  • This week's lab: place a pencil or straw in a clear glass of water and view it from the side at an angle -- the apparent "bend" right at the surface is refraction, visible in under a minute with things already in most kitchens.
  • If a magnifying glass is available, hold it at different distances from a printed page or a window and watch the image change from upright-and-enlarged (object closer than the focal length) to upside-down (object farther than the focal length) -- avoid aiming it at direct sunlight, to prevent any fire risk.
  • Work this week's converging-lens example directly alongside last week's concave-mirror example, side by side on paper, so the matching numbers make the "same equation, different geometry" point impossible to miss.
  • Mention eyeglasses as the most relevant everyday lens application: diverging lenses correct nearsightedness, and converging lenses correct farsightedness -- a real, wearable example most students already have some experience with.
Talk about it:
  • Why does light bend when it crosses from one medium into another?
  • How is the lens equation the same as the mirror equation, and how does the resulting image geometry differ?
  • What everyday device or situation have you noticed refraction or lens behavior in?

Mastery looks like: They apply Snell's law to find an angle of refraction, and apply the thin lens equation to find an image's location, orientation, and size for a converging or diverging lens.

Common stumbles: Forgetting that light bends AWAY from the normal when entering a LESS dense medium (the reverse of the worked example's case); mixing up which side of a lens a real image forms on compared to a mirror.

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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