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ScientistGrades 10–12⏱ 20 min

Refraction and lenses

Shine a laser into water and glass, watch it bend by Snell’s law or reflect completely, then move a candle to see the image a lens makes.

Standards: HS-PS4-1

1Wonder

Put a straw in a glass of water and look from the side: the straw seems broken at the surface. A magnifying glass makes an ant look huge. Both tricks come from light bending.

2Explore

Light lab

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Turn your phone sideways, or open this lesson on a tablet or computer. The explanation and quiz below work here.

Air · n = 1Water · n = 1.33normalθ₁θ₂n₁ sin θ₁ = n₂ sin θ₂1 × sin 40°= 1.33 × sin 28.9°θ₂ = 28.9°SillyChili
What to do
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Try this
  • Shine the laser from air into water. Does the light bend toward the dashed normal or away from it?
  • Try glass instead of water. Does it bend more or less?
  • Swap them: water on top, air below. Turn the laser slowly. What happens past the critical angle?
  • Choose Lens. Move the object far away, then close to the lens. When is the image upside down? When is it bigger?
  • Put the object closer than F. Where does the image appear now?

3Explain

SillyChili
Step 1 of 5

Light bends at a boundary

Light from the bottom of the straw changes direction when it leaves the water. Your eyes assume light travels in straight lines, so the straw looks broken. This bending is called refraction.

Refraction and lensesn1sin⁡θ1=n2sin⁡θ21f=1do+1di\begin{aligned} n_1 \sin\theta_1 &= n_2 \sin\theta_2 \\ \frac{1}{f} &= \frac{1}{d_o} + \frac{1}{d_i} \end{aligned}

Light bends toward the normal when it slows down. A convex lens makes an upside-down real image of far objects and an upright, bigger virtual image of objects inside its focal length.

4Practice

Question 1 of 5

Light goes from air into glass at a slant. Which way does it bend?

5Recap

  • Refraction: light bends when it crosses into a material where it has a different speed.
  • Snell’s law: n₁ sin θ₁ = n₂ sin θ₂, angles measured from the normal.
  • Slower material → bends toward the normal; faster → away.
  • Past the critical angle, light reflects completely (total internal reflection).
  • A convex lens focuses parallel light at F; 1/f = 1/dₒ + 1/dᵢ.