What is Refraction of Light (Snell's Law)?
Refraction is the bending of light as it passes between materials of different optical density, caused by a change in speed. Snell's law is the equation that predicts exactly how much the light bends.
Snell's law states n1 sinθ1 = n2 sinθ2: light bends toward the normal entering a denser medium, and away from the normal leaving one.
- •Speed decreases
- •Bends toward the normal
- •e.g., air → water
- •Speed increases
- •Bends away from the normal
- •e.g., water → air
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Step-by-step worked examples
Light passes from air (n1 = 1.00) into water (n2 = 1.33) at a 40° angle of incidence. Find the refraction angle.
n1·sinθ1 = n2·sinθ2 sinθ2 = (1.00×sin40°)/1.33 = 0.483 θ2 = sin⁻¹(0.483) ≈ 28.9°
Light travels from glass (n = 1.50) into air (n = 1.00) at a 25° angle of incidence.
sinθ2 = 1.50×sin25°/1.00 = 0.634 θ2 = sin⁻¹(0.634) ≈ 39.3°
Light in air (n1 = 1.00) hits an unknown medium at 60° and refracts to 35°. Find n2.
n2 = n1·sinθ1/sinθ2 n2 = 1.00×sin60°/sin35° = 0.866/0.574 n2 ≈ 1.51
Flashcards
Quick quiz
Q1.Light travels from air (n = 1.00) into glass (n = 1.50) at 30° incidence. What happens to the refracted ray?
Q2.In n1 sinθ1 = n2 sinθ2, the angles are measured from…
Q3.Air to water (n1 = 1.00, n2 = 1.33), θ1 = 40°. What is θ2 approximately?
Q4.Going from a denser to a less dense medium, if the angle exceeds the critical angle, light…
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Common mistakes
Assuming the angle of incidence is measured from the surface. — Correct: It's always measured from the normal, the line perpendicular to the surface.
Assuming light always bends toward the normal. — Correct: It bends toward the normal entering a denser medium, and away from it leaving one.
Mixing up which n goes with which angle. — Correct: Match each refractive index to the medium where its corresponding angle is measured.
Ignoring total internal reflection at large angles going from dense to rare. — Correct: Beyond the critical angle, light reflects completely instead of refracting.
FAQ
What is Snell's law formula?
n1 sinθ1 = n2 sinθ2, relating the incidence and refraction angles to the refractive indices of the two media.
What is refraction of light?
The bending of light as it passes between materials with different optical densities, caused by a change in speed.
How do you calculate the refraction angle?
Rearrange Snell's law: θ2 = sin⁻¹(n1 sinθ1 / n2), using the known refractive indices and incidence angle.
What are examples of refraction of light?
A straw looking bent in a glass of water, lenses focusing light, and rainbows forming as sunlight refracts through raindrops.