Apply angle conventions and Snell’s law at a boundary.
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Method and worked examples
For reflection, angle of incidence equals angle of reflection, measured from the normal perpendicular to the boundary. Refraction is a change of direction associated with a propagation-speed change. Refractive index is n=vc. At an ideal boundary, n1sinθ1=n2sinθ2. Use degrees consistently and measure both angles from the normal, not from the surface.
Entering a higher-index medium reduces speed and usually bends the ray toward the normal; leaving for a lower index bends it away. A normally incident ray changes speed without changing direction. Frequency stays fixed at the boundary, while wavelength changes according to v=fλ. These rules apply to the stated uniform media and a single boundary; they do not require a claim that all frequencies have identical refractive index.
Example 1
Light travels in glass of index 1.50. Use c=3.00×108m/s. Find speed.
v=nc=3.00×1.50108=2.00×108m/s.
Example 2
Light enters glass n=1.50 from air n=1.00 at 30.0∘ to the normal. Find refraction angle.
sinθ2=(1.501.00)sin30∘=31. θ2=19.47…∘, or 19.5∘.
Watch out:Snell’s law uses angles from the normal. The angle from the surface is its complement.
Reflection and refraction practice | Year 11 Physics | Ascent Arcade