Refraction of light — Physical education, 14–17 years
Why light changes direction when it crosses into a different material, and how to predict the new direction.
The idea
Refraction is the change in direction of light when it crosses a boundary, such as air to glass or air to water. The light changes speed in the new material, so its path usually bends. Angles are measured from the normal, an imaginary line at right angles to the boundary.
Why we need it
A straight-line model works while light stays in one material, but it cannot explain why a coin appears to move under water or why a lens focuses light. Refraction was studied to predict these changes at material boundaries. The key problem is that light does not travel at the same speed in every transparent substance.
A worked example
A ray travels from air into glass at an incidence angle of 30°. Take n_air = 1.0 and n_glass = 1.5, then use n₁sin i = n₂sin r. So sin r = 1.0 × sin30° ÷ 1.5 = 1/3, giving r ≈ 19.5°. The ray bends towards the normal.
The common trap
Many people measure the angle from the surface instead of from the normal. That is reasonable because the surface is visible and feels like the obvious reference line. But the equations use the normal: if the ray makes 30° with the surface, its angle to the normal is 60°.
Outside school
Eyeglasses use carefully shaped lenses to refract light so that images form sharply on the retina. Cameras, microscopes and telescopes use the same idea to control where light converges. Optical fibres guide light through repeated internal reflections, allowing information to travel over long distances.
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