Why does light bend when it enters glass?

Light travels more slowly in glass than in air. The side of the beam that reaches the glass first slows down first, so the beam swings round towards the normal. The amount it bends is set by the ratio of the two refractive indices.

What is the critical angle?

It is the angle of incidence, inside the denser material, at which the refracted ray grazes along the boundary at 90°. For glass to air it is 41.8°, for water to air 48.8° and for diamond to air 24.4°.

When does total internal reflection happen?

Only when light travels from a material with a higher refractive index into one with a lower index, and the angle of incidence is bigger than the critical angle. Then all of the light reflects back inside.

Why does a diamond sparkle?

Diamond has a very high refractive index (2.42), so its critical angle is only 24.4°. Most light that enters is trapped and bounces around inside before leaving through the top.

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Refraction and Snell's law

Light changes direction when it crosses into a material where it travels at a different speed. Past the critical angle it cannot get out at all.

Refraction angle vs incidence angle

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What's happening

Light slows down in a denser material by a factor n, the refractive index, and the change in speed bends the ray at the boundary. Going into a denser material it bends towards the normal; coming out it bends away. Snell's law links the two angles. When light leaves a denser material, the refracted angle reaches 90° at the critical angle; beyond that no light gets through and the whole ray reflects. That total internal reflection is how optical fibres carry light round corners. Some light always reflects at the boundary, and the reflected share grows sharply near grazing angles.

n₁ sin θ₁ = n₂ sin θ₂sin C = n₂ / n₁v = c / n

GCSE Physics (AQA, Edexcel, OCR): refraction and ray diagrams. A-Level Physics: refractive index, Snell's law, critical angle and optical fibres.

Work through the numbers with Physics Formulas.

Challenge

Predict first: light goes from glass (n = 1.50) into air. At what angle of incidence does the ray stop coming out? Work out sin C, then drag the ray and watch for the moment it vanishes.

FAQ

Why does light bend when it enters glass?: Light travels more slowly in glass than in air. The side of the beam that reaches the glass first slows down first, so the beam swings round towards the normal. The amount it bends is set by the ratio of the two refractive indices.
What is the critical angle?: It is the angle of incidence, inside the denser material, at which the refracted ray grazes along the boundary at 90°. For glass to air it is 41.8°, for water to air 48.8° and for diamond to air 24.4°.
When does total internal reflection happen?: Only when light travels from a material with a higher refractive index into one with a lower index, and the angle of incidence is bigger than the critical angle. Then all of the light reflects back inside.
Why does a diamond sparkle?: Diamond has a very high refractive index (2.42), so its critical angle is only 24.4°. Most light that enters is trapped and bounces around inside before leaving through the top.