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Refractive index
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Light bends when it enters glass. The refractive index tells you how much.
Reflection and refraction of light
Light waves are transverse and can be reflected and refracted.
Law of reflection: angle of incidence = angle of reflection, measured from the normal.
Entering glass from air, light slows down and bends towards the normal.
Law of reflection: angle of incidence = angle of reflection, measured from the normal.
Entering glass from air, light slows down and bends towards the normal.
Refractive index
n = sin i ÷ sin r
i is the angle of incidence in air; r is the angle of refraction in the material. n has no unit.
The bigger n is, the more the light bends. For glass, n is about 1.5; for water, about 1.33.
i is the angle of incidence in air; r is the angle of refraction in the material. n has no unit.
The bigger n is, the more the light bends. For glass, n is about 1.5; for water, about 1.33.
Investigating with a glass block
Shine a ray from a ray box into a rectangular glass block at several angles of incidence. Mark the rays, then draw the normal and measure i and r with a protractor.
Plot sin i (y-axis) against sin r (x-axis). The gradient is n.
Semicircular blocks and triangular prisms can also be used to study refraction.
Plot sin i (y-axis) against sin r (x-axis). The gradient is n.
Semicircular blocks and triangular prisms can also be used to study refraction.
Light enters glass with angle of incidence 40° and angle of refraction 25°. Calculate n.
- n = sin 40° ÷ sin 25°
- = 0.643 ÷ 0.423
Answer: n ≈ 1.52
Angle of incidence equals angle of reflection. Light slows and bends towards the normal on entering glass. n = sin i ÷ sin r. Find n for glass by measuring i and r with a glass block and plotting sin i against sin r.
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