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3.2.4Waves

Refractive Index — Cambridge IGCSE Physics 0625 Revision Notes & Topical Questions

5 min read 10 parts Lesson 14 of 35 1 quiz

📚 Cambridge IGCSE Physics 0625 Hub / Topical Revision
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Lesson summary

3.2.4 Refractive Index — IGCSE PHYSICS notes: Define refractive index as a ratio of speeds and of angles and use it with the critical angle.Definition — a number that shows how strongly a material bends light, always larger than 1 and different for different materialsNo unit — it…

  • Optical density
  • Speed in glass
  • Never faster
  • What the symbols mean
  • Memory aid
  • Reverse path

Section 1 of 10Refractive index

Definition
  • Definition a number that shows how strongly a material bends light, always larger than 1 and different for different materials
No unit
  • No unit it is a ratio
Values
  • Values glass is about 1.5, water 1.33, diamond about 2.4
Optical density
  • Optical density a more optically dense material has a higher refractive index

Section 2 of 10Ratio of speeds

Definition
  • Definition the ratio of the speeds of a wave in two different regions
Equation
  • Equation n = speed of light in a vacuum ÷ speed of light in the material, n = c ÷ v
Speed in glass
  • Speed in glass for n = 1.5 the speed is 2.0 × 108 m/s, since c is 3.0 × 108 m/s
Never faster
  • Never faster n is never below 1, so light is never faster in a material than in a vacuum

Section 3 of 10Ratio of angles

Definition
  • Definition the ratio of the sine of the angle of incidence to the sine of the angle of refraction
Equation
  • Equation n = sin i ÷ sin r, also known as Snell's law
    What the symbols mean: n = refractive index (no unit) · i or θ1 = angle of incidence · r or θ2 = angle of refraction
    • (both measured from the normal)
Angles
  • Angles i is the angle in air and r the angle in the material, both from the normal
Calculation steps
  • Find sin of the known angle
  • Rearrange the equation
  • Use the inverse sine function to find the angle
Warning
  • Warning sin i ÷ sin r is not the same as i ÷ r
Memory aid
  • Memory aid i comes before r in the alphabet, so i is on top

Section 4 of 10Finding n by experiment

1
Draw round a rectangular block on paper.
2
Send a ray at a measured angle of incidence i to the normal at one face.
3
Mark the entry and exit points and join them to find the path through the block.
4
Measure the angle of refraction r inside the block.
5
Repeat for several angles and plot sin i against sin r.
  • Result
    • a straight line through the origin whose gradient is n

Section 5 of 10Leaving the dense medium

Reverse path
  • Reverse path light going from glass to air has the ray paths reversed
Angles
  • Angles the larger angle is always in the medium with the smaller n
Check
  • Check the angle in glass must be smaller than the angle in air

Section 6 of 10Critical angle and refractive index

Critical angle c
  • Critical angle c the angle of incidence in the denser material for which the refracted ray travels along the boundary
Equation
  • Equation sin c = 1 ÷ n,
    • which rearranges to n = 1 ÷ sin c
      What the symbols mean: c = critical angle · n = refractive index of the denser material (no unit)
Meaning
  • Meaning the larger the refractive index, the smaller the critical angle
Values
  • Values glass about 42°, water about 49°, diamond about 25°
Method
  • Method find sin c from n first, then use the inverse sine to find c
Diamond
  • Diamond its small critical angle means more total internal reflection, so it sparkles

Section 7 of 10Apparent depth

Method
  • Method the pin method gives n as real depth divided by apparent depth for small viewing angles
Index matching
  • Index matching a glass rod in a liquid of the same refractive index becomes nearly invisible

Section 8 of 10More from the specification

More from the specification
Refractive index of optical fibres
  • Refractive index of optical fibres high refractive index glass cores ensure complete internal reflection within medical imaging optical fibres.
Converging Lens Ray Diagrams
  • A converging (convex) lens refracts rays parallel to the principal axis through the principal focus (focal point);
    • placing an object beyond 2F produces a real, inverted, diminished image,
  • while placing an object between F and 2F produces a real, magnified image.
Magnifying Glass and Virtual Images
  • When an object is placed closer to a converging lens than its principal focal length, light rays diverge after refraction;
    • projecting rays backward forms a virtual, upright, magnified image.
Diverging Lenses and Plane Mirrors
  • A diverging (concave) lens always forms a virtual, upright, diminished image between the lens and the object,
  • whereas reflection in a flat plane mirror forms a virtual image identical in size and upright.
Converging lens image formation
  • Converging lens image formation objects placed beyond the principal focus produce real, inverted images on the opposite side.
Diverging lens ray diagrams
  • Diverging lens ray diagrams diverging lenses always produce upright, diminished, and virtual images located between object and lens.
Principal focus
  • Principal focus the focal point on the principal axis where rays parallel to the axis converge or appear to diverge.
Distance
  • Total length of path travelled, a scalar.
Virtual image
  • Image formed where rays only appear to meet, not projectable on a screen
Optical fibre
  • Thin transparent strand that guides light by TIR
Principal focus
  • Point where parallel rays meet or appear to come from
Focal length
  • Distance from the lens centre to the principal focus
Plane mirror
  • Flat mirror.

Section 9 of 10Key terms

TermMeaning
Refractive indexRatio of the speed of light in a vacuum to its speed in a material
Optical densityHow strongly a material slows and bends light
Critical angleAngle of incidence in the denser medium that gives refraction along the boundary
Snell's lawn = sin i ÷ sin r
Inverse sineCalculator function that gives an angle from its sine

Section 10 of 10Exam technique

1
Cancelling the sine terms — sin i ÷ sin r is not i ÷ r.
2
Putting r on top — n = sin i ÷ sin r with i on top.
3
Giving refractive index a unit — it is a ratio with no unit.
4
Using the angle to the surface — angles are measured from the normal.

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