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3.5.1Stoichiometry

Refraction — Cambridge IGCSE Chemistry 0620 Revision Notes & Topical Questions

5 min read 5 parts Lesson 7 of 28 Unit quiz

📚 Cambridge IGCSE Chemistry 0620 Hub / Topical Revision
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Curriculum Equation Key قوانين المول والحسابات الكيميائية - Moles & Stoichiometry FormulasFast Exam Solution
n = m / M_r | N = n × N_A (6.022 × 10²³) | M = n / V | V_gas = n × 22.4 L (STP)
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Section 1 of 5Refraction Rules

Definition
  • Definition Refraction is the change in direction of a wave caused by a change in its speed as it crosses a boundary.
Denser medium
  • Denser medium When light enters a denser medium such as glass from air, it slows down and bends towards the normal.
Less dense medium
  • Less dense medium When light leaves glass for air, it speeds up and bends away from the normal.
Along the normal
  • Along the normal A ray travelling along the normal does not change direction, although its speed still changes.

Section 2 of 5Investigating Refraction with a Block

Investigating Refraction with a Block
Variables
  • Variables The independent variable is the material of the block, and the dependent variable is the angle of refraction.
Controls
  • Controls The width of the light beam and the frequency of the light are kept the same for every test.
Analysis
  • Analysis Plotting sin i against sin r gives a straight line whose gradient equals the refractive index of the material.
Accuracy
  • Accuracy A higher-resolution protractor and a sharpened pencil reduce random errors when marking the rays.

Section 3 of 5Light Passing Through a Glass Block

1
The ray bends towards the normal as it enters the glass, because light slows down.
2
Inside the glass the ray travels in a straight line at a reduced speed.
3
At the glass-to-air boundary the ray bends away from the normal as it speeds up.
4
The emerging ray is parallel to the original ray but displaced sideways.

Section 4 of 5Worked Example

Question

  • A ray of light in air meets a glass block of refractive index 1.5 at an angle of incidence of 30°.
  • Calculate the angle of refraction.

  1. Rearrange Snell's lawsin r = sin i ÷ n
  2. Substitutesin r = sin 30° ÷ 1.5 = 0.5 ÷ 1.5 = 0.333
  3. Inverse siner = sin⁻¹(0.333) = 19.5°

Answer r ≈ 19° (to 2 significant figures).

Section 5 of 5Exam Checklist & Tips

3.5.1 – Key Points for the Exam

  • Refraction The change in direction of a wave caused by a change in its speed at a boundary.
  • Normal A line drawn perpendicular to a boundary at the point of incidence.
  • Refractive index The ratio of the speed of light in a vacuum to its speed in the medium.
  • Snell's law The relationship n = sin i ÷ sin r between refractive index and the two angles.
  • Angle of incidence The angle between the incident ray and the normal.
  • Angle of refraction The angle between the refracted ray and the normal.

Common Mistakes

  • Saying light bends towards the normal when it leaves glass for air, when it actually bends away.
  • Inverting the ratio, writing n = sin r ÷ sin i.
  • Saying the frequency of light changes when it refracts.
  • Using radian mode on the calculator, which gives a wrong angle.

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