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IGCSEPhysicsEdexcelPhysics (Modular)Unit 2Revision Notes5. WavesReflection & RefractionSnell's Law

Snell's Law (Edexcel IGCSE Physics (Modular))

Revision Note

Katie M

Written by: Katie M

Reviewed by: Caroline Carroll

Snell's law

  • The angles of incidence and refraction are related to the refractive index of a medium by an equation known as snell's Law:

n space equals space fraction numerator sin space i over denominator sin space r end fraction

  • Where:

    • n = the refractive index of the material

    • i = angle of incidence of the light (°)

    • r = angle of refraction of the light (°)

  • 'Sin' is the trigonometric function 'sine' which is on a scientific calculator

  • You can revise the concept of refraction using the revision notes Reflection & refraction

A formula triangle can help rearrange the snell's law equation

Snell triangle (2), IGCSE & GCSE Physics revision notes

Snell's law formula triangle

  • For more information on how to use a formula triangle refer to the revision note on Speed

Refractive index

  • The refractive index is a number which is related to the speed of light in the material (which is always less than the speed of light in a vacuum):

refractive space index comma space n space equals space fraction numerator speed space of space light space in space straight a space vacuum over denominator speed space of space light space in space material end fraction

  • The refractive index is a number that is always larger than 1 and is different for different materials

    • Objects which are more optically dense have a higher refractive index, eg. n is about 2.4 for diamond

    • Objects which are less optically dense have a lower refractive index, eg. n is about 1.5 for glass

  • Since the refractive index is a ratio, it has no units

Worked Example

A ray of light enters a glass block of refractive index 1.53 making an angle of 15° with the normal before entering the block.

Calculate the angle it makes with the normal after it enters the glass block.

 Answer:

Step 1: List the known quantities

  • Refractive index of glass, n = 1.53

  • Angle of incidence, i = 15°

Step 2: Write the equation for snell's law

n space equals space fraction numerator sin space i space over denominator sin space r end fraction

Step 3: Rearrange the equation and calculate sin (r)

sin space r italic space equals space fraction numerator sin space i over denominator n end fraction

sin space r italic space equals space fraction numerator sin open parentheses 15 close parentheses over denominator 1.53 end fraction

sin space r italic space equals space 0.1692

Step 4: Find the angle of refraction (r) by using the inverse sin function

r space equals space sin to the power of – 1 end exponent space left parenthesis 0.1692 right parenthesis space equals space 9.7 space equals space 10 degree

Examiner Tips and Tricks

Important: in snell's law fraction numerator sin space i over denominator sin space r end fraction is not the same as i over r. Incorrectly cancelling the sin terms is a common mistake!

When calculating the value of i or r start by calculating the value of sin i or sin r. You can then use the inverse sin function (sin–1 on most calculators by pressing 'shift' then 'sine') to find the angle.

One way to remember which way around i and r are in the fraction is remembering that 'i' comes before 'r' in the alphabet, and therefore is on the top of the fraction (whilst r is on the bottom).

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    Author
    Reviewer
    Katie M

    Author: Katie M

    Expertise: Physics

    Katie has always been passionate about the sciences, and completed a degree in Astrophysics at Sheffield University. She decided that she wanted to inspire other young people, so moved to Bristol to complete a PGCE in Secondary Science. She particularly loves creating fun and absorbing materials to help students achieve their exam potential.

    Caroline Carroll

    Author: Caroline Carroll

    Expertise: Physics Subject Lead

    Caroline graduated from the University of Nottingham with a degree in Chemistry and Molecular Physics. She spent several years working as an Industrial Chemist in the automotive industry before retraining to teach. Caroline has over 12 years of experience teaching GCSE and A-level chemistry and physics. She is passionate about creating high-quality resources to help students achieve their full potential.