Revision Notes

3.2.2 Refraction of Light


  • When light enters a glass block, it slows down, causing it to change direction
  • When it leaves the block it speeds up again, changing direction once more

Refraction of light, IGCSE & GCSE Physics revision notes

Diagram showing the refraction of light as it passes through a rectangular block

  • As the light enters the block it bends towards the normal line
    (Remember: Enters Towards)
  • When it leaves the block it bends away from the normal line
    (Remember: Leaves Away)

Investigating Refraction

  • In your examination you might be asked to write a method explaining how you might investigate the refraction of light through different shaped blocks
  • As part of this method you should describe:
    • What equipment you need
    • How you will use the equipment
    • How you will trace the rays of light before, while and after they pass through the block

Glass blocks experiment, IGCSE & GCSE Physics revision notes

Diagram showing a ray box alongside three different shaped glass blocks


  1. Place the glass block on a sheet of paper, and carefully draw around the block using a pencil
  2. Take a ray box and carefully aim the box so that a single ray of light passes through the block
  3. Using a pencil, mark some points along the path of the ray:
    Before it reaches the block;
    Where it hits the block;
    Where it leaves the block;
    After it has left the block
  4. Now remove the block from the paper and, using a ruler and pencil, draw straight lines connecting points: a and b; b and c; c and d. The resulting line will show the path of the ray
  5. Replace the block within its outline and repeat the above process for a ray striking the block at a different angle

Exam Tip

Key things to remember include:

    • Naming the apparatus that you need (remember the ray box)
    • Explaining how to trace the rays
Extended Only

Snell's Law

  • When light enters a denser medium (such as glass) it slows down and bends towards the normal

Snells law, IGCSE & GCSE Physics revision notes

Diagram showing the angle of incidence, i, and the angle of refraction, r, of a ray of light entering a glass block

  • Snell’s law gives the relationship between the angle of incidence i, and the angle of refraction r:

Snells law equation, IGCSE & GCSE Physics revision notes

  • Where n is the refractive index of the material
  • You can rearrange this equation with the help of the formula triangle:

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

Use the formula triangle to help you rearrange the equation

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

Refractive index equation, IGCSE & GCSE Physics revision notes

  • The refractive index is a number that is always bigger than 1 and is different for different materials (n is about 1.5 for glass)

Exam Tip

Important: (sin i / sin r) is not the same as (i/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) to find the angle.

Author: Jenna

Jenna studied at Cardiff University before training to become a science teacher at the University of Bath specialising in Biology (although she loves teaching all three sciences at GCSE level!). Teaching is her passion, and with 10 years experience teaching across a wide range of specifications – from GCSE and A Level Biology in the UK to IGCSE and IB Biology internationally – she knows what is required to pass those Biology exams.

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