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8. REFRACTION of waves and a scientific wave model

Theoretical model of a refracted wave.

• Waves travel at different speeds in different materials and this can result in a change of direction as the waves pass through a boundary from one material to another.

  • This change in direction at the boundary between two media is called refraction.

• To explain how waves will be refracted at a boundary in terms of the change of speed and direction, we need diagrams!

  • In fact, a scientific model of waves!

• The scientific model explaining refraction of waves at a boundary between two media

• The above diagram illustrates the phenomena of refraction by considering what happens to waves e.g. visible light or water waves.

• You can think of the parallel lines as representing a series of crests of waves eg think of waves on the sea or ripples in a pond on throwing a stone in or ripple tank experiments in the school laboratory.

• The vertical dotted line is NOT a wavefront, but as already mentioned in reflection, it is an important imaginary construction line at 90^o to the media boundary to help measure what we call the angles of incidence and refraction - see diagram of refraction below.

• Refraction A: When waves passing through a less dense medium, hit a boundary interface, and enter a more dense medium, the waves 'bend towards the normal' i.e. refraction occurs.

  • *

  • This happens because on entering the more dense medium, the waves are slowed down causing the change in wave direction at the boundary interface. Diagram above and left of diagram below. Diagram B is discussed later, but it is the opposite situation to refraction A.

• 

• Comparing refractions A and B

• The above diagram illustrates the scientific model of the wave theory of refraction.

• You can also see that in refraction A the wavelength is decreased as well as the velocity.

• The frequency does NOT change.

• wave speed = frequency x wavelength, in 'symbolic shorthand'   

  •  v = f x λ   (see Part 10 wave calculations)

• If the frequency (f) does not change, then the velocity (v) is directly proportional to wavelength (λ).

• The bigger the change in speed the bigger the change in direction - the greater the angle of refraction.

• You see this effect in ripple tank experiments when you abruptly go from deeper water to shallower water the waves will change direction towards the normal.

  • The waves slow down in shallower water and if they hit the shallower water at an angle, refraction will occur.

  • The waves slow down in shallower water because of increased friction with the bottom surface of the ripple tank.

  • In this example the refraction has nothing to do with density, but is caused by increased friction - increase in the 'drag' effect.

  • You can observe this in a ripple tank by placing a rectangular plate in to the water at an angle to the waves and you can see these changes in wavelength and speed. BUT, by using a stroboscope you can show the frequency does not change.

  • 

  • Effect of decreasing speed.

• For examples see Reflection, refraction and diffraction of visible light, examples with diagrams and explanations

• Refraction B: When waves from a more dense medium, hit a boundary interface, and enter a less dense medium, the waves 'bend away from the normal' i.e. refraction occurs.

  • *

  • This happens because on entering the less dense medium, the waves can speed up causing the change in wave direction at the boundary interface. Diagram above and right of diagram below. Diagram refraction A has been previously discussed, but here refraction B is the opposite situation to refraction A.

• 

• Comparing refractions A and B

• The above diagram illustrates the scientific model of the wave theory of refraction.

• You can also see that in refraction B the wavelength has increased as well as the velocity.

• The frequency does NOT change.

• wave speed = frequency x wavelength, in 'symbolic shorthand'   

  • v = f x λ  (see Part 10 wave calculations)

• If the frequency (f) does not change, then velocity (v) is directly proportional to wavelength (λ).

• The bigger the change in speed the bigger the change in direction - the greater the angle of refraction.

• You see this effect in ripple tank experiments when you abruptly go from shallower water to deeper water the waves will change direction away from the normal.

  • The waves speed up in deeper water and if they hit the deeper water at an angle, refraction will occur.

  • The waves speed up in deeper water because of decreased friction with the bottom surface of the ripple tank.

  • In this example the refraction has nothing to do with density, but is refraction caused by decrease in friction - reduction of the 'drag' effect.

  • You can observe this in a ripple tank by placing a rectangular plate in to the water at an angle to the waves and you can see these changes in wavelength and speed. BUT, by using a stroboscope you can show the frequency does not change.

• Effect of increasing wave speed

• For examples see Reflection, refraction and diffraction of visible light, examples with diagrams and explanations

• If the waves hit the interface at an angle of 90^o (perpendicular) to the interface between the two mediums, there is still a change in speed and wavelength, but there is NO change in direction, NO refraction and the wave frequency remains the same. See diagrams below where the waves strike the boundary along, or parallel to, the normal.

• 

• A: When the waves pass from a less dense medium to a more dense medium the waves decrease in velocity at the media boundary and the wavelength also decreases.

• B: When the waves pass from a more dense medium to a less dense medium the waves increase in velocity at the media boundary and the wavelength also increases.

• In both cases the frequency remains unchanged and in both cases no refraction takes place.

• You can observe this in a ripple tank by placing a rectangular plate in to the water parallel to the waves and you can see these changes in wavelength and speed. BUT, by using a stroboscope you can show the frequency does not change.

• 

• The effect of increasing or decreasing friction in a ripple tank - effectively decreasing speed and wavelength or increasing speed and wavelength,

INDEX notes: Investigating and introducing the properties of waves

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Key points Introduction to waves: A model of wave refraction at interface between two media

Information sources for Doc Brown's key points: IGCSE-GCSE physics are based on textbooks & syllabus-specifications for students taking the UK AQA, Edexcel, OCR 21st Century Science, OCR Gateway science suite, WJEC, CCEA and CIE GCSE physics 9-1 level science examinations

Here's a structured set of summary revision notes on a model to explain the refraction of waves at an interface between two media, aligned with the core requirements across major UK GCSE/IGCSE exam boards (WJEC, CCEA, CIE, AQA, Edexcel, OCR). This model helps students visualise how wave speed and direction change at boundaries - essential for mastering wave behaviour.

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Refraction of Waves: Conceptual Model

Definition: Refraction is the change in direction of a wave when it crosses a boundary between two media due to a change in wave speed.

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Refraction Model Components

Component	Description
Incident ray	The incoming wave approaching the boundary
Refracted ray	The wave that enters the second medium and bends
Normal line	A line drawn perpendicular to the boundary at the point of incidence
Angle of incidence (i)	Between incident ray and normal
Angle of refraction (r)	Between refracted ray and normal
Wave speed	Changes depending on the medium’s density
Wavelength	Changes with speed; frequency stays constant

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Refraction Behaviour at the Boundary

Transition	Wave Speed	Direction Change
Less dense → more dense	Slows down	Bends towards normal
More dense → less dense	Speeds up	Bends away from normal
Along the normal	No change	No bending

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Required Refraction Practical (Ray Box and Glass Block)

Students investigate refraction by:

• Shining light through a glass block
• Drawing incident and refracted rays
• Measuring angles i and r
• Observing bending and calculating refractive index (CIE only)

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Real-World Examples of Refraction

• Light entering water or glass
• Sound waves entering different materials
• Water waves slowing in shallow regions
• Lenses focusing light via refraction

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Typical Exam Board Coverage of Refraction

Refraction Model	Required Practical
Refraction at boundaries	Light box experiment
Refraction of light	Glass block practical
Refraction & wave speed	Ray tracing
Refraction in water/light	Ripple tank
Refraction of sound/light	Refraction experiment
Refraction & Snell’s Law	Ray box & prism (only CIE)

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 Student Tips about Refraction

• Always draw the normal line first when constructing ray diagrams.
• Use a sharp pencil and protractor for accurate angles.
• Remember: frequency stays constant during refraction.
• Practise identifying direction of bending based on medium density.
• Link refraction to applications like lenses, prisms, and fibre optics.

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Common Misconceptions about Refraction

•  “Refraction changes frequency.”
   Frequency remains unchanged; speed and wavelength change.
•  “Waves always bend at boundaries.”
   Only if the wave hits the boundary at an angle.
•  “Refraction only happens with light.”
   All waves can refract - sound, water, and EM waves included.
•  “Waves bend away when entering denser media.”
   They bend towards the normal when slowing down.

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Keywords, phrases and learning objectives for waves - refraction model

Be able to describe refraction of waves using a scientific wave model and explain wave refraction with suitable diagrams i.e. explain the theoretical modelling of refracted waves.

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WHAT NEXT?

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INDEX physics notes: Introducing the properties of waves

INDEX of all notes on waves, radiation, astronomy etc.

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INDEX notes: Investigating and introducing the properties of waves