8. REFRACTION of waves and a scientific wave model

• 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.

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  • 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.

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• 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.

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  • 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' ie 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.

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• 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.

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