Physics Topic 6A- Properties of waves

• You need a taut string and a piece of kit called a vibratiob transducer. A vibration transducer will convert an electrical signal from a signal generator into vibrations. When it's attached to a taut string, waves are formed on the string.
• To ensure the string is taut by connecting one end to the vibration transducer and passing it over a pulley with masses hanging from the other end. This will keep it pulled nice and tight.
• Set up the equipment as shown in the next card, then turn on the signal generator and vibration transducer. The string will start to vibrate.
• Adjust the frequency of the signal generator until there's a clear wave on the string. The frequency you need will depend on the length of string between the pulley and the transducer, and the masses you've used.
• You need to measure the wavelength of these waves. The best way to do this accurately is to measure the length of four or five half-wavelengths in one go, then divide to get the average half-wavelength. Then double this to get the average wavelength.
• The frequency of the wave is whatever the signal generator is set to. You can then find the speed of the wave usinng the formula.

• Waves travel through different materials. When they arrive at a boudnary between two differnet materials, 3 things can happen:
  • Absorption- the waves may be absorbed by the material the wave is trying toi cross into. This transfers energy to the material's energy stores.
  • Reflection- the waves may bounce back
  • Transmission- the waves may be transmitted. This means that they carry on travelling through the material. However, they often undergo refraction. 

• When a waves reaches the boundary between two different materials, it can be transmitted through the new material. If it hits the boundary at an angle, it changes direction- it's refracted. 

• A ray is a straight line showung the path a wave travels along. 
• You can show the path taken by a light ray on a ray diagram.
  • Draw the boundary between the two materials and then as in the normal (a straight line that is 90° to the boundary) normally hsown as a dotted line.
  • Draw an incoming ray that meets the normal at the boundary.
  • The angle between the ray and the normal is the angle of incidence.
  • Now draw the refracted ray on the other side of the boundary. The anlge that the refracted ray makes with the normal is called the angle of refraction.
  • The angle of refraction could be smaller that the angle of incidence or bigger.
  If a ray travels along the normal and hits the boundary at 90°, it'll pass through without changing direction. The angles of incidence and refraction will both be 0°.

• Refractio occurs because waves travel faster in some materials than others, so the speed of a wave can change as it crosses a boundary.
• When a wsve refracts, it changes speed, but its frequency remains the same.
• Speed, frequency and wavelength are all related by the wave equation so as speed changes and frequency stays the same, wavelength must change.
• If the speed of a wave increases so does the wavelength.
• If the speed of a wave decreases so does the wavelength.
• The optical density of a material is a measure of how quickly light travels through it - the higher the optical density, the slower light travels.

• Wavefronts are imaginary lines drawb trhough certain points on waves, e.g. through each crest.
• They're perpendicular to the direction in which the wave is moving.
• In the wavefront diagrm on the next card showing light waves travelling along a normal to a boundary. The wavefronts are closer together in the more optically dense material, sowing the decrease in wavelength.
• Wavefront diagrams are useful in explaining why refraction happens.
• When a wave crosses a boundary into a new substance at an angle to the normal, one end of it enters the new material before the rest of the wave. This means that the end changes speed before the rest of the wave, casuing the wave to change direction.
• Which way the wave beeds depends on two things:
  • If a wave slows down at a boundary, it will bend towards the normal. Here the angle of refraction is smaller than the angle of incidence.
  • If a wave speeds up at a boundary, it bends away from the normal. Here the angle of refraction is greater than the angle of incidence.

• A wave can also be reflected when it hits a boundary of a different material.
• To draw a ray diagram showing reflection, you need to draw the incoming ray and the reflected ray. Also draw a normal.
• The angle of incidence is between the incoming wave and the normal.
• The angle of reflection is the angle between the reflected wave and the normal.
• The law of reflection applied to reflected wave is:
  • angle of incidence=angle of reflection.

• Waves are reflected differently at different boundaries:
• Specular reflection happens when parallel waves are reflected in a single direction by a smooth surface. e.g. when light is reflected by a mirror you get a nice clear reflection.
• Diffuse reflection is when parallel waves are reflected by a rough surface (e.g. a piece of paper) and the reflected rays are scattered in lots of different directions.
• The angle of incidence is still equal to the angle of reflection for each ray. However, the tiny bumps on the surface mean that the normal for each ray is different so the angle of incidence for each ray is different.
• When light is reflected by a rough surface, the surface appears matt and you don't get a clear reflection.

• It's best to do these experiments in a dim room so you can clearly see the light rays.
  
• A ray box or laser should be used to produce thins light rays so you can easily see the middle of the ray when tracing and measuring angles from it.

• To investigate how boundaries refract different amounts of light you can look at how much a light ray is refracted when it passes from air into different transparent materials, such as glass.

Method:

• Place a transparent rectangular block on a piece of paper and trace around it. Then use a ray box to shine a ray of light at the middle of one side of the block.
• Trace the incident ray on the paper, and do the same for light ray that emerges on the other side of the block.
• Remove the block and, with a straight line, join up the lines for the incident ray and the emerging ray showing the path of the refracted ray through the block.
• Draw the normal at the point where the light ray entered and then measure the angle between the incident line and the normal (angle of incidence, I) and the angle between the refracted ray and the normal ( the angle of refraction,R).
• Repeat this experiment with a different material keeping the same incident angle throughout.

• Different surfaces relfect light in different ways depending on how smooth they are.
• To investigate this, take a piece of paper and draw a straight line across it. Place a straight edged object on the paper so that it lines up with this line.
• Shine a ray of light at the object's surface and trace the incoming reflected light beams.
• Add in a normal line that meets the surface a the point of incidence.
• Draw and label the incident and reflected rays.
• Measure the angle of incidence and reflection.
• Measure and record the width of the reflected ray and make note of how bright it is.
• Repeat this for a range of objects.
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