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Interference
Interference is a phenomenon that occurs when two waves, traveling along the same medium, interfere. This can have constructive and destructive effects. This is why the two forms of interference are Constructive and Destructive. Constructive interference occurs when two waves travelling along the same medium interfere and when they overlap, the amplitude is greatly increased and the shape of the medium changed. This occurs because both waves have a displacement in the same direction (up or down). When they combine, this amplitude or displacement is doubled, changing the shape of the medium. Destructive interference, on the other hand, is different. This interference occurs when two waves travelling along the same medium interfere, but in this case, the amplitude of the waves decreases, or the wave even stops. This occurs because the displacement of the two waves are opposite, and the resulting effects of this is that the wave decreased or is completely cancelled out. In conclusion, interference is the result of two waves that interfere, the amplitudes add or subtract, and the medium is therefore consequently changed.

See this website for a demonstration on the effects of Constructive Wave Interference, and this website for a demonstration on the effects of Destructive Wave interference.


Interference can be displayed in real life through the dropping of stones in water. When the first stone is dropped in water, a ripple effect, or surface wave, is created, when the second stone is dropped near the first set of ripples, or surface wave, another set of ripples or surface wave is formed. These two waves interfere, constructively and destructively. In some parts of the water, mini waves are produced, this is the two amplitudes of the waves interfering, and the amplitudes doubling. In other parts of the water, it appears that no stone has been dropped at all, this is because a destructive interference has occurred in this part of the water.

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The ripple effect. Even though this is displayed through more than two surface waves, it can still clearly be seen that there a some rises and some halts in the ripple


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Reflection

Reflection occurs when a wave hits a flat surface and bounces back at an angle equal to the angle the light hit the surface. This reflection always happens in a straight line. In the diagram below, it can be seen that the angle that the light hits the mirror is called the angle of incidence, and the angle of the resulting reflected ray is called the angle of reflection. The corresponding rays with the angles are called The incidence ray and the reflected ray respectively. The line perpendicular to the reflecting surface is called the normal, and this is the line that helps the angles of the incidence and reflected rays be calculated.

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The above example however, applies to a plane mirror, when light hits a mirror that is curved, different effects occur.
When light hits a curved mirror, it is focused inwards into the middle of the mirror, or the 'focus'. This is different to a plane mirror as the light hitting it does not have to be at an angle to create an angle of incidence and reflection. Concave mirrors are useful as they can focus things. Convex mirrors are similar to concave mirrors, but instead the light is reflected outwards. This helps in real life with some mirrors as it gives a wider picture of a room.
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Refraction
Refraction occurs when a wave passes through a surface that is more or less dense than the previous surface it passed through. This density is called optical density. Refraction is the bending of a wave.

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As it can be seen in these two diagrams (Science World 3) refraction is a strange phenomenon as it changes the way that objects are perceived. In the diagram to the left, the coin appears to be a lot further away from the mugs bottom than it actually is. In the second diagram, it shows the angle of incidence and reflection being changed dramatically from the point the light enters the glass block until the point that it exits it.



Diffraction
Diffraction is another phenomenon that occurs concerning waves. When a wave travels through an object with a gap in it or simply with a wave moving around an object, a concave semicircle is formed on the other side. This happens because of a waves ability to spread in circles or spheres. This happens in real life through water waves passing through a gap in a rock.

A demonstration on the way diffraction hurts can be seen here

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To see how some of this relates to real life, go to Real World Example Video!!!!!!!


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