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Waves (Progressive Waves
Waves move energy from one place to another. In…
Waves
Progressive Waves
Waves move energy from one place to another. In a progressive wave the wave front moves through the medium. There are two types of waves, transverse and longitudinal.
Transverse
are waves where the displacement of the particles in the medium is perpendicular to the direction the wave is travelling in. For example water waves.
- Electromagnetic waves are oscillating electric an magnetic fields they include radio waves, microwaves, infra-red, visible light, ultra-violet, x-rays and gamma rays.
- Electromagnetic waves are transverse waves and they all travel at the speed of light ( 3 x 108 ms⁻¹) in a vacuum.
Transverse waves can be polarised but longitudinal waves cannot. Unpolarised light is a mixture if waves in different planes. When this light is passed through a polaroid material only light waves in one plane are transmited and the light is now polarised.
Longitudinal
are waves where the displacement of the particles in the the same direction as the wave is travelling in. For example sound waves.
Describing Waves
- Amplitude (A) is the maximum displacement of a particle in a wave from its equilibrium position. It is measured in metres (m).
- Frequency (f) is the number of complete waves passing a point in one second. It is measured in hertz (Hz).
- Wavelength (λ) is the distance between two identical points on a wave (i.e. one full wave). It is measured in metres (m).
- Wave speed (c) is measured in metres per second (ms⁻¹).
Phase
- Points on a wave which are always travelling in the same direction, rising a falling together, are in phase with each other.
- Points on a wave which are always travelling in opposite directions to each other, one is rising while the other is falling, are in antiphase with each other.
Path difference
- If we measure the distance travelled by two waves and then compare those distances, any difference in the distances travelled is called the path difference. Path difference is measured in metres (m).
Diffraction
Diffraction happens when a wave hits an obstacle or gap, diffraction is greatest when the gap is about the same size as the wavelength of the wave. The waves bend round the object or spread out when they pass through the gap, this is called diffraction.
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Single Slit Diffraction
When monochromatic laser light is shone through a narrow single slit a diffraction pattern is produced consisting of light and dark fringes. It produces a wide central bright fringe. The other bright fringes get dimmer as you move away from the centre.
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Diffraction grating
A diffraction grating is a piece of glass with lots of closely spaced parallel lines on it each of which allows light to pass through it, this is a transmission diffraction grating.
Diffraction gratings are used in spectrometers. The diffraction grating splits up the light into a spectra.
If there are N lines per metre on a diffraction grating then d can be calculated using
Refraction
Refractive Index (n)
The refractive index (n) of a material is the ratio of the speed of light (c) in a vacuum to the velocity of light in the material (cₛ).
The refractive index of a material is always greater than 1 e.g.
- Water = 1.33
- Diamond = 2.42
- Glass = 1.5
- Air » 1
When a ray of light goes from material (1) into material (2), rather than from a vacuum into a material we talk about the relative refractive index.
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Critical Angle
- When a ray of light goes from a material into an optically less dense material like air. The angle of refraction can become 90° and the ray of light travels along the boundary between the two material. When this happens the angle of incidence is called the critical angle
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If the incident angle is greater than the critical angle then light reflects at the boundary between the two material and this is called Total Internal Reflection.
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Superposition of waves, stationary waves
Superpositon of Waves
When two waves pass through the same point they combine together to either constructively interfere with each other or destructively interfere with each other before passing on past each other and continuing their separate journeys.
Constructive interference The two waves are in phase with each other and constructively interfere to give a wave of greater amplitude.
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Destructive interference The two waves are out of phase (anti-phase) with each other and destructively interfere to give a wave of zero amplitude.
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