Refraction, Diffraction and Interference

• all waves diffract
• the amount of diffraction depends on the wavelength of the wave compared to the size of the gap

  • when the gap is bigger than the wavelength, diffraction is unnoticeable
  • when the gap is several wavelengths wide, you get noticeable diffraction
  • you get the most diffraction when the gap is the same size as the wavelength
  • if the gap is smaller than the wavelength then the waves are mostly reflected back
• to observe a diffraction pattern

  • monochromatic, coherent light source (such as a laser)
  • if the wavelength is around the same size as the aperture then you get a diffraction pattern
  • fringes are caused by destructive and constructive interference of light waves
• diffracted white light creates a spectra of colours

  since white light is a mix of different wavelengths of light

  • the different wavelengths are diffracted by different amounts
  • this gives you a spectra of colours
• increasing the slit width decreases the amount of diffraction

  • the central maximum is narrower
  • its intensity is higher
• increasing the wavelength increases the amount of diffraction

  • the central maximum is wider
  • its intensity is lower
• interference patterns become sharper with more slits

  diffracted light intensity is more intense

  because there are lots of different beams reinforcing the pattern, making for more accurate measurements
• for monochromatic light, all the maxima are sharp lines

  • there is a line of maximum brightness in the centre called the zero order line
  • for a grating with slits a distance of d apart (d=1/ the number of slits per metre), the angle between the incident beam and the nth order maximum is $d\sin\theta=$$n\lambda$
• diffraction gratings help to identify elements and calculate atomic spacing by using X-ray crystallography

  • the wavelength of X-rays are of a similar wavelength to the atom spacings in crystalline solids
  • the crystal acts as a diffraction grating, causing the X-rays to form a diffraction pattern
• light is fastest in a vacuum. it slows down in other materials because it interacts with the particles in them. the more optically dense a material is, the more light slows down when it passes through
• absolute refractive index of a material

  $n=$$c\div c_{s}$

  the absolute refractive index is a property of THAT MATERIAL ONLY

  the relative refractive index is a property of the interface BETWEEN TWO MATERIALS
• optical fibres use total internal reflection

  • thin flexible tube of glass or plastic fibre that can carry light signals over long distances and around corners
  • step-index optical fibres have a high refractive index but are surrounded by cladding with a lower refractive index to allow total internal reflection (also protects from scratches which could cause light to escape)
  • all of the light is totally internally reflected from boundary to boundary until it reaches the other end
• a signal travelling down an optical fibre can be degraded by absorption or dispersion. signal degradation can cause information to be lost
• absorption causes loss in amplitude

  • as the signal travels, some of its energy is lost through absorption by the material that the fibre is made from
  • this energy loss results in the amplitude of the signal being reduced
• there are two types of dispersion

  • modal dispersion is when the light rays enter at different angles and so take different paths. a single-mode fibre only allows for one path and stop this
  • material dispersion - some wavelengths of the light will reach the end faster. using monochromatic light can stop this.
  • dispersion causes pulse broadening which can overlap and confuse the signal
• an optical fibre repeater can be used to boost and regenerate the signal every once in a while which can reduce signal degradation caused by both absorption and dispersion
• the angle of incidence is between the normal to the boundary and the ray of light arriving at the boundary