Refraction, diffraction and interference

• Coherent waves have a fixed phase difference and the same frequency and wavelength.
• A laser produces monochromatic (same wavelength / colour) light so diffraction and interference patterns are more defined.
• Young’s double-slit experiment involves a single light source directed towards two slits, which each act as a coherent light source, the light interferes constructively and destructively to create an interference pattern.
• The interference pattern created using white light is a bright white central maximum flanked by alternating spectral fringes of decreasing intensity with violet closest to the zero order and red furthest.
• Interference patterns can be formed by sound waves and all EM waves, not just light.
• The position of a certain point on a wave cycle, measured in units of radians, degrees or fractions of a cycle, is referred to as 'phase'.
• An interference pattern forms when light is passed through a single slit because the light diffracts as it passes through the slit, where the waves are in phase constructive interference occurs making bright fringes and where the waves are completely out of phase destructive interference occurs making a dark fringe.
• Increasing the slit width increases the width of the central diffraction maximum, but this is not true as the slit is not so close to the wavelength in size so less diffraction occurs - the central maximum becomes narrower and more intense.
• When light enters a more optically dense medium, it bends towards the normal.
• Pulse broadening occurs when the received signal is wider than the original, causing overlap of signals leading to information loss.
• Total internal reflection occurs when light is at a boundary to a less optically dense medium and the angle of incidence is greater than the critical angle.
• Signal degradation by absorption in an optical fibre reduces the amplitude of the received signal as part of the signal’s energy is absorbed by the fibre.
• Material dispersion causes pulse broadening when light with different wavelengths slows down at different rates in the fibre, causing them to arrive at different times.
• Modal dispersion causes pulse broadening when light rays enter the fibre at different angles, some may travel down the middle while others are reflected repeatedly, causing the rays to take different times to travel along the fibre.
• The purpose of the cladding in a step index optical fibre is to protect the core from scratches which would allow light to escape and degrade the signal, and it allows total internal reflection as it has a lower refractive index than the core.
• Both absorption and dispersion can be reduced by using an optical fibre repeater to regenerate the signal now and then.
• Material dispersion can be reduced by using monochromatic light.
• When the angle of incidence is equal to the critical angle, a light ray goes along the boundary, the angle of refraction is 90°.
• Modal dispersion can be reduced by using a single mode fibre, which is a very narrow fibre, so the possible difference in path lengths is smaller.
• The critical angle can be found using the formula sinC = n2 / n1, where n1 > n2, and C is the critical angle, n1 is the refractive index of material 1, and n2 is the refractive index of material 2.
• The advantages of optical fibres over traditional copper wires include carrying more information as light has a high frequency, no energy lost as heat, no electrical interference, cheaper, very fast, and the signal can carry more information as light has a high frequency.
• Path difference is the difference in distance travelled by 2 waves.
• Diffraction and interference are wave properties hence the interference pattern of light shows light has wave properties.
• The formula associated with Young’s double slit experiment is: w = λD / s, where w is the fringe spacing, λ is the wavelength of light used, D is the distance from screen to slits, and s is the slit separation.
• The distance between each node is half a wavelength.
• Young’s double slit experiment provided evidence for the wave nature of light through diffraction and interference.
• Stationary sound waves can be investigated by placing a speaker at one end of a closed glass tube, laying powder across the bottom of the tube, it will be shaken from the antinodes and settle at the nodes.
• Four safety precautions that must be followed when using a laser are: wearing laser safety goggles, not shining the laser at reflective surfaces, displaying a warning sign, and never shining the laser at a person.