Refraction,Diffraction,interference Flashcards
what causes interference
when two waves are superimposed on each other. they must be coherent
define coherent
when the two waves have identical frequency and constant phase difference
what is the path difference
the difference in length the waves have travelled to reach a certain point
define monochromatic
light of the same wavelength
how can you create two coherent sources of light
shine a laser through a slit with a double slit behind it. the light diffracts when passing the first slit and then goes through the second two and since the laser produces monochromatic light the slits act like two coherent sources.
why is an average fringe spacing found
to ensure the value is accurate because the fringe spacing is usually very small
what is a light fringe
light or dark bands caused by interference or diffraction of light
what happens when white light is used instead of a laser in the double-slit experiment
the middle fringe will be bright white light, the fringes will be more spread out and side fringes will be a spectrum of visible colours (blue closest to the centre cause it diffracts less than red light)
risk of lasers
eye injury. the natural aversion reflex (the eye will automatically look away from the light source) is too slow to prevent retina damage
how to be safe with lasers
don’t point it in peoples eye,
wear protective eyewear,
stick to institutional safety requirements
theories of light
Newton- said that light was made of corpuscles (pockets of light) in the 1600s
Huygen- gave the wavefront theory in the 1600s
Young- the double-slit experiment gave evidence for Huygen’s theory
Einstein- provided the photoelectric effect as evidence for newtons theory
Compton scattering (a photon scattering of an electron) gave evidence for the quantisation of light in the 20th century
how can interference be observed from sound waves
place two sources facing each other. the sound needs to be coherent. (same sound needs to be played at the same time from each source). then move a microphone from one source to the other. constructive or destructive interference is measured by whether the noise is loud or quite
why do the sound waves have to be coherent when measuring interference
the get a constant interference pattern
what causes dark/light fringes from a single slit experiment
the light fringes are caused by constructive interference. the dark ones are caused by destructive interference
how does the slit width affect diffraction
an increase in the slit width will decrease the central maximum because diffraction effects will decrease. the intensity of the central maximum will increase because the photons are less spread out
how does the wavelength affect diffraction
increasing incident light wavelength will increase the width of the central maximum because diffraction effects will increase. the intensity of the central maximum will decrease because the photons are more spread out.
what happens when light is shone through a diffraction grating
if the slit widths are the same as the wavelength the light is diffracted into a pattern of bright and dark lines.
what is an order
the position of the maxima in a diffraction grating pattern. the central point is the ‘zero-order’. its the brightest of all the lines. either side of it is the first-order lines. the further from the central line the dimmer the lines and the more visible the orders
what happens if there are more slits in the diffraction grating
it causes a sharper line pattern on the screen. this makes them more easily measurable
how does changing the spacing between slits and the wavelength of light change Sin(θ) in a diffraction grating
increasing wavelength increases θ so Sin(θ) also increases.
increasing the slit spacing decreases θ so Sin(θ) also decreases
when does the first-order maximum occur
it happens at an angle when the path difference between the two sources is one λ
how do you derive the grating equation
- produce coherent light sources
- 1st order maxima happens at an angle that the path difference = λ (angle θ)
- creat the triangle from the diagram of the grating (the hyp is the slit spacing and the opp is λ)
- trig shows that d*Sin(θ) = λ
- generalise for all. nλ=d*Sin(θ) where n = a whole number
why can crystals be used as a diffraction grating
there are tiny spaces between atoms in a crystal for light to pass through. the spacing can be found by looking at how x-rays diffract through
how can a diffraction grating be used to analyse light
a diffraction grating can be used to separate wavelength of light from different substances. the present wavelengths can be used to learn about the substance
define refraction
the change in direction of light as it changes speed when passing from one medium to another
what is a refractive index
the ration of the speed of light in a vacuum to the speed of light in that material
whats the refractive index of air
its so similar to the speed of light in a vacuum that the refractive index of air is approximately 1. c↓air ≈ c
what does snell’s law do
relates the angle of incidence and refraction to the refraction indices of the mediums
what is optical density
the more optically dense a material is the slower light travels through it. When light travels from a less optically dense medium to a more optically dense medium, it bends towards the normal.
When light travels from a more optically dense medium to a less optically dense medium, it bends away from the normal.
what happens to light if the incidence angle is greater than the critical angle
when the angle of incidence is greater than the critical angle, no light is refracted. instead, it is reflected (this is called total internal reflection)
what is internal reflection
when light enters a less optically dense medium it speeds up. some light is refracted but some is reflected
what happens when the angle of incidence is exactly equal to the critical angle
the ray of light will travel along the boundary of the two mediums. the angle of refraction is at 90 degrees to the normal
how do optical fibres work
they use total internal reflection to transmit light long distances
how is an optical fibre designed
each fibre contains a core of optically dense material that is covered by a layer of cladding which is much lower optically dense giving a very small critical angle
what causes material dispersion
it’s due to different wavelengths of light arriving at the end of the fibre at different times. this is because the refractive index changes with frequency. this causes pulse broadening. To avoid this monochromatic light can be used
what does pulse broadening in fibre optic cables cause
it degrades the signal so they may now overlap with each other.
what is modal dispersion
this is due to the different paths a signal will take in a fibre. this means different parts of a signal arrive at different times. this results in pulse broadening. to avoid this a monomode fibre is used.
what is pulse absorption
a signal may be absorbed by the material. this reduces the signal amplitude.
how can signal absorption and broadening be stopped
signals can be boosted by repeaters to counteract signal absorption and broadening
define diffraction
the spreading of a wave as it passes through a gap. diffraction is maximised if the gap is roughly the same width as the wavelength of the wave
what are the 3 ways to make a standing wave
closed pipe. antinode and the start, node at the end
open pipe. anti node and start and end
string- node at start and end
define reflection
when light changes direction at a boundary and remains in the original medium
law of reflection
the angle of incidence = angle of reflection
what properties of waves change when they are reflected or refracted
reflected- none
refracted- wavelength
principle of superposition
when two waves of the same type meet at a point the resultant displacement is equal to the sum of the displacements of the individual waves
define constructive interference
superposition of two waves causing a larger displacement than the original waves
define destructive interference
superposition of two waves causing a smaller displacement than the original waves.
define phase difference
the difference between the displacement of particles along a wave
