refraction, diffraction, interference Flashcards
define refraction
Refraction is the change of direction that occurs when light passes at an angle across a boundary between two transparent substances.
what speed do visible light and other waves in EM spectrum travel
Visible light and other waves in the electromagnetic spectrum travel at a speed of 3.00X108 ms*’ in a vacuum. Light travels slower in all other substances.
what is the refractive index
The refractive index n of a substance is the ratio of the speed of light in a vacuum c and the speed of light in that substance cs
n= speed of light in a vacuum / speed of light in substance
n= c/cs
what is the refractive index of air?
1
what is the refractive index of water
1.33
what is the refractive index of glass?
1.50
the greater the refractive index….
The greater the refractive index of a substance, the more slowly light travels through it, and the higher its optical density.
what is snell’s law
When light travels from one substance into another, the angle of incidence, angle of refraction, and refractive index of each substance are related by Snell’s law:
n1 sin 01 = n2 sin 02
what do rays of light travelling from a less optically dense substance into a more optically dense substance do?
refract towards the normal
what do rays of light travelling from a more optically dense substance into a less optically dense substance do?
refract away from the normal.
when does total internal reflection occur?
Total internal reflection occurs when light is travelling from an optically-dense material
to a less optically-dense material, and the angle of incidence is greater than the critical angle.
n1>n2
i > c
what is the critical angle?
The critical angle is the angle at which the angle of refraction is 90° for light going from an optically-dense material to a less optically-dense material:
sin0c = n2/n1
where n2 <n1
what do optical fibres do?
optical fibres make use of total internal reflection to transmit information encoded in light over long distances and a round corners.
describe the structure of an optical fibre
The core of an optical fibre is made of glass with a higher refractive index than the cladding. The
cladding protects the core from being damaged and prevents light from leaking out.
what is signal degradation?
Signal degradation in an optical fibre can cause intormation to be lost. It can occur due to absorption or pulse broadening.
what is absorption in optical fibres
Some of the light is absorbed by the optical fibre every time it reflects, resulting in a decrease in the amplitude of the signal.
what is pulse broadening in optical fibres?
The signal pulse gets broader as it travels along the fibre, reducing the quality of the signal and leading to loss of information if the pulses overlap.
it can be caused by modal or material dispersion.
what is MODAL dispersion?
If the core is wide enough, rays of light entering it at different angles will take different paths through it and travel different distances.
how can modal dispersion be reduced?
by using an optical fibre with a narrower core , known as a single mode fibre.
what is MATERIAL dispersion?
if white light is used for the signal, the different wavelengths in it travel at slightly different speeds through the fibres, causing them to separate as they travel along the fibre.
how can material dispersion be eliminated?
by using mono chromatic light
when are waves COHERENT
when they have the same frequency and a constant phase difference
constructive interference…
constructive interference occurs when two waves in phase superpose to give a wave with a larger amplitude.
destructive interference….
destructive interference occurs when two waves out of phase superpose to cancel each other out.
when does an interference pattern occur?
an interference pattern occurs when waves with a constant phase difference superpose forming cancellation and reinforcement at fixed points
what is the path difference?
the path difference between two waves is the difference in the distance travelled by the waves to the point at which they superpose
what happens if the path difference is a whole number of wavelengths n λ?
if the path difference is a whole number of wavelengths n λ ,the waves always arrive in phase at that point and constructive interference occurs producing a maximum.
what happens if the path difference is (n+ 1/2 λ)
if the path difference is (n+1/2λ) then destructive interference occurs, producing a minimum
what is two source interference?
two source interference occurs when coherent waves from two sources superpose to produce an interference pattern
what does young’s double slit provide evidence for?
the wave nature of light, because the resulting fringe pattern is due to diffraction and interference which cannot happen with particles.
what happens in young’s double slit experiment?
a single beam of light is split into two coherent beams as it passes through the closely spaced double fringe. this produces a pattern of dark and bright fringes on a screen
where do bright fringes form?
bright fringes form where the path difference between light from each slit is n λ, so waves arrive in phase and superpose constructively
where do dark fringes form?
dark fringes form where the path difference is ( n + 1/2 λ ) so the waves arrive out of phase and superpose destructively
define fringe separation w
fringe separation w is the distance between the centres of two adjacent maxima or minima (bright or dark fringes)
what is the equation for fringe separation?
w = λD/ s
w is fringe separation
λ is wavelength
D is distance from screen
s is slit separations
define diffraction
Diffraction is the spreading of waves when they pass through a gap or round an obstacle
what happens to light passing through a narrow slit?
Light passing through a narrow single slit is diffracted, producing an intenterence pattern on a screen.
For monochromatic light:
• the central fringe is much brighter and is twice as wide as the outer fringes
• the outer fringes are the same width and their brightness decreases from the centre
for white light, the different wavelengths are diffracted by different amounts, resulting In
• a bright white cental maximum.
• less bright fringes that are spectra, with blue light on the inner edge and red light on the outer edge.
what’s the difference between young’s double slit and the single slit?
for the single slit, the central maximum is very bright compared to fringes and it’s double the width of the fringes
diffraction grating vs single or double slit
Monochromatic light passing through a diffraction grating will produce an interference pattern where the maxima are much sharper than for a single or double slit.
the central maximum is the brightest and is called the zero order maximum.
as light from one source is diffracted through each slit of a grating,
As light from one source is diffracted through each slit of a grating, the diffracted waves constructively superpose in certain directions only, and destructively superpose in all
other directions.
what is the equation used for diffraction grating?
n lambda = d sin theta
where
n is the order of maxima
d is the slit width
what is D for diffraction gratings?
lines per mm
eg 100 lines per mm
d = 1/D
d = 1/100 = 0.01mm
= 1x10^-5 m
how to find the maximum number of order?
find n at 90°
n = dsin theta/ lambda
n= d/ lambda because sin 90 is 1
to find the maximum order
n = d/ lambda
