3.3.2 refraction, diffraction + interference

define path difference and coherence

Path difference is the difference in the distance travelled by two waves.

A coherent light source has the same frequency and wavelength and a constant phase difference.

describe young’s double slit experiment

• use one coherent source of light and a double slit OR two coherent sources of light in order to produce an interference pattern

• 2 slits about the same size as the wavelength of the laser light so the light diffracts

• wave is diffracted from each slit/each slit acts as a source + these superpose

• produces a pattern of light and dark fringes

characteristics of interference pattern

• each bright fringe is identical and has the same width and intensity

• fringes are separated by dark narrow bands of destructive interference

why are alternate maxima and minima of intensity observed?

• bright fringes are formed where the light meets in phase → superpose → interferes constructively → path difference is a whole number of wavelengths (nλ) - where n is an integer → maxima

• dark fringes are formed where the light meet out of phase → superpose → interferes destructively → waves in antiphase → (n+1/2)λ → minima

young’s equation

w = fringe separation

λ = wavelength of light

D - distance from slits to source

S = slit separation/spacing

effect of using white light instead of monochromatic light

• central maximum is white because each wave interfered here constructively

• other maxima are composed of a spectrum

• shortest wavelength (violet) would appear nearest to the central maximum

• longest wavelength (red) would appear furthest from the central maximum

• wider maxima and less intense diffraction pattern → as it moves further away from centre, wavelengths of blue observed decrease, colours look blurry

issues associated with using lasers + safety precautions

can cause permanent damage to your eyesight

• wear laser safety goggles

• don’t look directly into laser

• don’t shine laser at reflective surfaces

what does young’s double slit experiment prove/how understanding of EM radiation has changed over time

• evidence that light is a wave because diffraction and interference are wave properties

• proves that EM radiation must act as a wave (at least some of the time)

• however, this is not always what people thought. knowledge and understanding of any scientific concept changes over time in accordance to the experimental evidence gathered by the scientific community.

what is diffraction

diffraction is the spreading out of waves as they pass through or around a gap.

- the greatest diffraction occurs when the gap is the same size as the wavelength

- less diffraction occurs when waves pass through a wide gap than through a narrow gap

single slit diffraction

• monochromatic light can be diffracted through a single slit onto a screen, which forms an interference pattern of light and dark fringes

• wavefront within the slit behaves as a series of point sources which spread circular wave fronts from them

interference pattern of a single slit:

• the patterns has a bright central maximum, which is double the width of all other fringes (secondary maxima) either side

• alternating dark and bright fringes on either side

• bright fringes → constructive interference → waves meet in phase

• dark fringes → destructive interference → waves out of phase

• the peak intensity of each fringe decreases with distance from the centre

• each of the outer fringes is the same width

explain why the minimum intensity between two outer maxima is not zero

1. the waves arriving at a minimum have varying amplitudes

2. they do not meet completely out of phase

3. path difference results in destructive interference - the waves partially cancel each other out, leading to a minimum intensity that is not zero.

4. intensity decreases from central maximum - intensities at minimum points are not equal

interference pattern on white light single slit

• central white fringe

• each subsidiary maxima are composed of a spectrum → violet nearest the centre and red furthest

• maxima are wider (than monochromatic light) and dark fringes are smaller

single slit diffraction equation

W = width of central fringe

D = distance from slit to screen

a = width of single slit

diffraction grating equation

dsinθ = nλ

n = the order

d = grating spacing

applications of diffraction gratings

• spectral analysis of light from stars

• analysing composition of stars

• measuring red shift/rotation

what is refraction

refraction is the change in direction/bending of light when it enters another medium due to the change in speed(which changes direction).

(partial reflection also occurs when a light ray in air enters glass or any other refractive substance)

what is refractive index + the equation

the ratio of speed in light in a vacuum ( c ) : speed of light in that substance (cs)

for a light ray travelling from air into a transparent substance,

n = sini/sinr

snell’s law of refraction for boundary

what properties of the light wave changes during refraction

• frequency remains the same

• speed changes - increases when entering less optically dense medium and vice versa

• wavelength changes - increases when entering less optically dense medium

define critical angle and total internal reflection

• critical angle is the angle of incidence at which a ray of light would have an angle of refraction of 90 degrees and remain inside the denser material

• total internal reflection is when a ray of light is completely reflected back into the medium of incidence

conditions required for total internal reflection

1. angle of incidence is greater than the critical angle

2. incidence substance has a larger refractive index than the other substance (i.e. entering a less optically dense medium)

critical angle equation:

what are optical fibres used for

used in medical endoscopes to see the inside of the body

communications to carry light signals

how is total internal reflection used to transmit information along an optical fibre?

1. light transferring information enters core

2. light travels from core to cladding, which has a lower refractive index

3. at an angle > critical angle

4. TIR occurs and continues throughout the fibre

purpose of the cladding (3)

1. causes total internal reflection

2. prevents light loss from the core (light going from one fibre to another)

3. protects the fibre

purpose of the outer sheath (2)

1. prevents physical damage to fibre i.e. scratches

2. strengthens the fibre

what is a step index profile

core has a uniform refractive index + there is a sudden change in refractive index at the core-cladding boundary

what are material and modal dispersion

material: different colour light travels at different speeds in any medium apart from a vacuum. if white source was used red would arrive first (travels fastest). to prevent this only monochromatic light is used

modal: occurs when light pulses in the optical fibre spread out due to the different angles of incidence in the original pulse. in wider cores, light travelling along the axis of the core travels a shorter distance than the light undergoing TIR at the core-cladding boundaries (image). prevented by making core narrow.

both lead to pulse broadening

what is pulse broadening

caused by modal and material dispersion

impurities/irregularities in the fibre can cause pulse to be spread out due to dispersion of light.

causes different pulses to merge, leading to a completely distorted final pulse

absorption occurs when.

part of the signal’s energy is absorbed by the fibre therefore signal is changed (reduced in amplitude) → loss of information

to reduce absorption (2)

1. use core which is extremely transparent: more transparent → less light is absorbed

2. use optical fibre repeaters so that pulse is regenerated before significant absorption has taken place

to reduce pulse broadening (4)

1. make core as narrow as possible → reduce possible differences in path length of the signal

2. use monochromatic light source so that the speed is constant

3. use optical fibre repeaters so that pulse is regenerated before significant pulse broadening has taken place

4. use single-mode fibre to reduce multipath modal dispersion