Interference and diffraction

measure larger angle

to reduce percentage uncertainty

d sin θ = nλ

sin 0.500 / 2 = 19

19 x sin 0.400

5.29

measure distance of grating to screen (l)

and from centre to dot (x)

tan θ=x/l

gradient

gradient=d/λ

path difference is zero

phase difference is zero

so superposition takes place

diffraction grating

set up grating parallel to screen

measure distance between grating and screen

measure the distance between 1st order images on the screen

how diffraction pattern is created

waves passing through a narrow gap spread out

light reaches the wall from each part of the slit with differing wavelengths

when waves meet constructively interfere if in phase so brighter region is seen

red v green wavelength

red has longer wavelength than green

means red diffracts more

more dark points would be closer to the centre

diffraction occurs when light is reflected from cd surface

each ring on cd acts as a diffraction centre scattering light in all directions

interference occurs

in directions in which there is a path difference equal to a whole number of wavelengths and constructive interference occurs

white light is a range of wavelengths

hence each wavelength of light reinforces in a different direction which explains why a spectrum is seen

coherent waves monochromatic

coherent waves have a constant phase relationship

coherent waves have the same frequency

however for each frequency present the two reflected wavelengths are coherent

with a non chromatic source a set of dark rings for each frequency would be produced

hence with a white light source you would see a set f coloured rings

light is diffracted

each point on the wave front acts as a source for secondary wavelets

Huygens construction through gap

wave spreads out

each points acts as a secondary source for wavelets

they superpose

constructive interference

in phase

max intensity

path difference = nλ

destructive interreference

anti phase

min intensity

path difference=(n+1/2)λ

wavefront

line joining points on a wave that are in phase

why monochromatic light is important in diffraction experiments

emits small range of wavelengths

so smaller variation at each angle

produces a clearer interference pattern

wave and reflection will meet and superposition will take place

where in antiphase destrucive interference takes place

minimum amplitude at nodes so mice wont hear

waves also arrive from other surfaces

complete cancellation unlikely

resolution would be same but distance measured would be greater

coherent

constant phase relationship

superposition

two or more waves meet

the resultant displacement at a point is the sum of individual displacements from the individual waves

why x model was accepted

-provided experimental evidence

-supported previous evidence

-can be repeated by others