physics chapter 5.4-5.5 optics - interference

coherent source of waves

emit light waves with a constant phase difference and the same frequency

Young's fringes

parallel bright and dark fringes observed when light from a narrow slit passes through two closely spaced slits

what would happen if the single slit is too wide in the double slit experiment

each part of the slit produces a fringe pattern, which is displaced slightly from the pattern due to adjacent parts of the single slit = dark fringes of the double slit pattern become narrower than the bright fringes = contrast is lost between the dark + bright fringes

when is a bright fringe formed

when the light from one slit reinforces the light from the other slit - the light waves from each slit arrive in phase with each other

when is a dark fringe formed

when the light from one slit cancels the light from the other slit - the light waves from the 2 slits arrive 180' out of phase

fringe separation

the distance from the centre of a bright fringe to the centre of the next bright fringe - depends on the slit spacing, s, and the distance, D, from the slits to the screen

fringe separation equation

w= λD/s

when do the fringes become more widely spaced (shown by the fringe separation equation)

when distance, D, from slits to screen is increased; when the wavelength of light used is increased; when the slit spacing, s, is reduced

how to measure the fringe spacing

measure across several fringes from the centre of a dark fringe to the centre of another dark fringe, as the centres of dark fringes are easier to locate than that of light fringes - then divide measurement by number of fringes measured across

how can loudspeakers be used to demonstrate interference

2 loudspeaker connected to the same signal are coherent sources of sound waves - you can detect points of cancellation + reinforcement by ear as you move across in front of the speakers

why are the double slits a coherent wave source

each wave crest/trough from the single slit always passes through one of the double slits a fixed time after it passes through the other slit = double slits emit wavefronts with a constant wave difference

why could light from 2 nearby lamp bulbs not form an interference pattern

the 2 light sources emit waves at random = the points of cancellation + reinforcement would change at random = no interference patter is possible

how does colour of light affect the double slit experiment

the fringe separation depends on the colour of light used

why is fringe separation greater for red light than for blue light

because red light has a longer wavelength than blue light

light from a vapour lamp / discharge tubes

a monochromatic light source, as its spectrum is dominated by light of a certain colour

light from a filament lamp / the sun

composed of colours of the spectrum, and so covers a continuous range of wavelength from about 350nm to about 650nm

how does light from a laser differ from non-laser light

laser light is highly monochromatic; + a laser is a convenient source of coherent light

why is a laser beam very dangerous if it enters the eye

a convex lens can focus it to a very fine spot as a laser beam is almost perfectly parallel and monochromatic = eye lens would focus beam on a tiny spot on retina = would destroy retina due to the intense concentration of light

why is a laser a coherent source of light

all photons are traveling in the same direction and frequency

colour of the central fringe produced by white light

white - because every colour contributes at the centre of the pattern

colour of the inner fringes produced by white light

tinged with blue on the inner side and red on the outer side - because the red fringes are more spaced out than the blue fringes, + the 2 fringe patterns don't exactly overlap

colour of the outer fringes produced by white light

merge into an indistinct background of white light becoming fainter with increasing distance from the centre - because, where the fringes merge, different colours reinforce + so overlap