How can the interference of light be observed?
illuminate two closely spaces parallel slits (double slits) using a suitable light source
What do the two slits act as in the double slit experiment?
act as coherent sources of waves
What does it mean for wave to be coherent?
emit light waves with constant phase difference and same frequency
What would have Young used instead of a light bulb to illuminate a narrow single slit in his double slit experiment?
would have used a candle
What is the double slit arrangement illuminated by?
light from narrow single slit
What can be seen on the white screen from the double slit experiment?
alternate bright and dark fringes, referred to as Young’s fringes
Why can Young’s fringes be seen on the white screen from the double slit experiment?
this is where diffracted light from the double slits overlap
How are the fringes spaces from the double slit experiment?
evenly spaced and parallel to the double slits
What happens if the single slit is too wide in the double slit experiment?
each part of it produces a fringe pattern, which is displayed slightly from the pattern due to adjacent parts of a single slit
What happens to the fringes when the slit is too wide in the double slit experiment?
dark fringes of double slit pattern becomes narrower than bright fringes, + contrast lost between dark and bright fringes
What can be used instead of a light bulb and a single slit for the double slit experiment?
a laser beam
Why must the fringes be displayed on a screen when a beam of laser light is used in the double slit experiment?
beam of laser light will damage the retina if it enters the eye
What are the fringes formed due to?
interference of light from the two slits
How are the bright fringes formed?
light from one slit reinforces light from other slit
light waves from each slit arrives in phase with each other
How are the dark fringes formed?
light from one slit cancels light from other slit
light waves from two slits arrive 180° out of phase
What is the fringe separation?
distance from centre of a bright fringe to centre of next bright fringe
What is the equation for fringe separation?
w = λD / s
How can the fringes become more widely spaced? (distance)
distance D from slits to screen increase
How can the fringes become more widely spaced? (wavelength)
wavelength λ of light used increased
How can the fringes become more widely spaced? (slit spacing)
slit spacing s reduced
What is the slit spacing?
distance between centres of slits
When only is the equation w = λD / s valid?
only if fringe separation much less than distance D from slits to screen
How would you measure the fringe separation?
measure across several fringes from centre of dark fringe to centre of another dark fringe
then divide measurements by number of fringes you measured across
Why would you use the dark fringes when measuring the fringe separation?
as centres of dark fringes easier to locate than centres of bright fringes
