5.5- MORE ABOUT INTERFERENCE Flashcards
Why are double slits described as coherent sources?
they emit light waves of same frequency with constant phase difference, provided we illuminate double slit with laser light, or with light from narrow single slit if we’re using non-laser light
How does each wave crest or wave trough from the single slit always pass through one of the double slits?
a fixed time after it passes through the other slit
As each wave crest or wave trough from the single slit always passes through one of the double slits a fixed time after it passes through the other slit, what does this show?
double slit emits wavefronts with constant phase difference
What does the straight waves from the beam vibrating on the water surface in the double slit arrangement do?
diffract after passing through two gaps in barrier + produce interference pattern where diffracted waves overlap
What happens if one gap is closer to the beam than the other in the double slit arrangement?
each wavefront from beam passes through nearer gap first
What is the same even if one of the gaps is closer to the beam in the double slit arrangement?
time interval between same wavefront passing through the two gaps always same so waves emerge from gaps with constant phase difference
Why can’t light from two nearby lamp bulbs not form an interference pattern?
as the two light sources emit light waves at random
points of cancellation + reinforcement would change at random, so no interference pattern possible
in the double slit experiment, what does the fringe separation depend on?
colour of light used
What is white light composed of?
continuous spectrum of colours, corresponding to continuous range of wavelengths from about 350nm for violet light to about 650nm for red light
Is fringe separation greater for red light or blue light?
red light
Why is fringe separation greater for red light than blue light?
red light has loner wavelength than blue light
formula for fringe spacing w
w = λD/s
What do vapour lamps and discharge tubes produce?
light with dominant colour
What glow does a sodium vapour lamp produce?
yellow/orange glow
Why does sodium vapour lamp produce a yellow/orange glow?
due to light of wavelength 590nm
Are other wavelengths of light also emitted from a sodium vapour lamp?
yes
If other wavelengths of light is emitted from a sodium vapour lamp, why is the colour yellow/organge?
colour due to light of wavelength 590nm much more intense
What sort of light source is a sodium vapour lamp?
monochromatic light source
How is a sodium vapour lamp a monochromatic light source?
its spectrum is dominated by light of a certain colour
What is light from a filament lamp or from the Sun composed of?
colours of spectrum and so covers continuous range of wavelengths from about 350nm to about 650nm
What happens if a beam of white light is directed at a coloured filter?
light from the filter is a particular colour
Why is that when a beam of white light is directed at a coloured filter, light from the filter is a particular colour?
as it contains a much narrower range of wavelengths than white light does
How does light from a laser differ from a non-laser light? (2)
laser light highly monochromatic
laser is convenient source of coherent light
What does the wavelength of laser light depend on?
type of laser that produces it
What light does a helium-neon laser produce?
red light of wavelength 635nm
Why can a convex lens be used to focus a laser beam to a very fine spot?
as a laser beam is almost perfectly parallel + monochromatic
What happens to the beam power when the beam light is focused to a very fine spot using a convex lens?
beam power concentrated in very small area
Why is it dangerous if a laser beam enters the eye?
eye lens would focus beam on tiny spot on retina + intense concentration of light at that spot would destroy the retina
Should you look along a laser beam, even after reflection?
no
Why can we use a laser directly to illuminate the double slit when demonstrating double slit interference?
laser is source of coherent light
What doesn’t need to be done in the double slit experiment when using a laser beam as it’s a coherent source of light?
don’t need to make light pass through narrow single slit
What does a light source emit light as a result of?
electrons inside its atoms moving to lower energy levels inside atoms
each such electrons emits a photon
What is a photon?
packet of EM waves of constant frequency
What happens inside a laser, when a photon is emitted?
each photon causes more photons to be emitted as it passes through light-emitting substance
As each photon causes more photons to be emitted as it passes through a light-emitting substance in a laser, what does this mean?
photons in laser beam are in phase with each other
What type of light source is a laser?
coherent source of light
How do atoms emit photons in a non-laser light source?
atoms emit photons at random so photons in such beam have random phase differences
What are the blue light fringes like in comparison to red light fringes?
blue light fringes closer
What does each component colour of white light produce?
its own fringe pattern
each pattern centred on screen at same position
What colour is the central fringe of white light?
white
Why is the central fringe of white light white?
as every colour contributes at centre of pattern
What are the inner fringes tinged with on the inner side and outer side?
inner side tinged with blue and outer side tinged with red
Why are the inner fringes tinged with blue on the inner side and red on the outer side? (white light fringes)
red fringes more spaced out than blue fringes and the two fringe patterns do not overlap exactly
What do the outer fringes merge into? (white light fringes)
into an indistinct background of white light becoming fainter with increasing distance from centre
Why do the outer fringes merge into an indistinct background of white light becoming fainter with increasing distance from the centre?
where fringes merge, different colours reinforce and so overlap
