WAVES AND QUANTUM Flashcards
When does maximum diffraction occur?
When the gap is equal to the wavelength
How are diffraction angles calculated?
sinƟ = λ/ b
sin of the angle = wavelength over the width of the gap
This also gives the smallest angle of separation of two sources that can be resolved
When are waves refracted?
think light
When it changes mediums
Light speeds up/ slows down in different mediums, this causes the wave to refract
What does it mean if the refractive index is less than one?
The light is speeding up so is refracting away from the normal
When does superposition happen?
When two or more waves pass through each other, when they cross
When waves cross, their displacement combines then each wave goes their own way
What does the principal of superposition say?
When two or more waves cross, the resultant displacement equals the vector sum of the individual displacement
How do you superimpose waves using phasors?
Add the arrows tip to tail
What does in phase mean?
The points are in step - their phasors point the same way
Two points in phase interfere constructively
What do antiphase waves look like on two phasors?
Their arrows point in different directions
How do you get interference patterns?
The sources must be coherent
Coherent means they have the same frequency and wavelength
Two coherent sources will be in phase
When do you get constructive interference?
When the path difference between two sources is a whole number
When do you get destructive interference?
When the path difference between two sources is half a number
What is a standing wave? and when do they occur?
A standing wave is the superposition of two coherent waves (same frequency and same wavelength)
You get standing waves when a progressive wave is reflected at a boundary
When a tube/ pipe is closed, what is at the end? (node or antinode)
A node
The lowest possible frequency is λ/4
When a tube/ pipe is open, what is at the end? (node or antinode)
Antinode
The lowest possible frequency is λ/2
What happens in a single slit experiment?
The light waves diffract as it passes through the single slit and is shown on a screen as a fringe pattern
Why should you use a laser for young’s double slit/ single slit?
A coherent source is required
Laser light is monochromatic
Where is the brightest part of a diffraction pattern (on a screen)?
Where the light ray is coming directly from the source/ laser
Light rays are almost parallel
Why are there dark fringes on a screen?
Because the sum of the phasors add up to give a resultant of 0
What is important about the spacing of slits in Young’s double slit experiment?
The gap needs to be smaller than the wavelength to ensure that there is diffraction
What is the formula for Young’s double slit?
X = Dλ/d
X= fringe spacing, D= distance between screen and slits, λ= wavelength, d= distance between slits
What was Newton’s theory of light?
Light is made up of tiny particles, called “corpuscles”
Corpuscular theory explained reflection and refraction BUT NOT diffraction and inteference
What was Huygen’s theory of light?
Light is a wave
Huygens’s Principle states that every point on a wave front is a source of wavelets, which spread forward at the same speed.
By applying his theory to a wave, we can prove that light is a wave
it proves that light reflects, interferes and refracts
Define ‘diffraction’
The bending of a wave around the edges of an opening or an obsticle
