wave model Flashcards
(28 cards)
What is diffraction?
When light passes through an aperture, slit or around a bend/edge of an object, it experiences a scattering process called diffraction.
In diffraction, what is the degree of bend dependent on?
The degree of bend is dependent on the relative difference between its wavelength and size of the slit.
What is the impact of a slit size smaller than or equal to the light’s wavelength, or larger?
If the slit size is smaller than or equal to light’s wavelength, considerable diffraction occurs. Smaller the slit, greater the degree of diffraction.
If the slit size is larger than light’s wavelength, diffraction still occurs but is unnoticeable to the naked eye.
What is Huygen’s principle?
Huygens proposed that when a wave reaches an opening, its wavefront can be perceived as individual points emitting spherical secondary waves.
Why does light create a diffraction pattern? (Fresnel)
Fresnel proposed that the diffraction pattern obtained from white light is a result of constructive and destructive interference between secondary ‘wavelets’ emitted from the original wavefront.
What do waves undergo when they are out of phase and in phase?
When waves are out of phase, they undergo destructive interference. This results in dark fringes or ‘minima’ of diffraction.
When waves are in phase, they undergo constructive interference. This results in spectral bands or ‘maxima’ of diffraction.
Why does the intensity of each spectral band decrease from the centre?
This is because the diffracted ray of light is lower in intensity the greater the angle of diffraction from the midline (θ).
Outline Young’s double slit experiment
A monochromatic light source was shone through two narrow slits separated by a very small distance. A viewing screen was set-up directly behidn the double-slit apparatus.
Which model of light did Young’s experiment support and why?
Young’s observations supported the wave model of light.
If light was a particle or consisted of particles, only light particles which pass through the slits would be observed on the viewing screen, producing only two bright bands.
However, Young instead observed multiple bright bands/spots on the viewing screen rather than just two. These spots spanned a larger distance than that between the slits and had alternating dark and bright spots (minima and maxima).
When will two waves undergo constructive interference? (+equation)
When the difference in phase between two waves are exactly one wavelength apart (dsinθ=mλ)
When will two waves undergo destructive interference? (+equation)
When the difference in phase between two waves are exactly or a multiple of half a wavelength apart (dsinθ=(m+0.5)λ)
How do you find the greatest order?
Substitute an angle of 90º into the diffraction equation, the greatest order is given by m=d/λ
How do you increase the number of maxima present?
For a given separation distance, we can decrease the wavelength of light to increase the number of maxima present.
What does increasing the number of slits do? (Diffraction grating)
Increased number of slits produces diffraction bands with greater resolution. The number of waves being directed at the same angle and position on the viewing board increases. This increases the intensity of bright spots as more waves experience constructive interference which leads to a greater amplitude.
Why is red furthest away from the centre when white light is diffracted?
Within each spectrum, longer wavelength of visible light (red) is further away from the centre. This is because the angle of diffraction increases for light of longer wavelengths.
Outline Newton’s corpuscular model of light.
He proposed that the propagation of light is caused by the rectilinear motion of light particles – light corpuscles. These light particles travel at a finite speed and their interaction with the external environment obeys Newtonian physics.
Why did Newton think light cannot be a wave?
He argued that light does not exhibit diffraction. Also, light’s ability to travel in a vacuum as at that time supported his model as waves required a medium to propagate.
What is an advantage of Newton’s Particle Model regarding reflection?
When spherical objects e.g. ball undergoes elastic collision with a rigid surface, the angle at which it collides with the surface equals to the angle at which it leaves the surface.
What is an advantage of Newton’s Particle Model regarding refraction?
Newton proposed that particles of a medium produce attractive forces. So, denser media have more particles and thus stronger attraction. When light corpuscles approach an interface, the stronger attraction of the denser medium produces on the corpuscles a net force and thus acceleration towards it. This produces the effect of light bending away or towards the normal.
Why was Newton’s explanation of refraction later experimentally disproved?
the speed of light was found to be slower, rather than faster, in water. (water is a denser medium than air or vacuum)
What was a limitation of Newton’s particle model (interference)?
As particles, the idea of light corpuscles ‘adding up’ or ‘cancelling’ each other was not feasible, therefore deemed as a major limitation of Newton’s theory.
What did Young’s experiment show to disprove Newton’s particle model?
Instead of producing two vertical bright bands right behind the slits, Young observed multiple bright spots spanning a large area than the distance between the slits.
Why does Newton’s particle model not explain diffraction?
When a beam of light passes through a small opening or slit, it scatters to form a large image.
What does Huygen’s model of light assume?
Huygens’ model of light assumes light behaves as a longitudinal wave.
