Light as a Wave Flashcards
What is Huygen’s principle?
That all waves consist of smaller wavelets.
They propagate a distance of the wavelength after one period, and create the next wave front.
What is refraction?
When the direction of light changes due to a change in the velocity.
When does refraction occur?
When light travels from one medium to another.
As light travels from a medium from a higher refractive index to a lower (water-air) it bends away from the normal, and vice-versa.
What is the refractive index?
The ratio of the speed of light in a vacuum to the speed of light in a medium.
n = c/v
What is dispersion and when does it occur?
It occurs due to refraction when white light passes through a prism.
When this happens, the light is split into all visible light ROYGBIV.
What is diffraction? What is the ratio for diffraction?
The bending of waves/light when it travels through slits.
This creates interference patterns which can be constructive (bright spot) or destructive (dark spot).
Diffraction = λ/w
Explain what happens with diffraction with the ratio of λ/w.
As the λ increases, diffraction increases.
As the λ decreases, diffraction decreases.
As the w increases, diffraction decreases.
As the w decreases, diffraction increases.
What is polarisation? How does it support light being a wave?
Where a transverse wave can only vibrate in 1D.
Light waves are EM waves (electro-magnetic).
Since light can be polarised, it must mean that it’s a transverse wave.
Which of the 2 competing models does Young’s Double Slit Experiment support?
The wave model.
Why do the results of the experiment support Young’s model?
Because an interference is caused by
- Alternating constructive & destructive interference
- There’s diffraction through the two slits
Interference and diffraction are both wave behaviours.
Why do the results of the Young’s experiment not support the competing model?
If light behaved as a particle, then we’d only expect to see 2 bright spots on the screen rather than an interference pattern.
When is light produced?
When charged particles accelerate. They consist of an electric field which in turn creates a changing magnetic field. Hence, light is an EM wave.
With reference to interference, explain why the central band observed in Young’s Double Slit experiment is bright and not dark.
- The light is diffracted as it comes through the slits, acting like 2 coherent sources of waves.
- Constructive interference occurs where crests and crests overlap, destructive where other.
- Since the central point is equidistant from both slits, it will be constructive and bright.
