# Interaction of Light with Matter¶

## Speed and Refraction¶

As light passes through a medium, it constantly interacts with atoms which causes them to absorb and re-emit them in the same direction.

• As a result, the more dense the medium, the slower light travels through it.
Medium Speed (m/s) Refractive Index
Vacuum $$3\times10^8$$ -
Air $$2.99\times10^8$$ 1.00028
Water $$2.25\times10^8$$ 1.33
Glass $$1.99\times10^8$$ 1.5
Diamond $$1.25\times10^8$$ 2.4

## Analysis of Light¶

Many properties of light can be used for analysis

• Emission
• The change in wavelength resulting from doppler shifts
• The amount of shift is dependent on the wavelength being emitted and the relative speed of the light
• Motion away from the observer causes a redshift (wavelength gets longer)

## Properties of light¶

• Reflection
• No light is absorbed, all the light hits the object and bounces off
• Refraction
• Some radiation interacts, and as a result, the light is slowed down
• Dispersion
• Light is refracted at different rates depending on the wavelength and the medium
• Interference
• Peaks of the waves interact destructively or constructively
• Diffraction
• The wave like properties of light cause them to spread out when passing through a narrow aperture. Also causes interference
• Scattering
• All the radiation is absorbed, and is released in all directions
• Polarisation
• Light waves can travel on any rotational plane

## Reflection and Refraction ¶

• When light hits the interface between two different substances with different refractive indexes, two things occur
1. Some light is reflected
• The light does not interact with the medium at all
2. Some light is refracted
• The higher the refractive index of the second medium, the closer the light will be to perpendicular the interface

## Dispersion ¶

• Materials with higher refractive indices tend to hold onto absorbed light for longer before reemitting it.
• Since refractive indices are $$\nu$$ dependent, the lower $$\nu$$ (higher energy), the higher the refractive index.

## Wave-Particle Duality ¶

See the dual slit experiment

• What are particles?
• Have well defined positions
• Are indivisible
• Quantised in structure. Defined edges
• What are waves?
• Waves are patterns
• Don’t have boundaries or edges
• Don’t come in packages
• Have $$\nu$$, $$\lambda$$ and $$E$$
• What is light?
• Comes in quantised packages called photons
• Each photon has $$\nu$$, $$\lambda$$ and $$E$$
• Photons only come in integer values (no half photons)
• The wave is the permutation in the electromagnetic field which exists only in quantised states
• The photon is the wave packet of the resultant waveform

## Speed of Light¶

• All light travels at the same speed through a vacuum ($$3\times10^8\:m/s$$)
$E=h\nu=\frac{hc}{\lambda}$
• If h and c are constant, than only $$\nu$$ and $$\lambda$$ can vary the energy