Light interaction with matter

Wavelength ~ 500nm When light interacts with systems of much

larger sizes (few cm, m): Geometric optics When light interacts with systems of similar

sizes: Wave optics When light interacts with systems of much

smaller sizes (atoms and molecules): Scattering: it wiggles the charges which in turn radiate light.

Light Scattering

When light travels in a medium, it scatters from individual atoms. When the arrangement of the atoms is uniform and regular (solids, glass), the light scattering from different atoms CANCELS. (no scattering)

In a gas, because its density fluctuates, the scattering is strong. Here are some of the empirical observations: Some wavelengths scatter more than others. Some polarizations scatter more than others.

Rayleigh Scattering

The rule for Rayleigh scattering: The shorter the wavelength of the incident light,

the more light is scattered. Blue light scatters more than red. (400nm

blue light is scattered 10 times as great as 700nm red light).

The sky is blue! And the sunlight is red at sunset.

RayleighScattering

Other examples

Fine black pigment mixed into white paint gives it a bluish cast (first noticed by da Vinci) Also blue eyes….

Fine smoke from wood fire, illuminated from the side, and viewed against a black background, looks blue. If viewed against a bright background, it looks red and brown.

Smoke from the end of a cigarette. (What is the color of the exhaled smoke? the cloud is white!)

Polarized Light

Light is a transverse wave in the sense that the electric field is transverse to the direction of light propagation.

If the electric field has a fixed direction, the light is linearly polarized.

If the electric field makes a 45 deg angle relative to the x-axis, it can be thought of as consisting of two in-phase waves with one polarized in the x direction and other in the y direction.

Unpolarized light

For unpolarized light, we can think of it as consisting of two lights with polarizations in x and y directions but with a random phase difference (incoherent).

Different ways to get polarized light

Scattering Reflection Absorption

Polarization due to Scattering

Light propagating in the z-direction has polarization in the xy plane. When scattered along the y-direction, it must be polarized in the x direction.

When the sun is just above the horizon, the light from the blue sky above is polarized horizontally. It is possible to determine the direction of the sun by measuring the polarization of the light.

Light from white clouds are not polarized.