Light as a Wave

• Fundamentals of Quantum Nature of Light

• Prepared by
• Mr. G. Shahin
• 22th of February, 2018

• Collège des Frère

Light as a Wave

• Light (or electromagnetic radiation), can be thought of as either a particle or a wave. Light has various properties:
• a wavelength,  (distance between waves)
• a frequency, f (number of waves passing you each second)
• a speed, c =  f (this is always the same: 300,000 km/s)
• an energy per photon, E = h f (where h is just a constant)
• Note that because the speed of light is a constant, , f, and E are linked: if you know one, you know the other two.

The Electromagnetic Spectrum

Light as a Particle

• Light can also behave as particle. Each packet of light is called a photon, and each photon carries a specific amount of energy (associated with the photon’s wavelength or frequency).

• Photons emitted from a source will spread out in all directions at the speed of light. Since the amount of area surrounding a source increases as the distance squared, the density of photons will decrease as 1/r2. This is the inverse square law of light.

Scattering of Light

• Dust in the Earth’s atmosphere (or in space) can scatter light. In general, short wavelength blue light gets scattered more than red light. That’s why the sky is blue.

Scattering of Light

• The long path through the atmosphere means all the blue photons are scattered away. That’s why the Sun is red at sunset.

First Type of Radiation: Blackbody Thermal Continuous Radiation

• Anything that is hot (i.e., above absolute zero) produces light at all wavelengths – a continuous spectrum. But the amount of light given off at each wavelength depends on the object’s temperature.

• For hotter objects:
• Peak intensity shifts to shorter (bluer) wavelengths
• peak  1/T
• more light is created