energy <=

There was a long debate in history as to what really is light, particles or waves. Now, we understand light much better. Light is a portion in the electromagnetic radiation (EM radiation). What are properties of waves?

• The EM Spectrum
Microwave 200 m - 0.1 mm
Infrared 0.1 - 0.0007 mm
visible (light) 0.7 - 0.4 micron
Ultraviolet 0.4 micron - 1 nm
X-rays 1 nm - 0.1 pm
Gamma rays < 0.1 nm
The EM radiation consists of radiowaves. microwaves, IR, visible, UV, X-rays, and gamma rays.

Color patterns on soap films is due to interference of waves.

EM radiation travels at constant velocity

c = 3x108 m s-1

EM radiation is the transmission of energy by EM wave without a medium.

As waves, EM radiation is characterized by frequency, wavelength, wavenumber, and energy.

• Max Planck (1858-1947) explained the energy aspect of EM radiation. For a radiation of wavelength l, the frequency, n is,

n = c / l

E = h n

h is the Planck constant, (= 6.62619x10-34 J), and E the energy of the bundle he called quanta, or photons.

# Photoelectric effect

• Photoelectric was discovered in 1887 by Hertz, who observed UV light changes the lowest voltage for sparking.

• Cathode rays (electrons) was discovered.

• Photoelectric current is proportional to the intensity of light.

• Using light of fixed wavelength, kinetic energy of photoelectrons does not depends on intensity of light.

• Einstein (1905) explained light as photons (particle property) whose energies agree with Max Planck's postulated quanta. Both Einstein and Planck received Nobel prizes for their contributions.

Exercise 1

The wavelength of a red light is 700 nm. Estimate its frequency, energy of the photon.,