Spectroscopy

The spectral colors correspond to different wavelengths of electromagnetic radiation

Is Light a Wave or a Particle?

Argument: Light is reflected according to the law of reflection, which is a property of waves. Therefore light is a wave.

Counterargument

px = mvx = m v sin(), py = mvy = m v cos()

Fx = m dvx/dt, Fy = m dvy/dt

Fx = 0 => vx is unchanged

Perfectly elastic collision =>

½ m (vx2 + vy

2) is unchanged

Fx = 0 => vx is unchanged

Perfectly elastic collision =>

½ m (vx2 + vy

2) is unchanged

Therefore vy (after) = - vy (before)

We cannot deduce, simply from the law of reflection, whether light is a wave or a particle.

We cannot deduce, simply from the law of reflection, whether light is a wave or a particle.

There is a long history of controversy in optics over whether light is a wave or a particle.

There is excellent evidence that light is emitted and absorbed from matter is discrete amounts, very much like particles.

Each “particle” of light carries an energy E. If the light has frequency f, the “particle” has energy

E = hf where h = 6.626 x 10-34 J s = 4.136 x 10-15 eV s h is called Planck’s constant and the

“particle” of light is called a photon.

classical physics: A mechanical system has states of motion at a continuous range of energies.

E m v k x 12

2 12

2

Quantum Physics

Atoms and molecules have states of excitation separated by discrete energies

E = 12.087 eV – 10.199 eV = 1.888 eV

The emitted photon has an energy of 1.888 eV

f = E/h = 1.888 eV/ (4.136 x 10-15 eV s)

= 4.565 x 1014 Hz

= c / f = (3 x 108 m/s) / (4.565 x 1014 s-1)

= 6.57 x 10-7 m = 657 nm

E = 12.087 eV – 10.199 eV = 1.888 eV

The emitted photon has an energy of 1.888 eV

f = E/h = 1.888 eV/ (4.136 x 10-15 eV s)

= 4.565 x 1014 Hz

= c / f = (3 x 108 m/s) / (4.565 x 1014 s-1)

= 6.57 x 10-7 m = 657 nm

The atom emits red light in this transition

E1 = 12.087 eV – 10.199 eV = 1.888 eV

f1 = E/h = 1.888 eV/ (4.136 x 10-15 eV s)

= 4.565 x 1014 Hz

1= c / f = (3 x 108 m/s) / (4.565 x 1014 s-1)

= 6.57 x 10-7 m = 657 nm

E2 = 2.549 eV, E3 = 2.855 eV

2 = c / f2 = hc / E2 = (1240 eV nm)/ 2.549 eV

= 486 nm

3 = c / f3 = hc / E3 = (1240 eV nm)/ 2.855eV

= 434 nm