Baby-Quiz

1. Why are diffraction effects of your eyes more important during the day than at night?

2. Will the converging lens focus blue light or red light at a closer distance to the lens? Explain.

3. Assume that you have the thinnest film that strongly reflects red light. Would you need to make the film thinner or thicker to completely reflect blue light? Explain.

4. If all the labels had come off the sunglasses in the drug store, how could you tell which ones were polarized?

Blackbody Radiation

All objects emit radiation whose total intensity is proportional to the fourth power of the Kelvin temperature.

Wien’s Law

KmTp 3109.2

The Sun’s Surface Temperature

Estimate the temperature of the surface of our Sun, given that the Sun emits light whose peak is in the visible spectrum at around 500 nm.

Energy distributed among the oscillating electric charges of the molecules is not continuous, but instead consists of a finite number of very small discrete amount, each related to the frequency of oscillation as:

sJh

hfE

34

min

106.6

;

Planck’s Quantum Hypothesis

Planck’s assumption suggests that the energy of any molecular vibration could be only some whole number multiple of hf:

....3,2,1

;

n

nhfE

1. If all objects emit radiation, why don’t we see most of them in the dark?

2. Rutherford’s model provided an explanation for the emission of light from atoms. What was this mechanism and why was it unsatisfactory?

3. Suppose you were a nineteenth-century scientist who had just discovered a new phenomenon known as Zeta rays. What experiment could you perform to define if Zeta rays are charged particles or e/m waves? Could this experiment distinguish between neutral particles and an e/m wave?

The Photoelectric Effect

Predictions of the Wave Theory

- the light intensity should influence the maximum KE of electrons;

- the frequency of the light should not affect the KE of ejected electrons.

Photon Theory

Light is a particle again…

E=hf

1. The energy of photon is the same for all photons in a monochromatic beam.

2. Increasing of the intensity of the light means increasing of the number of photons in the beam, but does not affect the energy of each photon as long as the frequency is not changed.

According to Einstein, electron is ejected from the metal by a collision with a single photon.

1. An increase in intensity of the light beam means more photons are incident, so more electrons will be ejected , but max KE in not changed.

2. If the frequency of the light is increased, the max KE increases linearly.

3. If the frequency f is less than the “cutoff’ frequency f0, no electrons will be ejected at all, no matter how big is the intensity.

1. If a metal surface is illuminated by light at a single frequency, why don’t all the photoelectrons have the same kinetic energy when they leave the metal’s surface?

2. What property of the emitted electrons depends on the intensity of incident light?

3. What property of the emitted photoelectrons depends on the frequency of incident light?

Bohr’s Model

Bohr’s First Postulate

The angular momentum of electron is quantized.

Allowed Angular Momenta

2

hnLn

Allowed Radii

12rnrn

Bohr’s Second Postulate

An electron doesn’t radiate when it is in one of the allowed orbits.

Bohr’s Third Postulate

A single photon is emitted whenever an electron jumps down from one orbit to another.