Black Body Radiation

What is a black body??

A black body is an idealized object that absorbs all electromagnetic radiation it meets. It then emits thermal radiation (vibration of particles producing heat) in a continuous spectrum according to its temperature.  The sun is a good but not perfect example of a black body.  A black charcoal briquette comes close to a perfect black body.

Black body radiation is the radiation (electromagnetic waves focused around the infrared spectrum) that is emitted by a very hot body. They are called black bodies because most or all the radiation lies outside of the visual spectrum, so a human looking at the body doesn't see any color.

As we heat it further, it will start to glow dull red (1000k), then yellow and eventually blue.  Heated further over 10,000 K it will radiate UV light but it will also emit blue so it will appear blue to us, infrared light is also being emitted. Most of the radiation though will be in the UV wavelengths.

A Blackbody is an object that absorbs all light.

·         Absorbs at all wavelengths.

As it absorbs light, it heats up.

• Characterized by its Temperature.

It is also the perfect radiator:

• Emits at all wavelengths (continuous spectrum)

• Energy emitted depends strongly on the Temperature.

• Peak wavelength also depends on Temperature.          

2.  Wave Model of Light/Classical Theory

James Maxwell’s EMR model of Light suggests;

·         electrons oscillating in the blackbody set up a propagation of electric and magnetic fields perpendicular to wave motion

·         producing electromagnetic radiation (EMR wave)

·         as the number of EM vibrational modes increases, energy increases and the wavelength should get shorter (frequency higher) and the intensity should reach infinity. 

·         this implies that a bb at a high enough temperature emits
EM radiation of infinite intensity. 

·         Our sun at 5000 K would have grown infinitely bright. 

·         Did NOT happen – Ultraviolet Catastrophe – could not explain the sharp decrease in intensity emitted at the shorter wavelengths – What was predicted (classical model) and what happened (quantum model) did not coincide.

3.  Max Planck (1900) explains the bb curve

·         EM radiation did not follow the classical laws, energy transfer is NOT continuous

·         Suggested that there is a minimum amount of energy that could be absorbed or emitted by objects

·         This minimum amount varied depending on the frequency of charges vibrating in the black body

·         Energy could only be transferred in discrete amounts he called QUANTA

·         Planck couldn’t explain why but comes up with an equation

Multiple Choice

In his explanation of the photoelectric effect, Einstein proposed that

• the speed of light is constant

• light energy is concentrated in distinct "packets"

• light energy is evenly distributed over the entire wave front

• metalic surfaces emit electrons when illuminated with short-wavelength EMR

Fill in the Blanks

Multiple Choice

• I only

• II only

• I and II only

• II and III only

·         When the intensity of the incident light increases, the maximum kinetic energy of the photoelectrons does NOT increase, just the number of photoelectrons emitted.

·         Einstein proposed that each photon gave all of its energy to just one electron.

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Stopping Potential

·         To determine the maximum kinetic energy of the photoelectrons a “stopping voltage” is applied in the opposite direction as the current produced by the photoelectrons

·         When the current is reduced to zero in the circuit, the following equation is true

                                                         E_{k (max)} = qV_o

How do we measure this?

Multiple Choice

Graphically speaking:
·        slope is the same regardless of metal and equal to Planck’s constant

·         x-intercept in the threshold frequency of the metal

·         y-intercept – work function of metal

Classical Theory could not explain why a very bright (high intensity) red light (1.77eV) could not eject an electron or why a very dim light (low intensity) of blue could eject an electron

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​Here is a good visual for the Photoelectric Effect