Modern Physics

Blackbody Radiation

The ultraviolet catastrophe and Planck's quantum hypothesis

Blackbody Radiation lesson illustration

 

 

What is the name of a particle of light or energy?

 

 

[ Flash ]

showmethephysics.com

 

 

 

Fill In The Blank Notes

 

Background

Electrons give off light when they return to the ground state after being excited by an energy source

 

 

Bohr studies the light given
off by a 'Black Body'

 

 

Blackbody Radiation lesson illustration

Neils Bohr, right

 

 

'Black Body' = heated body that gives off only the light from its internal glow - no reflected light

 

Blackbody Radiation lesson illustration

Material was heated and the light
 given off was analyzed

 

 

 

 

Spectroscopy

School blocks YouTube? Click below

| Onsite Version |

St. Mary's U. Astronomy and Physics Dept.

 

Spectral lines
Physics 2000

Emission Spectra

Blackbody Radiation lesson illustration

 

Whatever colors an element gives off, it also .....

 

 

 

 

 

 

... absorbs

 

 

Blackbody Radiation lesson illustration

Atomic Spectra for Iron

 

 

Full Spectrum

 

Hydrogen Emission
/Absorption Spectrum

[ Flash ]

Place your mouse over the image to see the absorption spectra

 

 

Full Spectrum

 

 

Helium

 Emission/Absorption 
Spectrum

Place your mouse over the image
to see the absorption spectra

[ Flash ]

Every Energy level transition creates a single spectral line

 

 

| Onsite Version |

 

 

 

"Science of the Silicon Solar Cell"

Don Ion

 

 

How many different spectral lines can be produced
by an atom with 5 energy levels?

Blackbody Radiation lesson illustration

 

 

 

Answer

 

    Blackbody Radiation lesson illustration

 

 

 

10 Spectral Lines

 

 

10 colors

 

 

10 frequencies

10 photons

 

 

 

 

 

Black Body Radiation Results:

 

1. Only the high energy objects gave off colors on the violet side of the spectrum 

 

 

 

R O Y

G

B

V

Blackbody Radiation lesson illustration

Low Frequency

 

High Frequency

 

Frequency related to Energy

 

 

 

2. Electrons in atoms are not capable of giving off all colors (energies)

 

 

 

3. Each element gives off a unique pattern of colors called a 'bright line spectrum'.

 

 

Blackbody Radiation lesson illustration  This knowledge is very useful to chemists and other scientists. Why?

 

 

 

Answer

 1. Spectral lines used to:

 

  • identify unknown elements.

 

  • determine the composition of distant stars

 

 

Blackbody Radiation lesson illustration 

 

 

 

To Identify an Unknown Element

 

 

- Electrify, Heat it up and analyze its colors.

 

- Compare emitted colors to a database of elemental spectral lines

 

 

 

 

 

 

 

The diagram below represents the bright-line spectra of four elements, A, B, C, and D, and the spectrum of an unknown gaseous sample.

Blackbody Radiation lesson illustration

Based on comparisons of these spectra,
which two elements are found
in the unknown sample?

Jan 2006

 

 

 

Blackbody Radiation lesson illustration

B and C

Blackbody Radiation lesson illustration

 

What are stars made of ?

 

 

Check Your Understanding

5 quick questions — pick an answer to see if you’re right.

1. An element absorbs light at certain colors. Compared to the colors it emits, they are…

2. How many different spectral lines can an atom with 4 energy levels produce?

3. Each element gives off a unique pattern of colors called a…

4. How do astronomers know what distant stars are made of?

5. Which statement about atoms is supported by blackbody results?

NYSSLS Regents Practice · Modern Physics · Blackbody Radiation

The Colors of Stars

Stars behave approximately like blackbodies: the wavelength at which they glow most brightly depends on their surface temperature. The red supergiant Betelgeuse has a surface temperature of about 3500 K, the Sun about 5800 K, and the blue-white star Rigel about 12,000 K.

Which star’s radiation peaks at the shortest wavelength?

▶ Want a challenge? Open 6 more Regents-style questions  (harder · mixed formats)

Challenge score: 0 / 6

Multiple choice

C1. An electric stove burner heats up. In what order does it change color?

Substitute values

C2. The wavelength of peak emission is given by Wien’s law, λmax = 2.90 × 10−3 m·KT. Substitute numerical values into this mathematical representation to determine the peak wavelength for Betelgeuse. Then calculate it.

λmax = ( × 10−3 m·K)( K) = × 10−7 m
Support or Refute · Justify

C3. A student makes the following claim.

Claim: The Sun’s radiation peaks in the visible part of the electromagnetic spectrum (about 4.0 × 10−7 m to 7.0 × 10−7 m).

Place a check mark in either the Support or Refute box. Justify your response using a mathematical relationship. (Wien’s law, λmax = 2.90 × 10−3 m·KT)

Determine the value

C4. A thermal camera detects a person whose skin temperature is about 310 K. Using Wien’s law, λmax = 2.90 × 10−3 m·KT, determine the person’s peak wavelength in micrometers (1 µm = 10−6 m).

λmax = µm
Multiple choice

C5. Classical physics predicted that a blackbody should emit enormous amounts of ultraviolet radiation, which does not happen. How did Max Planck fix this?

Constructed response

C6. An incandescent bulb filament glows at about 2800 K, so most of its radiation is infrared. An engineer proposes running the filament at a higher temperature. Identify one benefit and one drawback of this design change.

Bohr's Conclusions →

Also see: The Big Bang — evidence for the origin of the universe

Also see: Nuclear Physics — radioactive decay, fission, and fusion