How Matter Emits Light: 1. the Blackbody Radiation

How Matter Emits 

Light: 

1. the Blackbody 

Radiation

Announcements 

n Quiz # 3 will take place on Thursday, October 

20 th ; more infos in the link `quizzes’ of the 

website: 

¨ Please, remember to bring a pencil. 

n Solutions for Exam # 1 are available from the 

website, under `Exam 

n Looking ahead: 

¨ Homework # 3 is due on Thursday, Oct. 20 th 

¨ Homework # 4 starts on Thursday, Oct 20 th . It is due 

on Thursday, Oct. 27 th

Assigned Reading 

n Complete Unit 22; 

n Unit 23

Dimming with distance 

n As you move away from a light source (a 

light bulb, a street light, etc.) it becomes 

dimmer. Why? 

The energy emitted by 

the source is constant, 

but get spread over a 

larger surface at larger 

distance

Dimming with Distance 

n As distance R increases, the area over 

which the total light output L is distributed 

increases as 4πR 2 

n Thus: 

Brightness = ______ L 

4πR 2

How Matter and Light Interact

Matter interacts with light in four 

different ways: 

n Absorption – the energy in the photon is 

absorbed by the matter and turned into thermal 

energy 

n E.g., Your hand feels warm in front of a fire. 

n Reflection – no energy is transferred and the 

photon bounces off in a new (and predictable) 

direction 

n E.g., Your bathroom mirror 

n Transmission – no energy is transferred and the 

photon passes through the matter unchanged. 

n Emission – matter gives off light. Can be done in 

two different ways, as we will see.

Absorption 

Photon deposits energy into material. Thermal energy is increased 

and the material gets warmer.

Transmission 

Photon passes through material without depositing energy. Everything 

remains unchanged.

Reflection 

Photon reflects off of material. No energy is lost but outgoing photon 

has a new direction.

These processes depend on both 

the material and the wavelength of 

the photon

Survey Question 

Our eyes work via the process of: 

1) absorption 

2) reflection 

3) transmission 

4) emission 

5) none of the above


Leaves are green because: 

1) they only emit frequencies corresponding to 

green 

2) they only reflect frequencies corresponding to 

green 

3) they only transmit frequencies corresponding 

to green 

4) they only absorb frequencies corresponding 

to green

The Difference Between Black and 

White 

n White light – contains all the frequencies 

of the visible part of the spectrum. 

n White paint – reflects all frequencies of the 

visible part of the spectrum equally. 

n Black paint – absorbs all frequencies of the 

visible part of the spectrum equally.

Discussion Question 

Why does NASA paint spacecraft white? 

80% 

Absorption Spectrum 

of Black Paint 

Absorption 

40% 

0% 

Infrared 

Visible 

Absorption Spectrum 

of White Paint 

Frequency

Emission: How do objects make 

light in the first place? 

n There are two principal mechanisms for 

producing electromagnetic radiation 

¨ Blackbody radiation 

¨ Spectral line emission of atoms and 

molecules 

Both of these mechanisms result from 

accelerating/decelerating electrons! I.e., you 

accelerate or decelerate an electric charge to 

create EM radiation

Light from Objects 

n We perceive this `acceleration or `deceleration of 

electrons as `light from objects 

n Imagine to heat up a piece of metal in a furnace: 

¨ It will first turn red (temperature raising) 

¨ then orange 

¨ then yellow 

¨ then whitish-blue (highest temperature) 

The higher the temperature, the bluer the 

object will appear 

[you are linking temperature to color!]

What happens? 

n The higher the temperature, the faster the atoms/ 

molecules in the object are (T ~ v 2 ), thus more energetic 

collisions 

n More energetic collisions cause more sudden 

accelerations/decelerations of the electrons in the 

matter, thus light with shorter wavelength (higher energy)

What does it mean? 

n Higher Temperature = faster atoms 

n Faster atoms = more frequent and 

energetic collisions 

n more frequent and energetic collisions = 

more sudden electron accelerations/decel 

n more sudden electron accelerations/decel 

= higher photon energy 

n Higher photon energy = bluer light 

(E=hc/λ)

The color of light emitted is 

connected to the Temperature

How to Measure Temperature 

n 

n 

n 

 

¨ 

 

¨ 

 

¨ 

¨ 

 

¨ 

¨ 

 

¨ 

¨

Do not confuse Heat and 

Temperature! 

n 

n 

Temperature refers to the 

degree of motion of the 

particles in a material, i.e. 

the speed with which the 

particles move (T~kinetic 

energy~v 2 ). 

Heat refers to the amount of 

energy stored in a body as 

motion among its particles 

and depends on density as 

well as temperature.

Survey Question: 

n You heat an oven to 450 F, and you also, separately, 

boil some water in a pot (boiling point is 212 F). What 

happens if you stick your hand first in the oven and then 

in the boiling water [dont do that!]? Why? 

I get burned in both 

I only get burned in the oven, because of the higher 

temperature 

I only get burned in the water, because of the higher 

heat


n You heat an oven to 450 F, and you also, separately, 

boil some water in a pot (boiling point is 212 F). What 

happens if you stick your hand first in the oven and then 

in the boiling water [dont do that!]? Why? 

I get burned in both 

I only get burned in the oven, because of the higher 

temperature 

I only get burned in the water, because of the higher 

heat

What is a Blackbody? 

n An object that can absorb all the radiation falling on it 

(light at all wavelengths), so it appears black when cold 

n When it gets heated up, it can also emit radiation at all 

wavelengths (think of the heated piece of metal). 

n A stove burner (conducting material), a furnace, planets 

(radiating solids), and stars (dense gas) are excellent 

examples of close-to-blackbodies 

n Materials that are insulating, non-burning, or liquid are 

usually not good blackbodies

Blackbody Radiation 

n 

n 

n 

n 

n 

Take a blackbody (e.g., a piece of metal) 

and heat it up. 

After it becomes hot, keep the temperature 

constant (this is called thermal equilibrium) 

Then plot, on a graph, the intensity of the 

radiation (light) emitted as a function of 

wavelength: this is called a spectrum 

The shape of the spectrum and the maximum intensity of a 

Blackbody will only depend on the Temperature 

Think of Temperature as motion of the atoms/molecules in the 

blackbody; if T=constant, the motion does not change, and the 

acceleration/deceleration of the electrons also does not change. The 

`color of the B.B. will not change. 

Black Body radiation is the e.m. emission of matter at 

thermal equilibrium (constant T)

One Temperature=one spectrum 

Blackbodies are excellent thermometers

Wien’s Law 

n Hotter objects emit photons with a 

higher average energy = shorter 

wavelength. 

¨ The peak of the blackbody emission spectrum is 

given by 

" max 

= 

2.9 #106 

T(Kelvin) nm

Stefan-Boltzmann Law: 

n Hotter objects emit more total radiation per 

unit surface area. 

n The luminosity of a hot body rises rapidly 

with Temperature: L=A σT 4


The graph below shows the blackbody spectra of three different 

otherwise identical stars. Which of the stars is at the highest 

temperature? 

1) Star A 

2) Star B 

3) Star C A 

Relative 

Intensity 

B 

C 

Wavelength


Emitted power = σ A T 4 

You are gradually heating two rocks (one larger than the 

other) in an oven to an extremely high temperature. As 

they heat up, the rocks emits nearly perfect theoretical 

blackbody radiation – meaning that 

1) the larger rock is bluer and brighter. 

2) the larger rock is redder and brighter. 

3) the larger rock is bluer but the same brightness. 

4) the larger rock is the same color but brighter. 

5) the larger rock is the same color and brightness.


Emitted power = σ A T 4 

You are gradually heating two rocks (one larger than the 

other) in an oven to an extremely high temperature. As 

they heat up, the rocks emits nearly perfect theoretical 

blackbody radiation – meaning that 

1) the larger rock is bluer and brighter. 

2) the larger rock is redder and brighter. 

3) the larger rock is bluer but the same brightness. 

4) the larger rock is the same color but brighter. 

5) the larger rock is the same color and brightness.


The graph below shows the blackbody spectra of two totally 

different stars. What can you conclude from the plot about 

the two stars? 

1) Star A is hotter but smaller than Star B 

2) Star A is hotter and larger than Star B 

3) Star A is cooler and larger than Star B 

4) Star A is cooler and smaller than Star B 

Relative 

Intensity 

A 

B 

Wavelength

Summary 

n Blackbody Radiation (a.k.a. Thermal 

Radiation) 

¨ Many objects with a temperature greater than 

absolute zero (0 K) emit blackbody radiation. 

¨ Hotter objects emit more total radiation per 

unit surface area. 

¨ Hotter objects emit photons with a higher 

average energy.


n You emit radiation: 

¨ 

¨ 

True 

False


n You emit radiation: 

¨ 

¨ 

True 

False 

Your skin feels warm, you emit infrared 

radiation

How Matter Emits Light: 1. the Blackbody Radiation

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