# Lecture 4: Light and Matter

## 42 terms

### 4 ways in which light can interact with matter

emission, absorption, transmission, reflection/scattering

### emission

matter releases energy as light

### absorption

matter takes energy from light

### transmission

matter allows light to pass through it

### reflection/scattering

matter repels light in another direction

### Power

rate at which energy is used/emitted. aka luminosity. measured in Watts.

### Watts

1 joule per second. ex. a 100 watt light bulb radiates 100 joules of energy every second

### Force Field

describes the strength of a force that an object experiences at any point in space

### electric fields

surround objects with electric charges

### Magentic Fields

surround electrical currents (i.e. moving charges)

### an electrical field

a changing magnetic field produces

### magnetic field

changing electrical field produces

### electromagnetic fields

(self regenerating fields) the changing magnetic and electrical fields continue to produce each other until some object intervenes.

### Examples of Electromagnetic Fields

Radiowaves, microwaves, visiblelight, X-rays andγ- rays

### Light

A vibration in an electromagnetic field through which energy is transported. Can be thought of as a wave, or as a particle. Has dual nature.

### Wavelength

distance from one peak to the next

### Frequency

Number of peaks passing by any point each second. Units are cycles/sec or hertz

### Amplitude

The height of each peak

### Speed

the rate at which peaks move

### wave

pattern revealed by interaction with particles i.e. ripple traveling on surface of water

speed=f x λ

3x10^5km/s

### short wavelength =

high frequency (violet)

### long wavelength =

low frequency (red)

### photons

packets of energy. The energy carried by each photon depends on its frequency (color). E = hf = hc / λ ["h" is called Planck's Constant] h=6.626x10-34 joule x s. Bluer light carries more energy per photon

### Left of Electromagnetic Spectrum

shortest wavelength, highest frequency, and highest energy per photon found at the end. 400 nm

### Right of Electromagnetic Spectrum

longest wavelength, shortest frequency, and lowest energy per photon found at this end. 700nm

### temperature, composition, velocity

By studying spectrum of an object we can learn its

### Blackbody

ideal emitter, which absorbs all incident radiation and reradiates this energy with a characteristic spectrum. No electromagnetic radiation passes through, and none is reflected. Spectrum depends only on T, the temperature to which object is heated.

### Glass

light goes right through, not a blackbody

### shiny metallic surface

light gets reflected, not a blackbody

### piece of soot

black material, light gets almost completely absorbed and material heats up, blackbody

### continuous spectra

has energy at all wavelengths. most important example thermal (blackbody) spectrum.

### blackbody

most astronomical objects emit spectra that is nearly?

has specific spectrum shape (color) that depends on T. Has certain normalization (brightness). Isotropic.

### Isotropic

same emission in all directions

### 7000k (thermal spectra)

all colors appear brighter, but blue is brightest, objects look blue

### 6000k (thermal spectra)

all objects appear the same brightness, objects appear white

### 5000k (thermal spectra)

all colors fainter, but red is brightest, objects appear red

### Rules for Emission for Opaque Objects

Hotter objects emit bluer photons (with a higher average energy). -Wiens Law
Hotter objects emit more total radiation per unit surface area (each square meter of hotter object's surface emits more light at all wavelengths) - Stefan Boltzman Law

### Wiens Law

λmax =2.9x10^6 /T(K) nm

### Stefan Boltzman Law

F = σT^4 [J/s/m^2]