Light is a ParticlePhysics 12

What is Light?

-most information about the Universe is obtained through analysis of light

-a wave is rising and falling (oscillating) disturbance that transports energy from a source to a receiver

What is Light?

-a light wave is an electromagnetic (EM) disturbance consisting of changing electric and magnetic fields

-light waves transport energy from moving electric charges in stars (source) to electric charges in the retina of human eye (receiver)

-light waves are distinguished by their lengths

-Wavelength (λ) – the distance from any point on a wave to the next identical point (from crest to crest or trough to trough)-measure these waves in billionth of a meter (x 10-9 m) nanometers, nm, or the angstrom unit, Å-visible light has wavelengths of 4000 Å (400 nm) to 7000 Å (700 nm)

-visible light of different wavelengths perceived as colors (R-O-Y-G-B-I-V)

Relative sensitivity of the human eye to different colors and wavelengths of visible light.

Most sensitive

Least sensitive

Speed of Light-all EM waves move through empty space at the same speed, c = 300 000 000 m/s-no known object can be accelerated to move faster than “c”-one of the most important and precisely measured numbers in astronomy-a light-year (ly) is the distance light travels through empty space in one year-frequency (f) of a wave motion - the number of waves that pass by a fixed point in a given time, measured in cycles per second or Hertz (Hz)

c = λ f

Example:

What is the wavelength of light with a frequency of 7.5 x 1016 Hz?

-light (EM) waves or radiation outside of visible range exist:

radio, microwaves, infrared (IR), ultraviolet (UV), X rays, gamma rays

-the range of all EM waves ordered according to decreasing wavelength (or increasing frequency) is called the EM spectrum

-the lower the wavelength, the higher the frequency the higher the energy carried in the EM wave

-for example, blue light is more energetic than red light

-UV light is more energetic than infrared light this is why UV causes sunburns and cancer

Radiation Laws-stars, like other hot bodies, radiate electromagnetic energy of all differentWavelengths-energy due to temperature is called thermal radiation thetemperature of a star determines which wavelength is brightest-stars radiate energy almost as a blackbody, or theoretical perfectradiator-the intensity (or amount of energy) of radiation emitted over a range of wavelengths depends only on the blackbody’s temperature Wien’s law of radiation

T

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Example:

What is the peak wavelength of light that emanates from the surface of the Sun, which has a temperature of 5778 Kelvin? What color is the Sun?

•The Sun’s thermal radiation spectrum•All blackbody radiation spectrums have the same shape•Hotter objects emit more energy at all wavelengths, and the peak shifts to shorter wavelengths.

Radiation Laws

-Stefan-Boltzmann Law:

F = σT4

-where…•F energy flux (joules) per square meter of surface per second (or Watts per m2)•σ a constant 5.67 x 10-8 W/m2/K4

•T temperature in Kelvin

Example:

What is the flux at the surface of the Sun?

Compare this value to the solar constant 1360 W/m2 at the upper atmosphere of Earth? Why are the two values different?

Quantum Theory Max Planck was able to determine an

empirical mathematical relationship between the intensity and frequency of blackbody data

In order to develop a theory that described his relationship, Planck was required to use discrete mathematics

Quantum Theory This lead to the idea that there was a

minimum amount of energy that could be exchanged

This minimum amount of energy lead to the idea that energy was quantized meaning that it can only exist is specific “packets”

E = nhf where n = 0, 1, 2, 3, …

Quantum Theory Einstein improved on Planck’s ideas Einstein concluded that to conserve energy

a blackbody radiator must emit light with “packets” of energy or photons or…

E = hf

Example:

Calculate the energy of light with a frequency of 7.5 x 1016 Hz.

Estimate how many visible light photons a 40-W light bulb emits per second. Assume λ = 500 nm and the efficiency of the bulb is 10%.

Quantum Theory Using Boltzmann’s statistical models

Planck was able to model the behaviour of blackbody radiation exactly

This was published in 1900 and was the birth of modern physics

However, these results were not immediately accepted (even by Planck) as they were contrary to previous work

Photoelectric Effect The photoelectric effect

eventually provided support for the idea of quantization of energy

The photoelectric effect occurs when photoelectrons are emitted from a metal when exposed to certain frequencies of light

Photoelectric Effect Experiments with the photoelectric effect

led to two key conclusions: When the intensity of light increases, the number

of electrons emitted increases The maximum kinetic energy of the electron

ejected from the metal is determined only by the frequency of light and is not affected by intensity

Einstein and the Photoelectric Effect Einstein saw the link between Planck’s

quantization of energy and the photoelectric effect

He proposed that not only would light be emitted as quanta but must also be absorbed as quanta

By considering these quanta (or photons) he was able to explain the photoelectric effect

Einstein and the Photoelectric Effect By using the concept of

the photon (and its associated energy), Einstein proposed the following:

hf = W + Ek(max)

Despite the fact this equation worked, it was not widely accepted (even by Planck)

Millikan and the Photoelectric Effect

Because the charge on the electron was not known when Einstein published his paper on the photoelectric effect, his result could not be proven

Millikan, having determined the charge on the electron, improved on the photoelectric effect experimental design and was able to confirm Einstein’s assumptions

Millikan and the Photoelectric Effect Using his experimental design,

Millikan was able to produce data that supported Einstein’s equation:

Ek(max) = hf – W Ek – kinetic energy of

photoelectron h – Planck’s constant f – frequency of EM radiation W – work function of metal

Example:

Light with a wavelength of 571 nm strikes a cesium metal surface inside a vacuum tube. What is the maximum kinetic energy of the emitted photoelectrons? What is the threshold frequency, fo, for cesium?

The electron volt (eV) Since the energies involved in Quantum

Physics are so small, instead of using joules to describe energy, the electron volt is used instead

One electron volt is equal to 1.60x10-19J