2 Black Body Eadaitodn And Photoelectricity

Question 1

What is perfect black body

Answer 1

-A theoretical object that absorbs all of the radiation incident on it and does not reflect or transmit any radiation

Question 2

What is a black body spectrum

Answer 2

-The spectrum of electromagnetic radiation that would be emitted from this hypothetical object.
-Higher temp of black body means higher intensity and smaller Wl of EM
-A cube of metal at room temperature emits invisible infrared radiation
When heated to 3000 K, however, it emits a large amount of visible light and we see it glow red, orange or whit

Question 3

Ultra Violet Catastrophe summarised

Answer 3

-Classical theory was able to measure the larger wl black body radiation…
-but they weren’t expecting to see the UV spectrum (shorter WL) to decrease in intensity
-their data suggested the intensity increased for smaller WLs using wave theory despite experiments disproving that

Question 4

How did Planck resolve the ultraviolet catastrophe

Answer 4

Oscillators were responsible for emitting electromagnetic radiation, and he assumed the energy emitted was quantised
This meant it could only be emitted in integer multiples of these packets (or quanta) of energy
Therefore, the energy emitted by an oscillator of frequency f was given as:
E = nhf
n represents the integer multiple of the packet hf
h is Planck’s constant and has the value of 6.63 × 10−34 J s
This allowed Planck to develop a theory that explained the observed spectra of almost-perfect black-bodies

Question 5

How did photoelectricity contradict wave theory??

Answer 5

-Incident radiation on a metal surface emitted electrons only if it was above f0 and if below.. no matter what intensity photoelectrons won’t be emitted

-wave theory predicted that an electromagnetic wave transferred energy continuously
According to wave theory, if low frequency radiation was aimed at the metal at a high enough intensity, then enough energy would be transferred to remove photoelectrons
However, as soon as a radiation above the threshold frequency was shone on the metal’s surface, even at low intensities, photoelectrons were immediately emitted
Additionally, the intensity of incident radiation only affected the number of photoelectrons emitted, but not the energy they left with
All of this evidence contradicted the idea that electromagnetic radiation transfers energy like a wave does

Question 6

Einsteins explanation of photoelectricity

Answer 6

-Einstein proposed EM was made from discrete quanta or packets of energy called photons
E = h f

These photons were massless
-Only to one interaction
-so if hf was less than work function photons could not combine to energise that electron
-To explain why the energy of emitted photoelectrons increased with the frequency of incident light:
Photons transferred all of their energy hf to electrons
If this was greater than the energy needed to emit the electrons, the rest of the energy was transferred to the kinetic store of the electrons.