Identify Planck’s hypothesis that

radiation emitted and absorbed

by the walls of a black bodycavity is quantised

Colours change as a body is heated

To understand how the wavelength of the radiation varies with temperature, scientists use a ‘black body’

A BLACK BODY ABSORBS ALL INCOMING RADIATION AND IS A ‘STANDARD’ FOR EXPERIMENTAL COMPARISONS

e.g. an oven with a small hole drilled into the wall The walls absorb all incoming radiation and only emits (through the hole) radiation which is characteristic of the oven temperature

Classical wave theory suggested that as the walls get hotter and shorter wavelengths are emitted, radiation intensity should continue to increase without

limit

However, in experiment the radiation was found to have a definite peak for a given temperature

Planck explained that the radiant energy was absorbed by atoms (until a peak was reached) and then emitted in a ‘lump’, so energy was always a multiple of ‘lumps’ having energy E=hf

Knowing c=f and E=hf, the energy of each ‘lump’ or ‘quantum’ (a photon) could be calculated

Identify the relationships between photon energy, frequency, speed of light and

wavelength:E hf

and c=f

Solve problems and analyseinformation using:

E = hfand

c = f

f=c/

E = hf

This occurs when electrons are released from a surfacewhich is exposed to electromagnetic radiation

Hertz first noticed the effect when he observed that stronger sparks were produced in the receiving loopwhen it was exposed to UV radiation from the transmitter. Materials which blocked UV reduced the spark

J. J. Thomson set up a metal surface and a collector in a vacuum tube and detected current flowwhen light was directed onto the metal surface - photoelectrons were being liberated

SO THE LIGHT FREQUENCY DETERMINED THE K.E. OF THE ELECTRONS

Explain the particle model of light in terms of photons with

particular energy and frequency

Classical Physics predicted an absorption of light energy, regardless of frequency, until there was enough KE to liberate the electron. Experiment showed that different frequencies liberated electrons with different K.E’s

LIGHT E=hfPHOTONS

MORE PHOTONS MORE LIGHT INTENSITY

HIGH FREQ. LIGHT HIGH ENERGY PHOTONS

The energy required to release the electron is called the WORK FUNCTION

Photon Energy, E = hf = W + (K.E)electron

If the photon has more energy than the work function,the electron will have KE equal to the excess

From Planck’s work with black bodies:

Identify data sources, gather, process and analyse information and use available

evidence to assess Einstein’s contribution to quanta and their relation to black body

radiation Identify Einstein’s

contribution to quanta and its relation to black body

radiation

The following diagram shows Hertz’ experiment in measuring the speed of radio waves.

Describe how Hertz’s observations lead to an understanding of the photoelectric effect.

4 marks

Hertz observed that when current bridged the gap in the circuit with the induction

coil, a current also flowed in the receiving loop. When the potential difference from

the induction coil was not large enough for current to flow by the voltage alone, a

current could be established when a UV light was shone on the gap. Lenard showed

that there was a frequency threshold below which no current would flow. Einstein

explained that the energy comes in photon packets. The higher the frequency of the

electromagnetic radiation, the more energy the photon has. This energy could be

transferred into kinetic energy for the electrons at the cathode. They would then have

enough kinetic energy to bridge the gap between the cathode and the anode.

Process information to discussEinstein and Planck’s differing views about

whether science research isremoved from social and political forces

Physics FactJacaranda p.216

(a) With reference to the diagram, discuss how Einstein explained the photoelectric effect.

(b) If 5.68 x 10-19 J is needed to allow current to flow, determine what wavelengths of light will

result in the ammeter registering a current.

The following diagram shows light incident upon a cathode in a discharge tube.

(a) 4 marks

Einstein explained that the electrons at the cathode do not have a sufficient maximum kinetic energy to reach the anode. Light

consists of packets of energy called photons. The higher the frequency the more energy a photon has. When a photon strikes an

electron, all the energy is converted into extra kinetic energy for the electron. If the photon had a high enough frequency, the

electron would gain sufficiently high kinetic energy to bridge the gap to the anode and current would flow.

(b) 2 marks

E = hc/ = hc/E

= 6.626 x 10-34 x 3.00 x 108/5.68 x 10-19 = 350 nm

350 nm

Identify data sources gather, process and present information to

summarise the use of the photoelectric effect in:

– solar cells– photocells

SUNLIGHT(LIGHT ENERGY)

ELECTRICALENERGY

USING THE PHOTOELECTRIC EFFECT

A type of photocell where light is focused onto the junction of p-type silicon (not exposed to light) and n-type silicon (exposed). The photoelectric effect causes photoelectrons to be released which are collected by a fine metal grid on the n-type silicon surface.Electrons (electricity) flow from n-type to p-type silicon via a circuit.

In a photoconductive cell, the photoelectric effect causes additionalelectrons to be released which increases conductivity (decreases resistance) i.e. a material may conduct electricity in light but not in dark.In a breathalyser a light source is shone through condensed alcoholvapour - more alcohol means less light is transmitted to the detectorso the current (and hence the reading) changes

The diagram below shows an intoxilyser which can measure the blood alcohol content of

a person.

A person breathes into the sample chamber. Alcohol absorbs specific wavelengths of

infra red light. The filter wheel filters out wavelengths other than those absorbed by

alcohol. The microprocessor can measure current.

Explain how light is used to determine the blood alcohol level of a person. In your

answer explain how the photocell works.

The bonds in the alcohol absorb specific wavelengths of infra red light. The filter filters out other wavelengths so that only the relevant ones

reach the photocell. A person’s breath who has a high blood alcohol content will absorb much of the infra red light. Light that does reach

the photocell will excite electrons in the cathode of the photocell so they have enough kinetic energy to reach the anode. A microprocessor

registers the amount of current from the electrical pulse and converts this to a blood alcohol content reading. The higher the current the less

blood alcohol they have because the light was not absorbed by the alcohol.