QATs VCE Physics SCHOOL-ASSESSED COURSEWORK UNIT 4 …QAT s Q VCE Physics Q SCHOOL-ASSESSED COURSEWORK, UNIT 4 OUTCOME 2 QATs VCE Physics SCHOOL-ASSESSED COURSEWORK UNIT 4 – OUTCOME

QATsQuality Assessment Tasks UNIT 4 ■ OUTCOME 2

VCE Physics

SCHOOL-ASSESSED COURSEWORK

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Introduction

NAME:

QATs VCE Physics SCHOOL-ASSESSED COURSEWORK UNIT 4– OUTCOME 2

©2012 Ser4PHYU4O2C

Published by QATs. Permission for copying in purchasing school only.

QATs

Quality Assessment Tasks

UNIT 4 OUTCOME 2

VCE PHYSICS

SCHOOL ASSESSED COURSEWORK

Introduction

Outcome 2

Use wave and photon models to analyse, interpret and explain interactions of light and matter and the quantised energy levels of atoms.

Task

Summary of Practical Reports

This task will be marked out of 40. It will contribute 50% of the marks (30) allocated for this outcome. A second task will be completed for this outcome.

The task has been designed to allow achievement up to and including the highest level in the Performance Descriptors.

You will complete a series of experiments as preparation for this task.

You have 90 minutes to complete the questions related to your summary. You can access ALL practical reports on Light and Matter experiments and one page of A4 notes.

Answer in spaces provided.

This task covers a broad range of key knowledge and key skills.

Your teacher will advise you of any variation to these conditions.

NAME: _________________________________

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Task

QATs ■ VCE Physics ■ SCHOOL-ASSESSED COURSEWORK, UNIT 4 OUTCOME 2

Ser4PHYU402C


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Task Preparation

Before completing the written task (Summary of Practical Activities), the following four experiments must be completed. Modifications to the experiments should not affect the Summary Report.

Each experiment will be marked and returned to you with feedback prior to the Summary Report being attempted.

In some situations a simulation can be used to replace an experiment.


Task



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laser whiteboard Retort stand

L

Diffraction grating Retort

stand

Task preparation

Experiment #1: Double slit Interference NAME: _____________________ Safety note: Ensure this experiment is always conducted below eye level to avoid accidental exposure

of the eye to the laser beam.

Aim:

To find the wavelength of a pencil laser.

Apparatus:

2 × retort stand/bosshead/clamp

1 × pencil laser

1 × diffraction grating (IEC double slit PA1670-006 or similar)

1 × white board or graph paper

1 × 1m ruler or tape

Method:

1. Use a retort stand and boss head clamps to hold the diffraction grating parallel to the white board. Make sure the label side (as shown below) is facing the laser.

2. Use another retort stand and boss head clamp to hold the laser pointer in position so that it beam passes through the diffraction grating and the diffraction pattern can be clearly seen on the whiteboard or graph paper.

3. Measure the distance (L) from the diffraction grating to the whiteboard. (hint set L = 1.0m). 4. Aim the laser pointer through the upper left most set of slits (d = 0.175 mm) 5. From the diffraction grating marked PA1670-006 record the slit spacing (d) in table over page. 6. Use a whiteboard marker to mark the bright points on the whiteboard. (Or a piece of graph paper) 7. Measure the distance (w) between adjacent bright points. 8. Calculate the wavelength of the laser pointers light beam. (w = L/d) 9. Repeat steps 4 8 for 3 other slits on slide PA1670-006.

d

L

w

Top view


Task


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L (m) d ( 10-3 m) W (m) ( 10-9 m)

0.175

error

Average

Discussion/Conclusion:

The wavelength of this laser is stated as 630 ~ 680 nm. How accurate were your results?

IEC DOUBLE – slits

PA1670-006

d = 0.175mm

d = 0.150mm d = 0.125mm

d = 0.100mm

d = 0.075mm

d = 0.050mm

d = 0.200mm

d = 0.225mm

d = 0.250mm

d = 0.325mm

d = 0.300mm

d = 0.275mm


Task



©2012 Ser4PHYU4O2C


laser whiteboard Retort stand

L

Diffraction grating Retort

stand

Experiment #2: Double slit Interference NAME: __________________

Safety note: Ensure this experiment is always conducted below eye level to avoid accidental exposure of the eye to the laser beam.

Aim:

To investigate the effects of varying L, and d on fringe spacing on Young’s double slit experiment.

Apparatus:

2 × retort stand/bosshead/clamp

1 × pencil laser

1 × Educational diffraction grating (with line spacing 100/mm, 300/mm & 600/mm)

[Paton Hawksley Education Ltd , Rockhill Laboratories , Keysham, Bristol, UK]

1 × white board or graph paper

1 × 1m ruler or tape

Method:

1. Use a retort stand and boss head clamps to hold the diffraction grating parallel to the white board. 2. Use another retort stand and boss head clamp to hold the laser pointer in position so that it beam

passes through the diffraction grating and the diffraction pattern can be clearly seen on the whiteboard.

3. Measure the distance (L) from the diffraction grating to the whiteboard.

4. From the diffraction grating marked 100lines/mm calculate the slit spacing (d). 5. Repeat for the grating of 300 and 600 line/mm. Record these in the table below. 6. Using your value for the wavelength of the light from the laser pointer calculate the fringe

spacing (w) expected for the above set up. Record these in the table below. 7. Aim the laser pointer through this set of slits. 8. Use a whiteboard marker to mark the bright points on the whiteboard. 9. Measure the distance (w) between adjacent bright points. 10. Repeat steps 7 9 for the other 2 gratings on the slide.


Task


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L = m = 10-9 m

Lines/mm d ( 10-6 m) Theoretical

W (m)

Experimental W (m)

100

300

600

Discussion:

Q.1 What is the effect of decreasing the slit spacing (d)?

Q.2 State two other ways the same effect can be achieved.

Q.3 The first dark fringe from the central maxima for the interference pattern produced by

the 100 lines/mm grating is the result of a path difference of:

a) how many wavelengths?

d

L

w


Task



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b) how many metres?

Conclusion: Briefly explain how L , d and affect the fringe spacing and why there was such distinct difference in the patterns seen in Experiment #1 compared to Experiment #2.


Task


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Experiment #3: Measurement of Planck’s Constant NAME: ____________

Aim:

To experimentally determine the value of Planck’s Constant.

Apparatus:

IEC Photo Electric Effect AP 2341-001.

Theory: The maximum kinetic energy, EK(max) , of an emitted photoelectron from a surface of work function, W, when illuminated with photons of energy hf is given by:

EK(max) = hf – W where W = hf0

When the photocell is operated in reverse bias, the magnitude of the reverse potential difference (backing voltage), Vs , between the cathode and the anode required to just prevent photoelectrons of energy EK(max) from reaching the anode (zero current) is given by:

EK(max) = qVs where q is the magnitude of the electron charge and equals 1.6 x 10-19 Coulombs.

Hence: qVs = hf – W

Vs = (hf – hf0)/q

A graph of Vs versus f for each filter will appear similar to figure 1. From this graph h/q and f0 are obtained, thus allowing Planck’s Constant to be deduced.

Figure 1 Graph of stopping potential, Vs , versus incident photon frequency, f . The slope of the graph equals h/q.

Vs

f

f0


Task



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The photocell is inside the metal enclosure. The inside surface is coated with a matte-black material to absorb any stray light not directly incident on the photo cathode. A strip of black tape on the front of the photocell screens the anode from the incident light from the shielded lamp.

Light from the lamp is emitted over a broad range of frequencies. (From near UV to IR.) Sharp cut-off optical filters are used to define a minimum wavelength, min , (of maximum frequency fmax = c/min . c is the speed of light and equals 3.00 x 108 ms-1) which will produce photoelectrons of maximum kinetic energy, EK(max) . These can be inserted in the slide holder in front of the photocell at the rear of the apparatus between the light source and the backing panel.

Shown below is the light absorption characteristic of an ideal sharp cut-off optical filter. The shaded region represents all wavelengths of incident light which are less than min and are totally absorbed by the filter. All wavelengths greater than min are transmitted to the photocell. Details of each filter are given in Table 1.

Table 1 Parameters of sharp cut-off filters

Filter colour Cut-off wavelength min (nm)

Cut-off frequency fmax (Hz)

Blue 428 + 2 7.01 x 1014

Yellow 492 + 2

Green 460 + 2

Orange 530 + 2

Red 590 + 2

Q1. Use c = f to complete the last column in table 1 above.

Abso

rptio

n %

0

100

min Wavelength


Task


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Method:

Calculate the frequencies, fmax , for the remaining filters in table 1.

Measurement of Planck’s Constant

1. Set the meter reading to the 0-20 A scale 2. Insert the Blue slide into the slide holder. 3. Turn on the power to the lamp. 4. Adjust the backing volts knob until the ammeter reads zero. 5. Switch the meter reading scale to 0-200 nA 6. Adjust the backing voltage knob until the ammeter reads zero. 7. The stopping potential Vs , is found by switching the meter reading scale to ‘Backing voltage’ Do

not touch the ‘Backing volts’ knob! 8. To double check your value repeat steps 17. Each partner should do this for the filter and

record the results in Table 2. 9. Repeat the above procedure with each of the remaining filters.

Table 2 Stopping Potentials

frequency

Filter (Hz) 1st 2nd 3rd 4th Average

Blue Yellow

Green

Orange

Red

10. Plot a graph of Vs versus fmax using the data from Tables 1 and 2. Ensure that the scale is such that f0 will be able to be read off the graph and hence w = hf0 can be calculated.

Stopping Potential Vs (volts)


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Data Analysis:

The gradient of the line of best fit equals h/q determine the error associated with this value.

h/q = +

Using q = 1.6 x 10 –19 Coulomb, the best estimate of Planck’s Constant is:

h = + Js

The best estimate of the threshold frequency f0 is:

f0 = + Hz

The value of the work function w of the photo-cathode material is:

w = + eV

Discussion:

Q.1 How does your calculated value of Planck’s constant compare with the accepted value of 6.626 10-34 Js?

Find using

(fo ,0) is the

X-intercept

error


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Q.2 Using the table below what is the most likely metal used for the photo-cathode?

Metal Work Function w (eV)

Cesium 2.1

Potassium 2.3

Sodium 2.8

Magnesium 3.7

Gold 5.1

Q.3 Unfortunately inside the photo-tube some light is reflected from the cathode onto the anode and this will result in the anode releasing some electrons as well.

Explain how this produces a lower than expected value of Planck’s constant.

Conclusion:

Briefly summarise THREE ways in which the photoelectric effect contradicts the wave model of light.


Task



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Experiment #4: Analysis of Atomic Emission Spectra NAME: __________

Aim:

To observe the spectral emissions of elements and determine the energy of the emitted photons.

Method:

Use the spectroscopes to observe the light produced by the different sources and record the wavelength of the shortest and longest spectral lines.

Light Source

Symbol

No. of spectral

lines observed

Wavelength (nm) Photon energy E = hc/ (J)

Shortest Longest Shortest Longest

Hydrogen H2

Helium He

Argon Ar

Mercury Hg

Neon Ne

Use the chart provided to compare your spectral lines to those shown.


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Discussion Questions:

Q.1 For which of the elements did you detect the correct number of spectral lines?


Task



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Q.2 Offer an explanation as to why you may have not detected the correct number for each of the viewed elements.

Q.3 Use the spectral chart for Hydrogen below to calculate the designated energy levels in an atom of Hydrogen.

wavelength () Photon energy

(E = hc/)

orbital level

that the electron starts in

energy of orbital level

(3.4 – E)

(x 10-10 m) (x10-19) J eV n eV

4100

4400

4900

6700

Q.4 Why do we not record any emission spectra for jumps back to the ground state?

Only visible light can be detected by the spectroscope. This occurs when excited electrons drop back to the n=2 orbital (1st excited state). This is called the Balmer series, refer to p.454 of the Heinemann. This energy level is based at E = -3.4eV. So all starting energy levels of the excited electrons are above this and are equal to the photon energy (E = hc/) more than -3.4eV.

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QATs VCE Physics SCHOOL-ASSESSED COURSEWORK UNIT 4 …QAT s Q VCE Physics Q SCHOOL-ASSESSED COURSEWORK, UNIT 4 OUTCOME 2 QATs VCE Physics SCHOOL-ASSESSED COURSEWORK UNIT 4 – OUTCOME