Ideas about Light and Matter
Experiments
1.
2.
3.
4.
5.
Diffraction through a Single Slit
Interference of light through a Double Slit
Photoelectric Effect
Light Sources and Spectra
Planck's Constant and LEDs
2
3
4
7
8
Selected experiments from Camberwell Grammar School
written by Dr Murray Anderson and Dan O'Keeffe
Light & Matter Prac 1: Diffraction through a Single Slit
Aim
To investigate the existence of diffraction of light as it passes through a single narrow slit and to
observe any trends in the amount of diffraction as the width of the slit varies or when different
coloured light is used.
Equipment
1 × diffraction of light kit with 35 mm slide of single slits, 1 × DC power supply, 1 × magnifying
glass
Tasks
1. Connect up the light source to a 6 to 8 V DC power supply.
2. View the 35 mm slide through a magnifying glass and draw
a sketch of the slide to illustrate the group of narrow slits
that appear on the slide
3. Using a blue filter in front of the light filament, view the
light passing through each of the 12 narrow slits from widest
to narrowest. The effect is best when your eye in along the
principal axis of the apparatus and you hold the slide directly
in-front of the iris of your eye.
4. Observe the optical pattern and the changes in the pattern as
the blue light is systematically passed through widest to
narrowest of slits. Draw a picture of the pattern.
5. Now repeat the process this time with the red filter in place
of the blue filter and observe the pattern and any variations
to the pattern when the light passes through the widest to the
narrowest slit. Again draw a picture of the pattern.
6. You will find a filter that is half blue and half red. Position it
in-front of the light filament. This is used to compare any
variations in the pattern due only to colour. Pass blue and red
light through the narrowest of slits.
7. Observe the two-coloured pattern. Draw and describe what
you see.
2)
4)
5)
7)
8.
a.
Complete the following sentences:
The amount of diffraction as indicated by the spreading out
of the light pattern ______________________ as the width
of the slit is decreased.
b.
The amount of diffraction as indicated by the spreading out of the light pattern
___________________ as you change from blue light to red light.
c.
Red light has a wavelength that is _______________ the wavelength of blue light.
d.
The colour-order of visible light from greatest wavelength to smallest wavelength
is:_______________________________
9.
Finally remove the filter and observe white light passing through a single slit. Explain the
coloured image that you see.
2
Light & Matter Prac 2: Interference of light through a Double-Slit
Aim
To investigate the existence of the interference of light as it passes through a pair of narrow slits [often
referred to as a double-slit or 2-slit] AND to observe any trends in the interference pattern as the
distance between the slits varies or when different coloured light is used.
Equipment
1 × diffraction of light kit with 35 mm slide of double slits of variable separation, 1 × DC power
supply, 1 × magnifying glass
2)
Tasks
1. Connect up the light source to a 6 to 8 V DC power supply.
2. View the 35 mm slide through a magnifying glass and draw a
sketch of the slide to illustrate the group of double slits that
appear on the slide.
3. Using a blue filter in front of the light filament, view the light
passing through each of the narrow double slits from furthest separation to least separation.
4. As in Prac 1, the effect is best when your eye is aligned along
5)
the principal axis of the apparatus and you hold the slide
directly in-front of the iris of your eye.
5. Describe the optical pattern that you observe and the changes
in the pattern as the blue light is systematically passed through
furthest separation to least separation. Draw a picture of the
pattern.
6. Now repeat the process this time with the red filter in place of the blue filter and observe the
pattern and any variations to the pattern when the light passes
6)
through the widest to the narrowest slit. Again draw a picture
of the pattern.
7. You will find a filter that is half blue and half red. This is used
to compare any variations in the pattern due only to colour.
Pass the light through the double-slits. Observe the twocoloured pattern. Draw and describe what you see.
8. Complete the following sentences:
7)
a.
When light passes through 2-slits an
________________________ pattern is produced.
b.
Increasing the separation between the slits causes the pattern
to be more ________________.
c.
Changing the light from blue to red causes the pattern to ________________________
9.
Finally remove the red/blue filter and allow white light to pass through a pair of slits. Explain the
coloured interference pattern that you observe.
3
Light & Matter Prac 3: Photoelectric Effect
Aim
The purpose of this experiment is to demonstrate Einstein’s relationship for the photoelectric effect
and as a result calculate Planck’s Constant h, the Threshold Frequency f0 and Work Function W for
the metal surface of the photo cell.
Equipment
The equipment is all contained in the one box. It contains the photocell, D.C. power supply and a
meter to measure the so-called stopping voltage and detect current in the photocell.
The extra equipment includes a light source to provide a broad range of frequency and a selection of
filters to select specific frequencies of light.
The filters have been especially constructed so that they have a sharp frequency cut-off. Each filter
will transmit light of all frequencies up to a specified frequency. Each filter has the wavelength of the
cut-off frequency printed on the slide.
filter
light source
light
photo cell
emitted
electrons
A
V
to measure
photo current
to measure
stopping voltage
Tasks
1. The light source is placed at the aperture at the rear of the instrument and light and the instrument
is turned on. Its position is then adjusted for maximum current reading.
2. Light is then switched off and a filter is placed in the instrument, and then the light is turned back
on. The current sensitivity is adjusted to read mid-scale.
3. The stopping voltage is now increased to bring the current almost to zero, which is to just stop the
electrons that have been emitted with kinetic energy from the photo-cell.
4. The current sensitivity is now increased to maximum value and the stopping voltage increased
until the current JUST reads zero.
5. Now switch the meter to read the backing voltage. This is the value of the stopping or cut-off
voltage. Record its value and the cut-off wavelength of the filter in the table below.
6. Repeat steps 2 - 5 with the same filter to obtain an average value for the stopping voltage.
7. Turn the current sensitivity to minimum and then replace the filter. Repeat steps 2 - 6 with each
of the other filters.
4
Results
Filter Wavelength Cut-off
(nm)
Frequency (Hz)
Stopping Voltages (V)
3 repeat measurements
Average Voltage
(V)
average 
0.5×range
Useful equations
c
where for light c = 3.00 × 108 m s-1 and E  Vq where for electrons, the charge q = 1.6 × 10-19

C, V is the potential difference through which the charge has moved and E is the energy transferred to
the charge.
Energy has two units: the correct SI one: the joule and the convenient one: an electron volt.
1 eV = 1.6 × 10-19 J and is equal to the energy acquired by 1 electron whose charge is q = 1.6 × 10-19 C
when passing through a potential difference of 1.0 volt. It is also the energy lost by an electron when
passing through a voltage difference but in the other direction.
Analysis and Questions
1. Draw and attach a graph of Average stopping voltage Vstop against frequency f. Include a line of
best fit.
f 
The Photoelectric Effect is described by the relationship: Vq = E = hf - W, where E is the maximum
Kinetic Energy of the emerging electrons, hf is the Energy of the incident single photon and W is the
Work Function of the metal.
The maximum Kinetic Energy of the emerging electrons, E, is given by the product Vq, where V is the
Average Backing Voltage and q the charge on the electron. (Note q = 1.6 × 10-19 C)
2.
In your graph what quantity is represented by the gradient of the line?
__________________________
3.
In your graph what quantity is represented by the y-intercept? __________________________
4.
Use your answers to 2) and 3) to determine Planck’s Constant in eV s and then in J s and the
Work Function of the metal in eV and then in J .
5
5.
Why does the current decrease as the stopping voltage becomes more negative? [note: “more
negative” means increasing the magnitude of the stopping voltage]
6.
What happens to the most energetic electrons at the stopping voltage?
7.
What happens to the stopping voltage as the frequency increases?
8.
What is the threshold frequency for the metal surface?
9.
What would happen if light of wavelength 1000 nm was shone on the photocell?
10. What 2 key indicators in this experiment demonstrate that light is a particle?
6
Light & Matter Prac 4: Light Sources and Spectra
Aim
The aim of this demonstration exercise is to observe a number of light sources and sketch the spectra
of visible light using a spectroscope – for the record a spectroscope could be a prism, but we use a
diffraction grating. This is a filter consisting of thousands of slits so that the pattern formed is a multislit interference pattern.
Equipment
1 × diffraction grating, light sources – sodium, neon, mercury, hydrogen, a candle, the sun,
incandescent globe, classroom fluoro tube
Tasks
1. For each of the spectra observed, sketch with coloured pencils the spectrum and categorize as
either continuous or discrete. For each case state the means by which the sample emits light.
Light Source
Sketch of Spectrum
using coloured pencils
Continuous
or discrete?
Sodium
Neon
Mercury
Hydrogen
Candle
Sun
Incandescent
globe
Classroom
fluorescent
light
7
Emission Means
Light & Matter Prac 5: Planck's Constant and LEDs
Aim
To determine Planck's Constant from the excitation voltage of LEDs of various colours.
Theory
In an LED circuit, a power supply gives electrons energy. This energy can send an electron in a diode
from the valence band to the conduction band. In an LED, the electron can then drop back emitting a
photon. The energy of the photon is give by E = hf. The value of the voltage (in volts) at the point
when the LED first glows is equivalent to the energy gap (in electron volts) between the valence and
conduction bands, and so to the energy of the photon, also in electron volts.
The measurements of the excitation voltage combined with the known frequency of the light from the
LED can be used to determine Planck's Constant.
Equipment
1 battery, a variable resistor, a 1 kohm resistor, a set of LEDs, a multimeter
Tasks
1. Design a circuit to measure the excitation voltage of an LED. Constraints: i) The voltage across
the LED must range from zero to a value above the excitation voltage, at least 3V. ii) The current
through the LED must be limited by a resistor, that is the LED must never receive the full voltage
of the battery, otherwise the LED will blow.
2. Show your design to your teacher.
3. Staring from zero, increase the voltage until the LED first glows. Record the voltage in the table
below. Repat a few times to get an average.
4. Change LED and repat step 3.
5. Look up the frequency of the LEDs from the information supplied by the teacher. Note: LEDs of
the same colour do not necessarily emit light of identical wavelength and frequency. The
composition of the semiconductors can modify the value. The value needs to be determined from
the supplier's specifications.
6. Graph excitation voltage against frequency. You should get a straight line through the origin, so
draw a line of best fit from the origin through your data points.
7. Determine the value of the gradient, Planck's Constant. The units are eV sec.
8. Use your graph to determine the maximum and minimum value of the Planck's Constant
9. Convert your value into Joule.sec.
Results
Colour
Wavelength (nm)
Frequency
Voltage Readings (V)
Experimental value for Planck's Constant = ........................ +/- ......................... eV.sec
Experimental value for Planck's Constant = ........................ +/- ......................... Joule.sec
8
Average (V)