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Chapter
5
Introductory photonics
Investigation 5.1
Light-emitting diodes (LEDs)
Aims
• To construct an I–V characteristic for an LED
• To determine how an LED behaves when it is forward
and reverse biased
Equipment
• voltmeter
• ammeter with mA settings
• red LED
• 150 Ω, 330 Ω, 680 Ω, 4700 Ω and 6800 Ω resistors
• 5 V DC power supply
• switch
• connecting leads
A diode allows current to flow through it in one direction. The direction is indicated by the arrow on the circuit diagram symbol. An LED emits light when it is
forward biased and the voltage is large enough.
Analysis
1. How does the LED behave when it is forward biased?
2. How does the LED behave when it is reverse biased?
Suggested answers to questions
1. The saturation voltage depends on the colour of the
LED. As the current through the LED increases, the
emitted light becomes brighter as more photons are
released.
2. Reverse biasing the diode will effectively stop the current from flowing.
Investigation 5.2
Light-dependent resistor
Aim
To examine the way in which the resistance of a lightdependent resistor (LDR) varies with light intensity
Equipment
•
•
•
•
•
9 V battery
LDR
voltmeter
ammeter (able to measure µA)
a bright desk lamp or light source
Alternatively, a datalogger could be used to obtain readings for the resistance of the LDR for different light
intensities.
Suggested answers to questions
2. The intensity of an LDR will decrease as the intensity
of light increases.
Investigation 5.3
Light-dependent resistor:
voltage divider method
Aim
To gather information to produce a graph of how the
resistance of a light-dependent resistor varies with light
intensity
Equipment
•
•
•
•
•
•
9 V battery
LDR
2.2 kΩ resistor
voltmeter
light meter or datalogger (if available)
a point light source, for example a 12 V, 36 W car
headlight globe in a holder
• connecting wires
Analysis
1. Plot log RLDR against log IL on graph paper.
2. Determine the relationship between RLDR and IL, if
possible.
Handy hints
For a point light source, the intensity varies inversely
with the distance from the light source:
1
I ∝ ----2 .
r
If the distance is multiplied by a factor of x, then the
1
light intensity will be ----2- of the original value.
x
If the plot of log RLDR against log IL is a straight line,
log RLDR = mlog IL,
log RLDR = log ILm,
log RLDR = ILm.
Analysis
1. Calculate the resistance of the LDR in each case,
using the formula V = IR.
2. How does the resistance of the LDR vary as the light
intensity increases?
jp22e-chap05-invest.pdf
© John Wiley & Sons Australia, Ltd 2004