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Using an LDR to detect the intensity of plane polarised light allowed through a Polaroid.

Extracts from this document...


David Simons

Physics Coursework: Effectiveness of an LDR

Physics Coursework

Using an LDR to detect the intensity of plane polarised light allowed through a Polaroid.

Physics Coursework

Using an LDR to detect the intensity of plane polarised light allowed through a Polaroid.



This coursework has two major aims. The former is to detect the intensity of plane polarised light let through a Polaroid as I rotate it. However, my main aim of this coursework is to detect the effectiveness of using an LDR as a sensor to measure this intensity, and hence its effectiveness at measuring the angle between two polaroids.


Circuit Diagram

The way that I will accomplish this is by using the following circuits:



The circuit on the right is just a simple 12V DC power supply, with a light bulb connected to it. This will be the light source whose intensity we will be measuring.

The circuit on the right is the sensor. There is an LDR, which is connected in series with a voltmeter and a variable resistor, which are then connected to each other in parallel. As light intensity increases, the resistance of the LDR will decrease. Thus, since the resistance of the variable resistor will remain constant, the ratio of potential differences will increase and cause a higher voltage being read across the voltmeter.

To put this mathematically:



Reading Across voltmeter for E.M.F ε,resistance of LDR R, and resistance of variable resistor r

As we increase light intensity, R decreases, becoming R - δR. However, r remains constant.

Light Source

However, we will also need to set up things outside of the circuit. For example, we will need to plane polarise the light before we can then use a rotating Polaroid in order to change its intensity.

...read more.


The final problem is sensitivity. With the resistor in the circuit, I am able to alter it in order to increase the sensitivity of the results that I get. With this circuit, however, that is not possible.

Since I have decided on the existence of the resistor, the next step would be to work out the value. Since I am using a resistance substitution box, I am limited to certain values. However, given the values of the resistor that are possible, and the EMF of 12V which we would achieve for a perfect battery, I can form the following formulae for the output potential difference, in terms of the values of resistance in the LDR, R. Firstly, I will work out the formulae for orders of magnitude, and then I will be able to further narrow it down.

Value of Resistor

Value of Output p.d









Naturally I will have to measure it for my LDR, but internet reports on the internet seem to suggest that the intensity of light emitted from a 60W bulb is approximately 50 lux. Therefore, the intensity that I would be interested in is the value of resistance from 25 lux downwards. This graph, from https://wwws.ee.ucl.ac.uk/facilities/teachlab/info/data/ldr seems to indicate that the resistance in the values that I need will be between 10 and 0.1 kΩ


As such, I am going to plot a calibration curve showing how equal changes in light intensity affect the voltage output over my desired range for each of the values I can choose from. The way values for resistance were calculated were by approximating the graph above with a regression line y = 100x-0.76

From this graph, you should be able to see the best resistor to pick.
...read more.


“measure of the minimum difference that can be detected or viewed.” Thus, “A system with high sensitivity can detect very small temperature differences”

What do I want in terms of sensitivity?

Naturally in any sensor sensitivity is good, because this means that you are able to use it to a higher degree of accuracy. While this is very much appreciated, however, I feel that on my sensor, this is not absolutely vital. Although having a sensitive device is good as it will improve accuracy and precision, it is a little redundant having a device which is more accurate than another part of your experiment.

Is my Light Intensity sensor sensitive enough?

In my experiment with the light intensity, I obtained my numerical results from an approximation from a graph which is not particularly accurate in the first place. Even if my sensor was very sensitive, the numerical results that I will achieve will be made inaccurate when I try to convert them to lux using this graph.image23.png

Is my Angle sensor sensitive enough?

For the angle sensor, this is much more important as, although there are some minor inaccuracies, we are doing this experiment much more carefully, and we actually have figures that can back each other up, as shown from the detail in the differing graphs. As such, for this, sensitivity is important. If our results were ideal (i.e. there was no ambient light entering the system) then it is fair to say that it is sensitive, as seen from the Ideal Results graph on page 18. However, as it is, our results are not really sensitive for the lower values and the higher values. Up until 35o, the results all fall within 0.4V of each other, as do the results from 70o to 90o. Thus, in terms of sensitivity, this sensor was not very good. I could have improved this by using either a whetstone network; or by blocking out ambient light more effectively.

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