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

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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.

PLAN

Aim

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.

Method

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:

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. The simplest way to do this will be to use another Polaroid, as shown in the following diagram:

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Since the photons’ vibration before going through a Polaroid are omnidirectional, before I am able to see the effects of a Polaroid, I must plane polarise it. Since a Polaroid plane polarises the wave, I will be able to use that. Unfortunately, this has the effect of halving the intensity of the light; increasing the need for a more sensitive resistor. There were two other methods I considered:

Lasers are already polarised, so I could have used a laser to pass through a rotating Polaroid. However, the problem with this is that although I am not losing any of the original light by passing it through a Polaroid, the laser’s initial intensity is much lower than the intensity of the original light bulb.

The other option was to use an infra-red emitter, which would then require an infra-red receiver on the other side. Although this would also solve the problem of ambient light, the problem is that it is a potentially a lot more problematic as infra-red receivers and emitters look exactly the same, and you may be unable to know which was which, especially since neither emits any visible light, unlike a laser or a light bulb.

The other reason I chose a light bulb is because of its price and availability, which also reduces the significance of causing a short circuit, or having too high a current in the circuit.

Ambient Light

As I was describing why I did not use an infra-red transmitter, I mentioned ambient light, which will cause a lot of problems when I am going my coursework:

As you can see, the intensity of the ambient light will be much greater than the intensity of the light which has just passed through two Polaroids. Naturally, using infra-red would be one way to solve this problem, but there were two others which I considered:

Firstly, by using an ice cream tub to contain the entire experiment, you have solved the problem of ambient light. There are disadvantages, for example, the need to cut holes, but the problem of adjusting the Polaroids is not a big problem due to the lid.

The above diagram shows the idea of creating a small container for purely the LDR which will stop the ambient light, and maintain access. This, though perhaps easier to create, also has many more problems than the ice cream tub method. For example, there is still the problem of needing holes for the LDR and also the fact that the small shade will shade the light entering the LDR. It also doesn’t stop the problem of creating a shadow when trying to read the voltmeter, if I try to lean over it.

Components

Power Supply

Another component that I think is important to consider is our power source. I have chosen to use a 12V DC power supply. The reason I chose to use a DC power supply is because had I used an AC power supply, then the bulb’s light would have oscillated, and using a sensor with a fast reaction time may have picked up on this very fast flickering. The reason I used 12V is because there is very little reason to go above this; since I am using a light bulb and an LDR, a higher EMF may just have caused problems with safety and high currents.

Detector

Now that I have considered my choice of emitter, I think it is necessary to discuss why I chosen my particular sensor. There were four different components I had the chance of using. The advantages and disadvantages of all four are contained in the table overleaf.

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The reason that I chose an LDR is because, most importantly, it is a passive sensor and this means that I am able to change its sensitivity by using whetstone networks, amplifiers or by inserting it into a potential divider. The fact that it has a slow reaction time could perhaps be an advantage, since it means that I will be able to use moving coil meters or digital multimeters without having to worry about it change before I can take a reading. Finally, I ...

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