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Investigating how distance affects the light shining on the L.D.R.

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RachaelSmith                                                                                                 Nov 2003

G.C.S.E Physics LDR Coursework


I am investigating how distance affects the light shining on the L.D.R.

An L.D.R is a light dependent resistor (LDR) is not so easy to see. It is a component that has a small transparent end which is sensitive to light. A change in the amount of light falling on it changes its resistance. This change in resistance can be used to switch a circuit on or off.

They are used in street lamps to switch them on in the evening and off in the morning. One other use is on bowling alleys. If your foot slides past the foul line, then you break a beam of light across this line. The LDR reacts to this, and a buzzer sounds. Similarly LDR’s can be used in burglar alarm circuits.

The Input is the distance of the bulb from the L.D.R. The out put is the mA.

These are some variables that could affect my results.


What effect will they have on the outcome?

What voltage the bulbs at.

If it’s at the same each time recorded at a certain length. Then the results will be recorded correctly, but if the power of the bulb isn’t the same it won’t be.

Amount of daylight

It would be an unfair test if there were different amounts of day-light at different readings.

Shadows from people & objects. E.g. wires.

Shadows will cover up light from the bulb, making the mA decrease.

Distance between the bulb and L.D.R.

Go down in 5’s to make it a fair test and keep it at that while recording so it doesn’t move (the mA).

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Find out what the ammeter reads without the bulb light (this is the daylight).

Then record what the ammeter reads as you move the bulb 5cms nearer to the L.D.R each time, but take the reading of no bulb light away from them.  


Distance from L.D.R   (cm)

1st reading with daylight (mA)

2nd reading with daylight (mA)


































We found out the reading for daylight which is 6.8mA and took this away

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If the light beam is more intense, then more bonds are broken. So the LDR’s resistance decreases when you bring the light source closer. And the resistance increases as you move the light away. This is because it all depends on how much the light spreads out, when the LDR is near to the light there is a small distance so it misses less light. But when the LDR is further away from the light there is a bigger distance and so it misses more light. See diagram on next page.


I think maybe at 45cms we had shadows covering the light a bit so the recordings (mA) varied.

Overall the experiment was fairly accurate. A thing that may vary the recordings to make the points off the best fit line is the angle of the bulb as it could be a few millimetres off. To extend this investigation we could put tissue paper or something in front of the bulb to measure how much light is absorbed.

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