# A Study On Light Intensity Biology Essay

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## Abstract:

To investigate whether light intensity of incoherent lights varies inversely with the square of the distance holds true in real-life situations and under laboratory conditions. Techniques utilised in the experiment included measuring the distance from the light source to the light sensor, recording the light intensity of a light source and recording observations with accuracy. The light from the light source that passes through the cardboard cylinder tube changes as the distance from the light sensor increases. The results show that light intensity of incoherent lights varies inversely with the square of the distance in real-life situations and under laboratory conditions.

## Introduction:

The purpose of this experiment was to investigate whether light intensity of incoherent light varies inversely with the square of the distance in real-life situations and under laboratory conditions. The light of an incoherent light source will spread out uniformly in all directions. In other words, any point source which spreads its influence equally in all directions without limit to its range will obey the inverse square law. Luminous intensity is proportional to the inverse square of distance, I ∝1/r2. The calculations below show how this formula was derived:

Light intensity = power / area.

The light intensity at a given distance from the light will be equal to the power output of the light source divided by the surface area of the sphere through which the light has spread.

Surface area of a sphere = (4πr2) / 3

So, the luminous intensity on a spherical surface a distance r from a source radiating a total power P is:

I = 3P / 4πr2

As P and π remain constant, the luminous intensity is proportional to the inverse square of distance:

I ∝1/r2

The light intensity is the power of the lighting energy and its unit is candle (cd). The unit for lighting reflux is lumen (lm).

It is hypothesised that if the distance increases the light intensity of incoherent lights will decrease and therefore it will obey the inverse square law.

The graph above demonstrates that as the light sensor is further away from a light source, the less bright the source is. This means that as the light sensor moves away from the source, the less light reaches the light sensor.

The picture above can be used to prove that the surface area of a sphere is proportional to the square of the radius. The light source spreads out over an area that is proportional to the square of the distance from the light source as shown below:

Substitute equation 1 into equation 3.

Substitute equation 2 into equation 5.

Now, divide equation 4 by equation 6.

Therefore, the ratio of intensities at distances r1 and r2 are:

Light bulbs and any light sources that utilise reflectors or modifiers to direct light into a conical beam will obey the inverse square law. The reason for this is because the light source emits light in all directions. An incoherent light will spread even if the light is directed with a reflector. The picture below demonstrates the behaviour of light waves of an incoherent light. However, a laser does not obey the inverse square law. This is because lasers are coherent beams of light and dimming will not occur in a vacuum as the beam does not spread. Coherent light occurs when all the waves are identical and in phase. Thus, laser light does not diverge much as it travels compared to incoherent lights such as a torch or a spot light. The picture below demonstrates the behaviour of light waves of a coherent light.

Car lights obey the inverse square law because the brightness of the headlights increases when the vehicle gets closer. This happens because light waves tend to spread out as they move away from their source. As a result, intensity decreases quickly as the distance from a light source increases.

If the power of a light source is increased then the light intensity will increase. This is because brightness can tell the power of the light produced. The brightness also changes if the voltage applied to a filament in a light bulb is increased.

The light sources that were used for this experiment were all incoherent lights. The incoherent lights that were used are microscopic, fluorescent and LED light.

Visible light consists of several different colours. The different colours represent different wavelengths of the radiation spectrum. One lumen is equivalent to 1/680 Watt of light with a wavelength of 555 nm. The relationship between illumination and visual response renders the lumen the preferred photometric unit of luminous flux for practical applications. For example, the electric light bulb emits light which consist of many different wavelengths. The visible light waves comprises of a continuous range of wavelengths or frequencies. Numerous of situations can happen when a light wave with a single frequency strikes an object. An object could absorb the light wave. In this case, the energy will be converted to heat. However, the object could reflect the light wave depending on the material of the object. Furthermore, the object could transmit the light waves. The object's light frequency and the nature of the atoms determines the manner in which visible light interacts with the object. Therefore, visible light affects the luminous intensity, thus it might not obey the inverse square law.

## Materials:

Cardboard cylinder tube (80cm)

Light sensor

A ruler

Microscopic Light

LED light

Fluorescent light

Lab quest

Laptop

USB cable

## Real-life situations:

The logger pro was opened by turning on the laptop first.

The USB cable was used to connect the lab quest with the computer.

The light sensor was attached to the end of a ruler using masking tape.

The ruler with the light sensor attached to it was inserted into the cardboard cylinder tube.

The microscopic light was placed in the mouth of the cardboard cylinder tube, facing the light sensor.

The light sensor was connected to the lab quest.

The distance from the light sensor to the microscopic light was measured using the ruler. The distance was recorded into a data table.

The microscopic light was turned on by plugging it into the wall socket.

The light sensor was turned on in order to measure the light intensity of the microscopic light. The data for the light intensity of the microscopic light was saved with an appropriate name onto a USB flash drive. The data was recorded into a table at home.

Step 4, 5 and 6 was repeated three times but with different distances, different light sources such as LED light, and fluorescent light.

## Under laboratory conditions:

Repeat the procedure for the real-life situation but in a dark room.

## Discussion/Analysis/Interpretation:

By looking at Table A in the microscopic light results, it can be seen that the results obeyed the inverse square law. This was because as the distance increases the light intensity of the microscopic light decreases. In the results table, the constant value shows how the light intensity of the microscopic light decreases as the distance increases. Figure 1 and 2 demonstrated and obeyed the inverse square law. Thus, it is evident that the relationship between light intensity varies inversely with the square of the distance in real-life situations and under laboratory conditions. The reason that this is evident is that figure 1 was tested under laboratory condition whilst figure 2 was tested with other sources of lights (real-life situation) such as sunlight. Therefore, the results obtained from the experiment supported the hypothesis which was if the distance increases, the light intensity of the light source will decrease.

The results obtained from the LED light experiment supported the hypothesis. The results showed evidence as the distance increases the light intensity of the LED light decreases. The LED light phase 1 results strongly demonstrate that an incoherent light obeyed the inverse square law under laboratory conditions and real-life situations. As LED lights are incoherent light. Figure 1 and 4 showed that the light intensity varied inversely with the square of the distance. In the results table, the constant value shows how the light intensity of the LED light decreases as the distance increases under laboratory conditions and real-life situations. Therefore, the results obtained from the experiment supported the hypothesis.

Figure 5 and 6 graphs for the fluorescent light clearly demonstrate that it obeyed the inverse square law. It can be seen from the graph as the distance increased the light intensity of the fluorescent light decreased. Furthermore, the fluorescent light was tested in the sunlight and in a dark room. Thus, it is evident that the results for the fluorescent light followed the inverse square law in real-life situations and under laboratory conditions. In the results table, the constant value shows how the light intensity of the fluorescent light decreases as the distance increases. The hypothesis was supported with the findings from this experiment.

The possible error in conducting this experiment was the light source not facing parallel to the cardboard cylinder tube. The effects of this would be not all of the light of the light source will travel to the light sensor, thus altering the result. The reason why this might alter the data is because the light of the light source will spread out and form an arc shape and since it is not facing parallel to the cardboard cylinder tube then some of the lights will not travel to the light sensor, thus the light intensity will decrease even more. Also, incorrect measurements could be recorded for the light intensity of the different light sources observations. Furthermore, another possible error in doing this experiment would be measuring the distance between the light sensor and the light source. This is because uncertainties are inherent in any measuring instrument. There are a range of possibilities for producing error but in order to minimise such incidents, a close watch on the instructions and asking for assistance from teachers will greatly enhance results. All the possible errors mentioned above were avoidable; these possible errors were avoided in our experiment.

The limitations for this experiment cannot be avoided which are:

When light enters a transparent materia, then some of its energy will be dissipated as heat energy. Thus, this will lose some of its intensity. When this absorption of energy occurs, the light of the light source will get transmitted through the material for different wavelengths of the light. This will then show only those wavelengths of light that are not absorbed.  The transmitted wavelengths will then be seen as colour, called the absorption colour of the material.

The grey cardboard cylinder tube reflected almost all of the light that falls on it by the material. This was because all of the light of the light source would be reflected back towards the light source. Thus, this will alter the results obtained from this experiment. However, if the cardboard cylinder tube was black then it will absorb almost all of the light that falls on it into the material. Thus, this will not alter the results obtained from this experiment, since little or no light will be reflected back toward the light source. That light that is absorbed ultimately becomes heat.

The air inside the cardboard cylinder tube must be purified as it might alter the data. This was because dust particles inside the cardboard cylinder tube might reflect the light of the light source. Thus, re-bounding the light in all direction. Therefore, the data of the light intensity will change.

The light source must be incoherent light as incoherent light obeys the inverse square law. The reason for this was because the light source emits light in all directions. Whilst, a coherent light such as lasers does not obey the inverse square law. This was because coherent light occurs when all the waves are identical and in phase. Thus, laser light does not diverge much as it travel compared to incoherent lights such as a torch or a spot light.

The power source can affect the light intensity of the light source such as the microscopic and fluorescent light. This was because the power coming out of the wall socket is not always 240 volts as the power will fluctuate. Whilst, if batteries were used then the light intensity of the light source will not be affected. This was because the power will be always be constant when using batteries. If the power of a light source was increased then the light intensity will increase. This is because brightness can tell the power of the light produced. The brightness also changes if the voltage applied to a filament in a light bulb is increased.

This experiment can be compared to a similar experiment which was done by The University of Queensland (UQ). However, the UQ experiment was measuring the intensity of the radiation instead of the light intensity of the light. The intensity of the radiation obeyed the inverse square law. This was because the results demonstrated that as the object's temperature increases, then it emits most of its light at higher and higher energies. As the source moved further away, the emitted particles were dispersed and therefore the chances of it striking the radiation measurement device will be unlikely. Therefore, the radiation intensity follows the inverse square law as one moves away from the source. This can be implied that the light intensity of a light source will obey the inverse square law given that the light source is non-coherent.

This experiment has the potential to test whether the light intensity of a coherent light varies inversely with the square of the distance under laboratory conditions and in real-life situations. This was because this experiment only tested the light intensity of an incoherent light varies inversely with the square of the distance in real-life situations and under laboratory conditions. If this experiment were to carried on and the results showed that the coherent lights such as laser did not obey the inverse square law, then this will support the theory that only incoherent lights obeys the inverse square law.

## Conclusion:

Light intensity of incoherent lights varies inversely with the square of the distance holds truth in real-life situations and under laboratory conditions.

## Constant value = 0.005961 +/- 0.0002592

Microscopic Light Graphs

LED Light Graphs

Fluorescent Light Graphs