Light Intensity Of Incoherent Lights Essay

2381 words - 10 pages


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.

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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:

S1 = (πr12)

S2 = (πr22)

I1 =

= I / ( (πr12))

I2 =

= I / ( (πr22))

= (I / ( (πr12))) / (I / ( (πr22)))


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

Light bulbs and any light sources that employ 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 beam 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 as the vehicle gets closer, the brightness of the headlights increases rapidly. 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 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.

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 consists of a continuous range of wavelengths or frequencies. There are numerous of situations that could 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...

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