# Light intensity and wave behavior of light

• ViolentCorpse
In summary, the two light bulbs produce twice as much brightness as a single light bulb would. This is because the waves emitted from the two bulbs interfere, and the intensity is proportional to the square of the amplitude. This is only observable for visible light, as the waves are too small for other wavelengths.
ViolentCorpse
Hello PF'ers,

Why is it that two light-bulbs produce twice as much brightness as a single light-bulb would? If we restrict our attention to a single point in space where light from the two sources meet, the waves would cancel half of the time and reinforce half of the time, producing on average no greater brightness than a single bulb working alone, unless the two sources produce all waves exactly in phase all the time, which is highly improbable. The brightness would be constantly fluctuating, but I guess our brains are too slow to observe that (not claiming that this is indeed what actually happens).

I know that coherent light is a prerequisite for interference effects and everyday sources of light aren't coherent, but still light behaves like a wave in most ordinary circumstances so there should be some discernible interference on average at least?

Photons don't interact with each other. There is a huge, messy network of photons in between you and your computer screen going every which way but you only notice the photons that hit your pupil. A photon's wave function can destructively interfere with itself, but not between two photons.

The (very) simplified argument: Intensity is proportional to the square of the amplitude. In your case you would have zero amplitude in the destructive spots and twice the amplitude in the constructive. The average intensity would be (0+2^2)/2 = 2 times that of a single source.

Of course, visible light wavelength would be too small to observe this interference even if you had two bulbs sending out coherent light.

1 person
I believe it boils down to the fact that the light emitted from a lightbulb is not coherent, so the interference is completely random, leading to no observable pattern of fringes. In other words, some waves would interfere constructively, some destructively, and most somewhere in between.

1 person
Orodruin said:
The (very) simplified argument: Intensity is proportional to the square of the amplitude. In your case you would have zero amplitude in the destructive spots and twice the amplitude in the constructive. The average intensity would be (0+2^2)/2 = 2 times that of a single source.

Of course, visible light wavelength would be too small to observe this interference even if you had two bulbs sending out coherent light.
That's a neat, intuitive explanation. Though, I wonder if things wouldn't be the same if there's a single source with no destructive spots at all i.e something like (1+1)^2/2=2?

Thank you so much, Orodruin!

Last edited:

## What is light intensity?

Light intensity can be defined as the amount of energy that is contained within a specific area of light. It is often measured in units such as watts per square meter or lumens.

## How does light intensity affect the behavior of light waves?

The intensity of light can affect the behavior of light waves in several ways. Higher intensity light can cause more energetic and frequent oscillations of the light waves, leading to a greater amplitude and shorter wavelength. It can also cause light to scatter more, leading to changes in color and brightness.

## What factors can affect the intensity of light?

The intensity of light can be affected by several factors, including the source of the light, the distance from the source, and any obstructions or filters that the light passes through. Additionally, the type of light (such as visible, infrared, or ultraviolet) can also impact its intensity.

## How can light intensity be measured?

Light intensity can be measured using a variety of instruments, such as a photometer, spectrometer, or lux meter. These devices measure the amount of light energy at a specific point or over a certain area and can provide a numerical value for the light intensity.

## What is the relationship between light intensity and the speed of light?

The speed of light is not directly affected by its intensity. However, changes in intensity can alter the frequency and wavelength of light, which can impact its speed through different mediums. For example, light travels slower through denser materials, so a higher intensity light may appear to travel slower than a lower intensity light in the same medium.

• Optics
Replies
17
Views
2K
• Optics
Replies
4
Views
3K
• Optics
Replies
8
Views
1K
• Optics
Replies
4
Views
4K
• Optics
Replies
4
Views
1K
• Optics
Replies
1
Views
1K
• Optics
Replies
12
Views
1K
• Optics
Replies
68
Views
12K
• Optics
Replies
3
Views
1K
• Optics
Replies
14
Views
2K