Question about physics of light / light travel

In summary, to calculate the amount of light at your doorstep, you need to take into account the power of the light bulb, the distance between the light and the door, and the angle at which the light falls.
  • #1
switchex
3
0
I have a question which relates to the physics of light. I hope I can explain it well enough for a proper answer!

Let's say there is a 15 feet tall light pole 50 feet from my door step. The light pole has a 300W light bulb. Given these info, is there anyway to calculate the amount of light at my door step? Can I say that the light in front of my doorstep, for example, is equal to a 20W light bulb?

Can anyone please provide a general formula for me to plug in the numbers. Thanks in advance.
 
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  • #2
the distance of the bulb from your doorstep is (225 + 2500)^0.5 feet. the power falls off with the square of the distance. so it would work out to 300/(225 + 2500) watts.
 
  • #3
Thanks very much for the reply. I hate to ask, but can u please explain more.
I am trying very hard to remember my math and physics from 20 years ago but my brain is useless!

How is the distance between the light and door = (225 + 2500)^0.5 .. that would equal to like 7425625! Shouldn't it be a^2 + b^2 = c^2 or 50^2+15^2= 52^2.

So C is equal to about 52 feet which is the distance from lightbulb to the bottom of the door step.

but anyway, are you saying the light infront of the dorr step is equal to about .11 watts? that is 300/(225 + 2500)= .11 watts.









I need to calculate this to prove something to my insurance company!
 
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  • #4
switchex said:
How is the distance between the light and door = (225 + 2500)^0.5 .. that would equal to like 7425625! Shouldn't it be a^2 + b^2 = c^2 or 50^2+15^2= 52^2.


(225 + 2500)^0.5 = 52
 
  • #5
The brightness of a bulb at distance D from the source is equal to:

[tex]B=\frac{L}{4\pi{D^2}}[/tex]

The distance is indeed ~52 foot,

B = 5.8x10^-4 W/square foot.

It may bode you well to convert to metres from feet first though!
 
  • #6
switchex said:
Let's say there is a 15 feet tall light pole 50 feet from my door step. The light pole has a 300W light bulb. Given these info, is there anyway to calculate the amount of light at my door step? Can I say that the light in front of my doorstep, for example, is equal to a 20W light bulb?

The vital information which is missing and nobody seems have asked for is that the light from the 300W bulb at your doorstep is equal to that from a 20W bulb placed where? On the top of the door step? If so, what is the distance from the 20W bulb to the door step?

In both cases, you also need the angle at which the light falls from either bulb on the flat doorstep. The first is given, and the light falls slanted on the step. The second one will be most probably almost directly on the top of the doorstep. (Of course, this I think may not be important, because you are probably estimating the light in the vicinity of the doorstep, not exactly on it.

Another complication is that the percentage of energy converted to light in a 300W bulb is different from that in a 20W bulb. But this can be ignored -- both will be around 4-5%.
 
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  • #7
Shooting Star said:
The vital information which is missing and nobody seems have asked for is that the light from the 300W bulb at your doorstep is equal to that from a 20W bulb placed where? On the top of the door step? If so, what is the distance from the 20W bulb to the door step?

i think what was meant in the OP was how he can compare a 300W bulb placed 52ft away with a 20W bulb placed 'x' feet away. so if the power received at doorstep by a 300W bulb is 0.11W then for an equivalent power to be received by a 20W bulb at the doorstep it would have to be placed (20/0.11)^0.5 feet away. works out to roughly 13.5 feet.

:uhh: now that i look at the numbers, they somehow don't seem right..
 

1. How does light travel through space?

Light travels through space in the form of electromagnetic waves. These waves do not require a medium to propagate, unlike sound waves which need a medium such as air. This is because the electric and magnetic fields in light can sustain each other, allowing it to travel through a vacuum.

2. What is the speed of light?

The speed of light in a vacuum is approximately 299,792,458 meters per second, or about 670 million miles per hour. This is the fastest speed possible in the universe and is denoted by the letter "c" in physics equations.

3. How does light interact with matter?

Light can interact with matter in three main ways: reflection, absorption, and transmission. Reflection occurs when light bounces off a surface, like a mirror. Absorption is when light is absorbed by matter, converting its energy into heat. Transmission is when light passes through a material, such as glass.

4. What is the difference between visible light and other types of electromagnetic radiation?

Visible light is just one small portion of the electromagnetic spectrum, which also includes radio waves, microwaves, infrared, ultraviolet, X-rays, and gamma rays. The main difference between these types of radiation is their wavelength and frequency, which determine their properties and how they interact with matter.

5. How does light behave as both a particle and a wave?

In the field of quantum mechanics, light is described as having both particle-like and wave-like properties. This is known as wave-particle duality. The double-slit experiment demonstrates this phenomenon, where light behaves like a wave when passing through two slits, but like a particle when measured at the screen behind the slits.

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