The beginners guide to light!

A

amber

Registered Member
Hello all, I am not sure where to post this topic so if the moderators feel it is in the wrong section, please relocate to the appropriate section.


Light, a generalised term we use for electromagnetic radiation. Light can be classed in two categories:

1)Visible light

This is light you can see with your eyes that has a wave-length between 400nm-700nm. More simply the colours you see.
(Do not worry I will explain wave-lengths later)

2)Detectable light

This is light that can be detected by device that our eyes cannot see, such as the infra-red range of light that is beyond 700nm.

In understanding light, I feel it is firstly important to understand what we call the inverse square law.

The inverse-square law, in physics, is any physical law stating that a specified physical quantity or intensity is inversely proportional to the square of the distance from the source of that physical quantity. The fundamental cause for this can be understood as geometric dilution corresponding to point-source radiation into three-dimensional space
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You can read more here: https://en.wikipedia.org/wiki/Inverse-square_law


What this means in simple terms , is that the further away an observer is from a light source, the weaker the light becomes, it becomes more stretched and a longer wave-length.
In science terms we would state there is less intensity and magnitude of the light.

In the link I provided you will notice a diagram on the right of the page that shows you in pictorial form how the inverse square law works.

Earlier on I said I would explain wave-length and this I shall do now. At the narrow end of the diagram the light has a shorter wave-length, the light being much more compressed in less space. At the wider end of the diagram the light will have a longer wave-length being stretched out to cover more space.
In colours, just remember purple is the shortest visible wave-length and red is the longest visible wave-length,
the colours of the rainbow filling between.

Any questions about this?
 
amber said:
Any questions about this?
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Is there any particular colour that humans generally prefer over another colour and is that preference related to wave length.
No need to answer I know all about that aspect.

Nice OP to start off.

I like light it is an intetesting subject...do you cover optics...I like optics...cause I like astronomy...and I like stars and galaxies and nebula and double stars but not movie stars.

Alex
 
Xelasnave.1947 said:
Is there any particular colour that humans generally prefer over another colour and is that preference related to wave length.
No need to answer I know all about that aspect.

Nice OP to start off.

I like light it is an intetesting subject...do you cover optics...I like optics...cause I like astronomy...and I like stars and galaxies and nebula and double stars but not movie stars.

Alex
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I try to cover most aspects of light, the incident rays of distant galaxies do interest me. You claim to know your science but I obviously do not know you, so I have a question for you to see what sort of intelligent answer you give.


The Universe is expanding, do you consider this is an observation expansion or an expansion of space itself?
 
amber said:
You claim to know your science
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No I am a layman and maybe I exagerated about knowing everything.
amber said:
The Universe is expanding, do you consider this is an observation expansion or an expansion of space itself?
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If this is a test I would answer that it is space that is expanding according to the current model and various physical observations have been provided in support.
But I dont understand red shift have you heard about it...apparently by using the princliples of red shift we can determine an objects distance and the rate the Universe is expanding.

However the great man himself saw the universe as static, he worked out his formulae with that pre condition in his thinking, he included a cosmological constant to hold the static universe from colapse but then Hubble says its expanding and he had a big telescope. But what if the great man was right.
The universe wad is static and not expanding...then they use his equations to prove the universe must be expanding...he had to say he was wrong..his gteatest blunder...but was he right and its static and rather than his greatest mistake the cosmological constant was his best idea...how do we know Hubble was right..he was a lawyer should we trust his astronomy?
Alex
 
Xelasnave.1947 said:
No I am a layman and maybe I exagerated about knowing everything.

If this is a test I would answer that it is space that is expanding according to the current model and various physical observations have been provided in support.
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Have you ever read Hubble's law before you make an assumption?

Hubble's law
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is the name for the observation in physical cosmology that:

  1. Objects observed in deep space - extragalactic space, 10 megaparsecs (Mpc) or more - are found to have a red shift, interpreted as a relative velocity away from Earth;
  2. This Doppler shift-measured velocity, of various galaxies receding from the Earth, is approximately proportional to their distance from the Earth for galaxies up to a few hundred megaparsecs away.[1][2]
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https://en.wikipedia.org/wiki/Hubble's_law



Notice one thing of the above, it states the red-shift observed is of objects observed and not of the space observed. We do not observe space is red-shifting and neither do we observe space interfering with the light passing through it. We neither observe space to be object like.


So what basis do you have to consider space is expanding?
 
amber said:
What this means in simple terms , is that the further away an observer is from a light source, the weaker the light becomes, it becomes more stretched and a longer wave-length.
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This is not what the inverse square law is stating. The inverse square law is simply a consequence of geometry. A point source that is emanating 'something' in all directions will follow the inverse square law. It could be a sound source or even a explosion throwing our shrapnel. The number of pieces per unit area would decrease with a inverse square relationship.

The inverse square law has nothing to do with wavelength decrease. I think you are confusing Hubble's law with the inverse square law.
 
amber said:
The same as a longer wave-length.
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No that is not correct.
The wavelength will not change. Go back and read your link no where in the article will there be any claim that the wavelength changes. You can look at it like the number of light waves per unit area decreases with the inverse square of distance NOT the wavelength itself. It is easier to visualize a light source emitting photons instead of waves for the inverse square relationshtp.

If you disagree with this, please support that contention with a link.
 
amber said:
My edit would be

What this means in simple terms , is that the further away an observer is from a light source, the weaker the light becomes.
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Yes, but to accurate and avoid confusion we would say that the intensity of the light decreased. A very simple experiment is to look at a green light from 10 feet away and then look at it from 2o feet away, the light will be 1/4 intense but the light will still be green (the same wavelength).
 
origin said:
Yes, but to accurate and avoid confusion we would say that the intensity of the light decreased. .
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I did state that in my opening post

What this means in simple terms , is that the further away an observer is from a light source, the weaker the light becomes, it becomes more stretched and a longer wave-length.
In science terms we would state there is less intensity and magnitude of the light.
Click to expand...

With edit is now reads

What this means in simple terms , is that the further away an observer is from a light source, the weaker the light becomes.
In science terms we would state there is less intensity and magnitude of the light.
 
amber said:
In science terms we would state there is less intensity and magnitude of the light.
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The language of science is very precise. With out using the correct terms you cannot effectively convey the proper meaning. So your definition above is not correct because you introduced the term magnitude.
The term magnitude as it relates to light is about the apparent brightness of a celestial object when observed with the naked eye.

Just stick with the proper term - intensity.

Edit to add: If you really want to put it in common language you could say the brightness decreases as the inverse square of distance.
 
origin said:
The language of science is very precise. With out using the correct terms you cannot effectively convey the proper meaning. So your definition above is not correct because you introduced the term magnitude.
The term magnitude as it relates to light is about the apparent brightness of a celestial object when observed with the naked eye.

Just stick with the proper term - intensity.

Edit to add: If you really want to put it in common language you could say the brightness decreases as the inverse square of distance.
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I thought magnitude meant strength, I stand corrected .
 
amber said:
What makes visible light , visible, compared to detectable light?
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Electromagnetic radiation is an entire range of frequencies of photons. Xrays, gamma rays, radio waves, microwaves and visible light are all electromagnetic radiation. What we call visible light is just the frequency of electromagnetic radiation that the rods and cones in our eyes can detect.
 
amber said:
What makes visible light , visible, compared to detectable light?

Is visible light a product or process of electromagnetic radiation and substance interaction?
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There are two questions here.

Visible light -indeed any light - is the product of an emission process from matter that generates radiation. There are two of these processes: spontaneous and stimulated. Spontaneous emission does not require radiation to be present - it is just given off by matter in an excited state. Stimulated emission is radiation given from matter in an excited state when pre-existing radiation interacts with it. A famous example of this is the laser.

However your first question is what makes certain wavelengths of light visible to us. That is the result of the electrochemical signal produced, when light in the range of wavelengths our retinas are sensitive to strikes it. The sensitivity range is determined by the photochemistry of the retina, specifically the "photopigments" present in the light detection cells of the retina.

So this is to do, not with how light is emitted but how it absorbed, in the eye. I do not know exactly what happens to these photopigments after they absorb a photon of the right wavelength, but I imagine they will produce an excited state of the pigment molecule, which will then release its energy, somehow, in a chemical form that is ultimately converted to a signal in the optic nerve. (I am not a biochemist.)
 
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amber said:
I thought magnitude meant strength, I stand corrected .
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It can used that way, in certain circumstances( though it really means size or extent), but in astronomy it refers to a particular type of scale used to categorize celestial objects. It is a logarithmic scale where the larger the magnitude, the dimmer the star, and negative magnitudes are brighter than positive ones. It goes back 2000 years to a Greek astronomer, Hipparchus, Who classified stars by their apparent brightness or size(bright stars look larger to the naked eye. The brightest stars were "First magnitude" stars. then you had second magnitude stars, etc. Later astronomers refined the system, expanding is range and accuracy( under the newer system, Sirius, the brightest star in the sky has a visual magnitude of -1.46). This is called visual or apparent magnitude.
They also added a new classification called absolute magnitude which compensates for the varying distance to these objects. It is based on what the visual magnitude of the objects would if they were all at an equal distance. Under this system, Sirius, with an apparent magnitude of -1.46 has an absolute one of 1.42, and Betelgeuse, with an apparent magnitude of 0.5, has an absolute magnitude of -5.85.
 
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