What type of spectrum is a rainbow?

[Likith D]

maybe it is emission spectrum of the sun but, seems continuous... unlike few distinct lines of the hydrogen emission spectrum...few images i just checked about it are similar to absorption spectrum...then maybe difraction could have made emission spectrum seem so or maybe not...
Well I am very confused about it... also include why it is so in the answer
Anddd... also why the sun seems white during noon and red during sunset ( I am not sure if these two questions are linked to each other but i wanted them answered )

 

[dukwon]

It's a result of refraction and reflection in spheres of water (raindrops). The index of refraction depends on wavelength, so different colours are refracted through different angles.

http://apollo.lsc.vsc.edu/classes/met130/notes/chapter19/indrop.html

 

[Likith D]

sooo... what kind of spectrum IS sunlight... emission absorption or continuous?

 

[DrClaude]

sooo... what kind of spectrum IS sunlight... emission absorption or continuous?

It is mostly a continuous spectrum (black-body radiation), but there are some frequencies missing due to absorption by atoms in the Sun's atmosphere (this is how helium was discovered).

 

[Likith D]

It is mostly a continuous spectrum (black-body radiation), but there are some frequencies missing due to absorption by atoms in the Sun's atmosphere (this is how helium was discovered).

Why is sunlight "mostly continuous spectrum" ? if it IS continuous spectrum, why is that so...
i see that you have said "black body radiation" for that but i can't draw any inference from that, so an explanation would be much more convincing... which is a black body in this case...the sun?
orrrr maybe... is it so that nuclear reactions give a continuous spectrum...

 

[Dale]

i see that you have said "black body radiation" for that but i can't draw any inference from that

Here is a description of blackbody radiation. https://en.m.wikipedia.org/wiki/Black-body_radiation

It is a continuous spectrum and a good first approximation to a stellar spectrum.

 

[Likith D]

Here is a description of blackbody radiation. https://en.m.wikipedia.org/wiki/Black-body_radiation

It is a continuous spectrum and a good first approximation to a stellar spectrum.

SO...why is the sun "mostly continuous spectrum" ?

 

[DrClaude]

SO...why is the sun "mostly continuous spectrum" ?

Because it's a hot ball of plasma.

 

[Likith D]

Because it's a hot ball of plasma.

so hydrogen in plasma state gives out continuous spectra ? but light from hydrogen plasma is emission spectrum, right? and what about the nuclear reactions of the sun ( maybe... i think they emit light too... ) ?

 

[Dale]

SO...why is the sun "mostly continuous spectrum" ?

Did you read the article? Especially the first paragraph in the section "Spectrum"

 

[Likith D]

Did you read the article? Especially the first paragraph in the section "Spectrum"

OOps my bad... so i read it since you specified the part about the sun...my doubts are all clear thanks a lot

 

[mfb]

As you mentioned that topic: the nuclear reactions have nothing to do with the sun spectrum. They happen in the core and keep it hot, but no direct radiation from there (apart from neutrinos - irrelevant here) ever reaches the surface.

 

[Khashishi]

SO...why is the sun "mostly continuous spectrum" ?

Because it is large and optically thick. Any photon emitted from inside the Sun will collide many, many times before it escapes from the Sun. IIRC, it can take millions of years for a photon from the core of the Sun to bounce around and randomly make its way to the edge. In this time, it has collided many times with electrons or protons or groups of particles. Each collision can shift the energy of the photon by a random amount. So a photon coming out can have just about any possible energy. If you plot it on a histogram, it looks like a continuum.

 

[Cutter Ketch]

Because it is large and optically thick. Any photon emitted from inside the Sun will collide many, many times before it escapes from the Sun.

Yes photons emitted in the core may take millions of years etc. etc. however the implication that any significant fraction of what we see is this deep radiation is wrong. Almost none of that ever does reach the surface. Almost all of the sun light we see is produced in a 100km shell of the sun called the photosphere. There is a thinner atmosphere above this that is cool enough to give the Hydrogen and Helium absorption lines. However at > 5000C the photosphere is a hot plasma and the free charges produce light in something close to a black body spectrum.

I think this is the answer to the OPs question. Given that the sun is made of hydrogen how does the photosphere produce anything but the hydrogen spectrum? Answer: its too hot for the charge to be bound in Hydrogen atoms and the plasma can make a continuous spectrum. Similarly for any black body, having the ability to interact over the full spectrum is implicit in its being black. If it can't interact with the whole spectrum it isn't black.

 

[mfb]

Answer: its too hot for the charge to be bound in Hydrogen atoms

This is a common misconception. 6000 K correspond to just ~1/2 eV, while the electrons are bound at 13.6 eV, and the density is reasonable for recombinations to happen. Most hydrogen (~99.99%) at the solar surface is neutral in the ground state - and transparent to nearly the whole solar spectrum. A small fraction of hydrogen is in ionized states. That still does not lead to emission or absorption lines because both processes are in equilibrium and the radiation mainly comes from the small fraction of ionized hydrogen.
The absorption lines are produced in layers where the fraction of hydrogen atoms not in the ground state is even smaller.

More discussion here and also here

 

[Cutter Ketch]

This is a common misconception. 6000 K correspond to just ~1/2 eV, while the electrons are bound at 13.6 eV, and the density is reasonable for recombinations to happen. Most hydrogen (~99.99%) at the solar surface is neutral in the ground state - and transparent to nearly the whole solar spectrum. A small fraction of hydrogen is in ionized states. That still does not lead to emission or absorption lines because both processes are in equilibrium and the radiation mainly comes from the small fraction of ionized hydrogen.
The absorption lines are produced in layers where the fraction of hydrogen atoms not in the ground state is even smaller.

More discussion here and also here

Oops. My bad. I didn't think hard enough about that obviously. Yes, ok. So you are saying enough of the hydrogen is ionized that the light is dominated by the plasma. Got it.

 

[Likith D]

So why can't we see the missing colors of helium absorption spectrum (emitted by the sun) in a rainbow ?

 

[Drakkith]

So why can't we see the missing colors of helium absorption spectrum (emitted by the sun) in a rainbow ?

The absorption lines are too narrow to be seen by the human eye.

 

[Cutter Ketch]

The absorption lines are too narrow to be seen by the human eye.

... but they are there. Search "Fraunhofer lines" or "Fraunhofer spectrum"

 

[davenn]

... but they are there. Search "Fraunhofer lines" or "Fraunhofer spectrum"

he didn't say they weren't.
They just cannot be easily seen without visual aide. For the casual observation, the spectrum looks continuous

I'm VERY sure Drakkith knows what Fraunhofer lines are Dave

 

[Cutter Ketch]

he didn't say they weren't.
They just cannot be easily seen without visual aide. For the casual observation, the spectrum looks continuous

I'm VERY sure Drakkith knows what Fraunhofer lines are Dave

I wasn't posting to Drakkith. I was completing his thought ( thus the ellipsis) and directing Lilith D who clearly doesn't know about the absorption lines in the solar spectrum where he can search to learn more. ("wink")

 

[davenn]

I wasn't posting to Drakkith. I was completing his thought ( thus the ellipsis) and directing Lilith D who clearly doesn't know about the absorption lines in the solar spectrum where he can search to learn more. ("wink")

that's why it's an awesome idea to quote people, so there is no misunderstanding

 

[JeffJo]

What type of spectrum is a rainbow?

It isn't any type of spectrum. Not if you take the word "spectrum" to mean a representation of the wavelengths of light where each theoretical wavelength appears at a unique position in a continuum.

maybe it is emission spectrum of the sun but, seems continuous... unlike few distinct lines of the hydrogen emission spectrum

The rainbow is continuous, even if a true spectrum produced from sunlight is not.

Raindrops reflect light, of wavelength λ, thru all deflection angles (measured relative to the line going directly back to the sun) from 0° to r(λ), often called the rainbow angle. The rainbow angle varies continuously between about r(λ)=40.6° for violet light, and r(λ)=42.3° for red light.

If the angle of incidence when sunlight hits a raindrop is A, and B(A) is the corresponding angle of refraction found by Snell's Law, then light that reflects one time internally is deflected thru the angle D(A)=4B(A)-2A. The rainbow angle is the maximum of this function. But since the intensity of the light at any deflection angle D(A) is inversely proportional to D'(A), each color becomes very bright near this maximum. As a result, we perceive different colors at different positions in an arc between 40.6° and 42.3° (the width of the sun, about 0.5°, also plays a part in what colors are perceived).

The point is that every position in the continuous rainbow consists of light of all visible wavelengths in the sun's spectrum, from the color you perceive to the red end of its spectrum. So even if your eye could detect different bands in a true spectrum, the rainbow itself is still continuous.