<p>Yes, visible light warms the earth.</p>
<p>Comprehensive explanations are in this cross-post to Physics Stack Exchange <a href="https://physics.stackexchange.com/questions/723331/why-do-dark-objects-absorb-visible-light-but-only-emit-infrared-radiation">Why do dark objects absorb physical light but only emit infrared radiation?</a></p>
<p>Briefly, when a visible light photon strikes a dark object, an electron will be excited. While the electron could immediately re-emit the photon, the molecular structure is such that it is much more likely for the electron to move into one of many other states and share the energy around other molecules in the material, thus increasing the temperature of the material. As the energy is spread around, when photons are emitted, they are more likely to have low energy (infrared).</p>

CC BY-SA 4.0
2022-08-18T06:57:25.577Z
Correct, this is called the Wien's displacement law: the colder an object the more it's maximum electromagnetic emission is shifted to infrared - the hotter the more the maximum will be shifted to shortwave radiation (blue).
squarespiral
CC BY-SA 4.0
2022-08-18T07:05:55.110Z
Thank you @squarespiral
Peter A
CC BY-SA 4.0
2022-08-18T18:34:46.103Z
I know you answered your own question, but I don't think this answers the question, "Does visible light warm the earth?"
Matt Hall
CC BY-SA 4.0
2022-08-18T19:17:21.450Z
Thanks, have edited the answer to make it much clearer
Peter A
CC BY-SA 4.0
2022-08-17T09:17:05.617Z
2022-08-18T19:17:02.517Z
2022-08-18T19:17:02.517Z
Peter A
<p><strong>The short answer is Yes, visible light does warm the earth.</strong> Then, the earth's surface <em>mostly</em> emits infrared radiation at normal temperatures. You got <a href="https://physics.stackexchange.com/a/723420/113377">a nice answer</a> about this aspect on the other thread.</p>
<p>It's not correct that &quot;most radiation reaching us from the sun is visible light&quot;. For example, <a href="https://ehs.lbl.gov/resource/documents/radiation-protection/non-ionizing-radiation/light-and-infrared-radiation/" rel="nofollow noreferrer">this resource from Lawrence Berkeley National Laboratory</a> has this statistic:</p>
<blockquote>
<p>It is estimated that more than half (54%) of the sun’s radiation is infrared, whereas 39% is the visible rays and the remaining 7% is the ultraviolet rays.</p>
</blockquote>
<p>This seems reasonable, given the spectrum here from <a href="https://en.wikipedia.org/wiki/Sunlight" rel="nofollow noreferrer">the Sunlight article</a> on Wikipedia. It's partly just the bandwidth of IR radiation compared to visible light:</p>
<p><img src="https://i.sstatic.net/64DfR.png" alt="CC BY-SA Robert A Rohde" /></p>
<p>To get back to your question, Earth's albedo (the amount of radiation reflected from the surface back into space) is about 0.3 on average, but it varies locally and depends on the wavelength of the radiation, per <a href="https://www.researchgate.net/figure/Spectral-and-effective-dashed-lines-albedo-of-several-widely-used-surfaces-The-ideal_fig1_320295066" rel="nofollow noreferrer">this figure</a>:</p>
<p><img src="https://i.sstatic.net/VzI9I.png" alt="The wavelength dependence of albedo" /></p>
<p>The 70% of radiation that is absorbed raises the temperature of the material (in general, absorbing radiation of any wavelength will). As the physicists explained, most radiation from bodies at about 20 deg Celsius is in the infrared band.</p>
<p>I hope I understood what you were asking. Be careful with sentences like this:</p>
<blockquote>
<p>why can the oceans [...] only radiate heat away as infrared?</p>
</blockquote>
<p>This begs the question, because to emit infrared radiation is to &quot;radiate heat&quot;: it's the same thing. In other words, you can't radiate heat away in any other electromagnetic band.</p>

CC BY-SA 4.0
2022-08-18T18:31:42.583Z
2022-08-18T18:31:42.583Z
Matt Hall
<p>I read that most radiation reaching us from the sun is visible light. Yet, heat can only be radiated away from earth as infrared.</p>
<p>Is this correct? If so, why can the oceans (say) absorb heat from visible light but only radiate heat away as infrared?</p>
<p>(I am excluding reflection for the purpose of this question. The light must be absorbed, warming the molecules to count.)</p>

CC BY-SA 4.0
2022-08-14T11:20:53.127Z
Yes, visible light warms the Earth: the sun is our primary source of heat. Quoting https://en.wikipedia.org/wiki/Black-body_radiation `A black body at room temperature radiates mostly in the infrared spectrum, which cannot be perceived by the human eye [...] As the object increases in temperature to about 500 °C (773 K; 932 °F), the emission spectrum gets stronger and extends into the human visual range, and the object appears dull red. As its temperature increases further, it emits more and more orange, yellow, green, and blue light (and ultimately beyond violet, ultraviolet).`
Barry Carter
CC BY-SA 4.0
2022-08-14T13:17:18.610Z
@BarryCarter You missed the point of the question.
Spencer
CC BY-SA 4.0
2022-08-14T13:20:29.080Z
@Spencer I was explaining why the heat Earth generates is infrared (lower temperature) while the heat the Sun generates is visible (higher temperature), which I believe was the question in the 2nd paragraph.
Barry Carter
CC BY-SA 4.0
2022-08-14T14:51:25.183Z
Thank you [@barry carter](https://earthscience.stackexchange.com/users/26103/barry-carter) [@Spencer](https://earthscience.stackexchange.com/users/6467/spencer) . I have edited the question to clarify. Indeed it is a physics question. Why can molecules absorb visible but only emit infrared
Peter A
CC BY-SA 4.0
2022-08-16T14:49:18.657Z
Cross posted to [here](https://physics.stackexchange.com/questions/723331/why-do-dark-objects-absorb-visible-light-but-only-emit-infrared-radiation) to Physics StackExchange
Peter A
<climate-change><greenhouse-gases>
Does visible light warm the earth?
Peter A
# Does visible light warm the earth?

(Asked by: username_0 on 2022-08-14)

I read that most radiation reaching us from the sun is visible light. Yet, heat can only be radiated away from earth as infrared.

Is this correct? If so, why can the oceans (say) absorb heat from visible light but only radiate heat away as infrared?

(I am excluding reflection for the purpose of this question. The light must be absorbed, warming the molecules to count.)

**Tags:** climate-change, greenhouse-gases

> **Comments:**
> **username_1**: Yes, visible light warms the Earth: the sun is our primary source of heat. Quoting https://en.wikipedia.org/wiki/Black-body_radiation `A black body at room temperature radiates mostly in the infrared spectrum, which cannot be perceived by the human eye [...] As the object increases in temperature to about 500 °C (773 K; 932 °F), the emission spectrum gets stronger and extends into the human visual range, and the object appears dull red. As its temperature increases further, it emits more and more orange, yellow, green, and blue light (and ultimately beyond violet, ultraviolet).`  
> **username_2**: @username_1 You missed the point of the question.  
> **username_1**: @username_2 I was explaining why the heat Earth generates is infrared (lower temperature) while the heat the Sun generates is visible (higher temperature), which I believe was the question in the 2nd paragraph.  
> **username_0**: Thank you [@username_1 username_1](https://earthscience.stackexchange.com/users/26103/username_1-username_1) [@username_2](https://earthscience.stackexchange.com/users/6467/username_2) . I have edited the question to clarify. Indeed it is a physics question. Why can molecules absorb visible but only emit infrared  
> **username_0**: Cross posted to [here](https://physics.stackexchange.com/questions/723331/why-do-dark-objects-absorb-visible-light-but-only-emit-infrared-radiation) to Physics StackExchange  

---

## 2 answers

### Answer by username_0 on 2022-08-17. Score: 1 | Accepted answer

Yes, visible light warms the earth.

Comprehensive explanations are in this cross-post to Physics Stack Exchange [Why do dark objects absorb physical light but only emit infrared radiation?](https://physics.stackexchange.com/questions/723331/why-do-dark-objects-absorb-visible-light-but-only-emit-infrared-radiation)

Briefly, when a visible light photon strikes a dark object, an electron will be excited. While the electron could immediately re-emit the photon, the molecular structure is such that it is much more likely for the electron to move into one of many other states and share the energy around other molecules in the material, thus increasing the temperature of the material. As the energy is spread around, when photons are emitted, they are more likely to have low energy (infrared).

> **Comments:**
> **username_3**: Correct, this is called the Wien's displacement law: the colder an object the more it's maximum electromagnetic emission is shifted to infrared - the hotter the more the maximum will be shifted to shortwave radiation (blue).  
> **username_0**: Thank you @username_3  
> **username_4**: I know you answered your own question, but I don't think this answers the question, "Does visible light warm the earth?"  
> **username_0**: Thanks, have edited the answer to make it much clearer  

---

### Answer by username_4 on 2022-08-18. Score: 1

**The short answer is Yes, visible light does warm the earth.** Then, the earth's surface *mostly* emits infrared radiation at normal temperatures. You got [a nice answer](https://physics.stackexchange.com/a/723420/113377) about this aspect on the other thread.

It's not correct that "most radiation reaching us from the sun is visible light". For example, [this resource from Lawrence Berkeley National Laboratory](https://ehs.lbl.gov/resource/documents/radiation-protection/non-ionizing-radiation/light-and-infrared-radiation/) has this statistic:

> It is estimated that more than half (54%) of the sun’s radiation is infrared, whereas 39% is the visible rays and the remaining 7% is the ultraviolet rays.

This seems reasonable, given the spectrum here from [the Sunlight article](https://en.wikipedia.org/wiki/Sunlight) on Wikipedia. It's partly just the bandwidth of IR radiation compared to visible light:

![CC BY-SA Robert A Rohde](https://i.sstatic.net/64DfR.png)

To get back to your question, Earth's albedo (the amount of radiation reflected from the surface back into space) is about 0.3 on average, but it varies locally and depends on the wavelength of the radiation, per [this figure](https://www.researchgate.net/figure/Spectral-and-effective-dashed-lines-albedo-of-several-widely-used-surfaces-The-ideal_fig1_320295066):

![The wavelength dependence of albedo](https://i.sstatic.net/VzI9I.png)

The 70% of radiation that is absorbed raises the temperature of the material (in general, absorbing radiation of any wavelength will). As the physicists explained, most radiation from bodies at about 20 deg Celsius is in the infrared band.

I hope I understood what you were asking. Be careful with sentences like this:

> why can the oceans [...] only radiate heat away as infrared?

This begs the question, because to emit infrared radiation is to "radiate heat": it's the same thing. In other words, you can't radiate heat away in any other electromagnetic band.


---

