William Herschel's light experiment

[sevensages]

TL;DR Summary

Why did the thermometer not in the sunlight absorb more infrared light than the thermometers in sunlight?

In Cosmos: A Spacetime Odyssey episode 5 Hiding in the Light, Neil Degrasse Tyson describes one of William Herschel's experiments with light in the year 1800. The episode shows Herschel's experiment as a cartoon. Herschel put two thermometers under a prism that was in the sunlight. Herschel put the first thermometer was in blue light. He put the second thermometer in red light. The third thermometer was not under the sunlight at all. The third thermometer was the control. At the beginning of the experiment, all three thermometers were 52 degrees fahrenheit. Three and a half hours later the thermometer under the blue light was still 52 degrees fahrenheit The thermometer under the red light had warmed to 56 degrees fahrenheit, which proved that red light holds more heat than blue light. The third thermometer was 58 degrees fahrenheit. Herschel starts shaking and examining the third thermometer to see if there is something wrong with it. Tyson says to Herschel (in the cartoon): "No, there's nothing wrong with your thermometer. You just discovered a new type of light."

Tyson then goes on to describe how our eyes are not sensitive to infrared light, but our skin feels infrared light as heat.

Why didn't the first and second thermometers absorb this infrared light as well?

Why did only the third thermometer absorb the infrared light?

 

[PeroK]

Why didn't the first and second thermometers absorb this infrared light as well?

Why did only the third thermometer absorb the infrared light?

Because the prism split the different light wavelengths into different paths.

 

[Ibix]

TL;DR Summary: Why did the thermometer not in the sunlight absorb more infrared light than the thermometers in sunlight?

Why did only the third thermometer absorb the infrared light?

Same reason there's only blue light in some places and only green light in others - because the prism bends light of different wavelengths by different amounts. All the infrared was passing through the "dark" (to our vision) region beside the red edge of the spectrum.

 

[sevensages]

Because the prism split the different light wavelengths into different paths.

But the third thermometer was not in the sunlight at all

 

[Ibix]

The third thermometer was not in the sunlight at all.

You mean, not in the visible sunlight. IR would be passing through the (apparently) dark region beside the visible spectrum.

 

[sevensages]

You mean, not in the visible sunlight. IR would be passing through the (apparently) dark region beside the visible spectrum.

Ok I just re-watched this part of Cosmos again. I think I have figured it out.

What made me figure this out is Tyson explains that infrared light is called infrared light because infra is Latin for "under" So infrared light means "under the RED light".

Cosmos depicts Herschel conducting his experiment with all three thermometers placed right next to each other on a small piece of paper. The second thermometer was only about an inch or two from the first thermometer. The third thermometer was only about an inch or two from the second thermometer. I think Herschel intended for the third thermometer to not be in the sunlight at all.

The second thermometer was the thermometer in the red light.

Is the reason that Herschel detected the infrared light with the third thermometer that the third thermometer was right next to the second thermometer?

It's now my understanding that if Herschel had placed the third thermometer, say, six feet away from the second thermometer, then Herschel would not have detected the infrared light at all.

Furthermore, if Hershel put the third thermometer right next to the BLUE light (as opposed to right next to the red light) , Herschel also would NOT have detected the infrared light at all.

Is my understanding on BOTH of these points correct ?

P.S. these are not rhetorical questions. I am still not 100% sure I fully understand this.

 

[Ibix]

Is my understanding on BOTH of these points correct ?

Yes. Not knowing about IR you would want the control thermometer as close to the experiment as possible, so it was measuring (as near as possible) in the exact same place under the exact same circumstances except for the light. We've all been in rooms where it's cold in one corner and warm in another, and we don't want that kind of thing to interfere, which is why he went as close as possible. Unfortunately (or interestingly) there was a type of electromagnetic wave passing through the region Herschel picked that the thermometer was sensitive to.

 

[PeroK]

Is my understanding on BOTH of these points correct ?

The refracted spectrum that we see running from red to violet is actually wider than that. Infrared (IR) is refracted less than red and UV is refracted more than violet. The hottest thermomenter must have been close enough to be exposed to the invisible IR part of the spectrum.

 

[sevensages]

The refracted spectrum that we see running from red to violet is actually wider than that. Infrared (IR) is refracted less than red and UV is refracted more than violet. The hottest thermomenter must have been close enough to be exposed to the invisible IR part of the spectrum.

It's not just about proximity though. It's the proximity of the third thermometer in relation to the other thermometers.

 

[Ibix]

It might be possible to see this for yourself if you have a prism and sun where you are. Your phone camera can see IR, although not very well. If you can generate a spectrum and mark the edges on a piece of paper, then take a snap with your phone camera, you may find that the spectrum it detects is a bit wider than the marks you made by eye.

I don't know if that'll work. You'll definitely need a bright sunny day for it to even be a possibility, so I can't try it for a few months.

 

[PeroK]

It's not just about proximity though. It's the proximity of the third thermometer in relation to the other thermometers.

You must have seen this:

 

[sevensages]

Yes. Not knowing about IR you would want the control thermometer as close to the experiment as possible, so it was measuring (as near as possible) in the exact same place under the exact same circumstances except for the light. We've all been in rooms where it's cold in one corner and warm in another, and we don't want that kind of thing to interfere, which is why he went as close as possible. Unfortunately (or interestingly) there was a type of electromagnetic wave passing through the region Herschel picked that the thermometer was sensitive to.

Thank you for answering this.

Yes, you would want the control thermometer as close as possible to the other two thermometers. But isn't it just serendipity (and nothing more) that the control thermometer was right next to the red light instead of right next to the blue light?

I still think it's partly happenstance that Herschel discovered infrared light with this experiment because to my knowledge, he could have just put the control thermometer next to the blue light thermometer.

 

[sevensages]

You must have seen this:

View attachment 321553

The infrared is right next to the red. I think if Herschel placed the control thermometer right next to the first thermometer in the blue light, he would not have discovered infrared light.

 

[PeroK]

I still think it's partly happenstance that Herschel discovered infrared light with this experiment because to my knowledge, he could have just put the control thermometer next to the blue light thermometer.

That's the history of scientific breakthroughs: from Herschel to Becquerel to Fleming. Luck, fortune, happenstance, serendipity. Call it what you will!

 

[sevensages]

That's the history of scientific breakthroughs: from Herschel to Becquerel to Fleming. Luck, fortune, happenstance, serendipity. Call it what you will!

Yeah. It was just a 50% chance that Herschel discovered infrared light with this experiment .

 

[Ibix]

But isn't it just serendipity (and nothing more) that the control thermometer was right next to the red light instead of right next to the blue light?

Yes.

I still think it's partly happenstance that Herschel discovered infrared light with this experiment because to my knowledge, he could have just put the control thermometer next to the blue light thermometer.

Yes. This is very common. For example, Roentgen noticed that his photographic plates were ruined when he left them near some rocks he was studying. He deduced that the rocks were emitting something he named "x rays".

My physics teacher commented that the difference between him and Roentgen is that he'd have sworn and thrown away the plates...