Question: What happens to absorbed light?

[Boltzman Oscillation]

Suppose I have two apples, one is white and the other is black. Both apples are exactly the same except for their colors. The black apple is darker than the white apple because it absorbs more energy than the white apple, correct? Would that mean that the black apple would have a higher energy? Or does light absorb not effect energy at all? I know for sure that a black apple would be hotter than a lighter apple if under the same light which would cause it to have more energy but is there another effect caused by the color change?

 

[Boltzman Oscillation]

Suppose I have two apples, one is white and the other is black. Both apples are exactly the same except for their colors. The black apple is darker than the white apple because it absorbs more energy than the white apple, correct? Would that mean that the black apple would have a higher energy? Or does light absorb not effect energy at all? I know for sure that a black apple would be hotter than a lighter apple if under the same light which would cause it to have more energy but is there another effect caused by the color change?

Maybe i answered my own question but any added information would help. Or any corrections!

 

[jbriggs444]

A black apple will absorb more visible light than a white apple. When bathed in white light, it will warm up more rapidly than a white apple. But the story does not end there.

The second law of thermodynamics applies. One of the implications is that if an object absorbs poorly in a particular range of frequencies then it will emit poorly in that range as well. So although the black apple warms up more rapidly, both apples can reach the same temperature. But the story does not end there either.

An apple at room temperature absorbs visible light from the sun and emits infrared radiation toward the ground and the rest of the sky. You may be able to make it black at visible frequencies and "white" at infrared frequencies, thus giving it a higher-than-expected equilibrium temperature. Or vice-versa for a lower-than-expected temperature.

 

[sophiecentaur]

An apple at room temperature absorbs visible light from the sun and emits infrared radiation toward the ground and the rest of the sky. You may be able to make it black at visible frequencies and "white" at infrared frequencies, thus giving it a higher-than-expected equilibrium temperature. Or vice-versa for a lower-than-expected temperature.

The fairest test situation would be to start with the apple in a box with walls of uniform temperature.

 

[jbriggs444]

The fairest test situation would be to start with the apple in a box with walls of uniform temperature.

Yes, though that idealization does not reflect everyday life on Earth.

 

[sophiecentaur]

Yes, though that idealization does not reflect everyday life on Earth.

The extreme in the other direction could be an object in space where the surrounding space is not far above 0K but the Sun, which only subtends 0.5degree is around 6000K. The net 'average' is around 300K - now how about that?

 

[Boltzman Oscillation]

A black apple will absorb more visible light than a white apple. When bathed in white light, it will warm up more rapidly than a white apple. But the story does not end there.

The second law of thermodynamics applies. One of the implications is that if an object absorbs poorly in a particular range of frequencies then it will emit poorly in that range as well. So although the black apple warms up more rapidly, both apples can reach the same temperature. But the story does not end there either.

An apple at room temperature absorbs visible light from the sun and emits infrared radiation toward the ground and the rest of the sky. You may be able to make it black at visible frequencies and "white" at infrared frequencies, thus giving it a higher-than-expected equilibrium temperature. Or vice-versa for a lower-than-expected temperature.

I have a question that has been messing with me. The apple is black because it absorbs less light than the white apple. This means that the chemical composition of the white apple and the black apple MUST be different. So my question, which assumes that both apples are the same composition, is incorrect right?

 

[Boltzman Oscillation]

Also is there any book that will teach me more about this?

 

[Boltzman Oscillation]

Sorry but what do you mean when you say this, "You may be able to make it black at visible frequencies and "white" at infrared frequencies, thus giving it a higher-than-expected equilibrium temperature. Or vice-versa for a lower-than-expected temperature." What do you mean by "it", the apple? I can make the apple black by shinning visible lights on it?

 

[jbriggs444]

I have a question that has been messing with me. The apple is black because it absorbs less light than the white apple. This means that the chemical composition of the white apple and the black apple MUST be different. So my question, which assumes that both apples are the same composition, is incorrect right?

The black apple absorbs more and reflects less. Hence the lack of reflected light. But yes, there is something different about the surfaces of the two apples.

 

[sophiecentaur]

So my question, which assumes that both apples are the same composition, is incorrect right?

They may taste similar so the composition of the insides will be much the same. The composition of the skins will be different.

 

[DrClaude]

Sorry but what do you mean when you say this, "You may be able to make it black at visible frequencies and "white" at infrared frequencies, thus giving it a higher-than-expected equilibrium temperature. Or vice-versa for a lower-than-expected temperature." What do you mean by "it", the apple? I can make the apple black by shinning visible lights on it?

Good question. I don't get it either. The equilibrium temperature should be the same.

Also is there any book that will teach me more about this?

A book on thermodynamics or thermal physics.

 

[jbriggs444]

I can make the apple black by shinning visible lights on it?

The proportion of light reflected versus light absorbed will usually depend on the frequency distribution of the light shining on it. An apple that reflects strongly in the red range will tend to look darker if viewed in a light which is devoid of reddish frequencies, yes.

The second law of thermodynamics has something to say if the apple is illuminated in a black body spectrum. It has less to say if the illuminating spectrum is skewed.

 

[DrClaude]

The second law of thermodynamics has something to say if the apple is illuminated in a black body spectrum. It has less to say if the illuminating spectrum is skewed.

It will still day that at equilibrium, the two objects will have the same temperature.

I have a feeling that you are thinking in terms of a steady state situation, not equilibrium.

 

[jbriggs444]

I have a feeling that you are thinking in terms of a steady state situation, not equilibrium.

Just so. An apple in the sun is not in thermal equilibrium either with the sun or with the rest of the sky.

 

[DrClaude]

Just so. An apple in the sun is not in thermal equilibrium either with the sun or with the rest of the sky.

We agree then!

A black apple will absorb more visible light than a white apple. When bathed in white light, it will warm up more rapidly than a white apple. But the story does not end there.

The second law of thermodynamics applies. One of the implications is that if an object absorbs poorly in a particular range of frequencies then it will emit poorly in that range as well. So although the black apple warms up more rapidly, both apples can reach the same temperature. But the story does not end there either.

An apple at room temperature absorbs visible light from the sun and emits infrared radiation toward the ground and the rest of the sky. You may be able to make it black at visible frequencies and "white" at infrared frequencies, thus giving it a higher-than-expected equilibrium steady state temperature. Or vice-versa for a lower-than-expected temperature.

 

[pinball1970]

Yes, though that idealization does not reflect everyday life on Earth.

Neither do black and white apples to be honest.