Does light lose energy when it reflects from a surface?

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When light reflects from a surface, at least the direction of its momentum changes. Since the total momentum must be conserved, there has to be something going on within the atoms of the surface.

So my question is that does reflected light increase the internal energy of the surface even if it is a really really tiny amount?

P.S. I am not talking about the fraction of light that gets absorbed by the surface. I know the energy of that fraction contributes to the internal energy of the surface. My concern is only about the photons being reflected.

energy photons momentum reflection

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edited 4 hours ago

asked 4 hours ago

physicsguy19

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up vote
3
down vote

favorite

1

When light reflects from a surface, at least the direction of its momentum changes. Since the total momentum must be conserved, there has to be something going on within the atoms of the surface.

So my question is that does reflected light increase the internal energy of the surface even if it is a really really tiny amount?

P.S. I am not talking about the fraction of light that gets absorbed by the surface. I know the energy of that fraction contributes to the internal energy of the surface. My concern is only about the photons being reflected.

energy photons momentum reflection

share|cite|improve this question

edited 4 hours ago

asked 4 hours ago

physicsguy19

583115

add a comment | 

up vote
3
down vote

favorite

1

up vote
3
down vote

favorite

1

1

When light reflects from a surface, at least the direction of its momentum changes. Since the total momentum must be conserved, there has to be something going on within the atoms of the surface.

So my question is that does reflected light increase the internal energy of the surface even if it is a really really tiny amount?

P.S. I am not talking about the fraction of light that gets absorbed by the surface. I know the energy of that fraction contributes to the internal energy of the surface. My concern is only about the photons being reflected.

energy photons momentum reflection

share|cite|improve this question

edited 4 hours ago

asked 4 hours ago

physicsguy19

583115

When light reflects from a surface, at least the direction of its momentum changes. Since the total momentum must be conserved, there has to be something going on within the atoms of the surface.

So my question is that does reflected light increase the internal energy of the surface even if it is a really really tiny amount?

P.S. I am not talking about the fraction of light that gets absorbed by the surface. I know the energy of that fraction contributes to the internal energy of the surface. My concern is only about the photons being reflected.

energy photons momentum reflection

energy photons momentum reflection

share|cite|improve this question

edited 4 hours ago

asked 4 hours ago

physicsguy19

583115

share|cite|improve this question

edited 4 hours ago

asked 4 hours ago

physicsguy19

583115

share|cite|improve this question

share|cite|improve this question

edited 4 hours ago

edited 4 hours ago

edited 4 hours ago

asked 4 hours ago

physicsguy19

583115

asked 4 hours ago

physicsguy19

583115

asked 4 hours ago

physicsguy19

583115

583115

add a comment | 

add a comment | 

2 Answers
2

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oldest

votes

up vote
2
down vote

Yes, this is the principle behind Doppler radar. The frequency/energy increases if the surface is moving towards the source. The frequency/energy decreases if the reflecting surface is moving away from the source.

The only time the frequency/energy will be unchanged is if the surface initially has the opposite momentum of the light.

share|cite|improve this answer

edited 3 hours ago

answered 4 hours ago

Dale

2,178415

• 
  
  
  

  
  

  
  
  
  
  
  

  
  So the frequency of reflected light is less than the incoming light evem if the surface is stationary?
  – physicsguy19
  3 hours ago
  
  

  
  
  
  


• 
  
  
  

  
  1
  

  
  
  
  
  
  

  
  If the surface is stationary then there will be a slight decrease. If the surface has the opposite momentum of the photon then the energy/frequency will be unchanged. I will add this to the answer
  – Dale
  3 hours ago
  

  
  
  
  


• 
  
  
  

  
  

  
  
  
  
  
  

  
  To clarify, upon reflection from a stationary object, the object recoils, giving it kinetic energy. The photon’s energy then needs to decrease (redshift) in order to account for the kinetic energy. The amount by which the frequency of the photon shifts is the Doppler factor. Thus both energy and momentum are conserved.
  – Gilbert
  20 mins ago
  

  
  
  
  

add a comment | 

up vote
0
down vote

When a photon is reflected there is no energy lost. There is a brief time when the photon interacts with the electron, momentum is deposited and then transferred right back to the photon. Only when there is absorption is momentum transferred to the material. Interestingly when photons are emitted there is a momentum change as well.

share|cite|improve this answer

answered 3 hours ago

PhysicsDave

61726

• 
  
  
  

  
  

  
  
  
  
  
  

  
  I think that explanation clearly contradicts the conservation of momentum.
  – physicsguy19
  3 hours ago
  

  
  
  
  

add a comment | 

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2 Answers
2

active

oldest

votes

2 Answers
2

active

oldest

votes

active

oldest

votes

active

oldest

votes

up vote
2
down vote

Yes, this is the principle behind Doppler radar. The frequency/energy increases if the surface is moving towards the source. The frequency/energy decreases if the reflecting surface is moving away from the source.

The only time the frequency/energy will be unchanged is if the surface initially has the opposite momentum of the light.

share|cite|improve this answer

edited 3 hours ago

answered 4 hours ago

Dale

2,178415

• 
  
  
  

  
  

  
  
  
  
  
  

  
  So the frequency of reflected light is less than the incoming light evem if the surface is stationary?
  – physicsguy19
  3 hours ago
  
  

  
  
  
  


• 
  
  
  

  
  1
  

  
  
  
  
  
  

  
  If the surface is stationary then there will be a slight decrease. If the surface has the opposite momentum of the photon then the energy/frequency will be unchanged. I will add this to the answer
  – Dale
  3 hours ago
  

  
  
  
  


• 
  
  
  

  
  

  
  
  
  
  
  

  
  To clarify, upon reflection from a stationary object, the object recoils, giving it kinetic energy. The photon’s energy then needs to decrease (redshift) in order to account for the kinetic energy. The amount by which the frequency of the photon shifts is the Doppler factor. Thus both energy and momentum are conserved.
  – Gilbert
  20 mins ago
  

  
  
  
  

add a comment | 

up vote
2
down vote

Yes, this is the principle behind Doppler radar. The frequency/energy increases if the surface is moving towards the source. The frequency/energy decreases if the reflecting surface is moving away from the source.

The only time the frequency/energy will be unchanged is if the surface initially has the opposite momentum of the light.

share|cite|improve this answer

edited 3 hours ago

answered 4 hours ago

Dale

2,178415

• 
  
  
  

  
  

  
  
  
  
  
  

  
  So the frequency of reflected light is less than the incoming light evem if the surface is stationary?
  – physicsguy19
  3 hours ago
  
  

  
  
  
  


• 
  
  
  

  
  1
  

  
  
  
  
  
  

  
  If the surface is stationary then there will be a slight decrease. If the surface has the opposite momentum of the photon then the energy/frequency will be unchanged. I will add this to the answer
  – Dale
  3 hours ago
  

  
  
  
  


• 
  
  
  

  
  

  
  
  
  
  
  

  
  To clarify, upon reflection from a stationary object, the object recoils, giving it kinetic energy. The photon’s energy then needs to decrease (redshift) in order to account for the kinetic energy. The amount by which the frequency of the photon shifts is the Doppler factor. Thus both energy and momentum are conserved.
  – Gilbert
  20 mins ago
  

  
  
  
  

add a comment | 

up vote
2
down vote

up vote
2
down vote

Yes, this is the principle behind Doppler radar. The frequency/energy increases if the surface is moving towards the source. The frequency/energy decreases if the reflecting surface is moving away from the source.

The only time the frequency/energy will be unchanged is if the surface initially has the opposite momentum of the light.

share|cite|improve this answer

edited 3 hours ago

answered 4 hours ago

Dale

2,178415

Yes, this is the principle behind Doppler radar. The frequency/energy increases if the surface is moving towards the source. The frequency/energy decreases if the reflecting surface is moving away from the source.

The only time the frequency/energy will be unchanged is if the surface initially has the opposite momentum of the light.

share|cite|improve this answer

edited 3 hours ago

answered 4 hours ago

Dale

2,178415

share|cite|improve this answer

share|cite|improve this answer

edited 3 hours ago

edited 3 hours ago

edited 3 hours ago

answered 4 hours ago

Dale

2,178415

answered 4 hours ago

Dale

2,178415

answered 4 hours ago

Dale

2,178415

2,178415

• 
  
  
  

  
  

  
  
  
  
  
  

  
  So the frequency of reflected light is less than the incoming light evem if the surface is stationary?
  – physicsguy19
  3 hours ago
  
  

  
  
  
  


• 
  
  
  

  
  1
  

  
  
  
  
  
  

  
  If the surface is stationary then there will be a slight decrease. If the surface has the opposite momentum of the photon then the energy/frequency will be unchanged. I will add this to the answer
  – Dale
  3 hours ago
  

  
  
  
  


• 
  
  
  

  
  

  
  
  
  
  
  

  
  To clarify, upon reflection from a stationary object, the object recoils, giving it kinetic energy. The photon’s energy then needs to decrease (redshift) in order to account for the kinetic energy. The amount by which the frequency of the photon shifts is the Doppler factor. Thus both energy and momentum are conserved.
  – Gilbert
  20 mins ago
  

  
  
  
  

add a comment | 

• 
  
  
  

  
  

  
  
  
  
  
  

  
  So the frequency of reflected light is less than the incoming light evem if the surface is stationary?
  – physicsguy19
  3 hours ago
  
  

  
  
  
  


• 
  
  
  

  
  1
  

  
  
  
  
  
  

  
  If the surface is stationary then there will be a slight decrease. If the surface has the opposite momentum of the photon then the energy/frequency will be unchanged. I will add this to the answer
  – Dale
  3 hours ago
  

  
  
  
  


• 
  
  
  

  
  

  
  
  
  
  
  

  
  To clarify, upon reflection from a stationary object, the object recoils, giving it kinetic energy. The photon’s energy then needs to decrease (redshift) in order to account for the kinetic energy. The amount by which the frequency of the photon shifts is the Doppler factor. Thus both energy and momentum are conserved.
  – Gilbert
  20 mins ago
  

  
  
  
  

So the frequency of reflected light is less than the incoming light evem if the surface is stationary?
– physicsguy19
3 hours ago

So the frequency of reflected light is less than the incoming light evem if the surface is stationary?
– physicsguy19
3 hours ago

1

1

If the surface is stationary then there will be a slight decrease. If the surface has the opposite momentum of the photon then the energy/frequency will be unchanged. I will add this to the answer
– Dale
3 hours ago

If the surface is stationary then there will be a slight decrease. If the surface has the opposite momentum of the photon then the energy/frequency will be unchanged. I will add this to the answer
– Dale
3 hours ago

To clarify, upon reflection from a stationary object, the object recoils, giving it kinetic energy. The photon’s energy then needs to decrease (redshift) in order to account for the kinetic energy. The amount by which the frequency of the photon shifts is the Doppler factor. Thus both energy and momentum are conserved.
– Gilbert
20 mins ago

To clarify, upon reflection from a stationary object, the object recoils, giving it kinetic energy. The photon’s energy then needs to decrease (redshift) in order to account for the kinetic energy. The amount by which the frequency of the photon shifts is the Doppler factor. Thus both energy and momentum are conserved.
– Gilbert
20 mins ago

add a comment | 

up vote
0
down vote

When a photon is reflected there is no energy lost. There is a brief time when the photon interacts with the electron, momentum is deposited and then transferred right back to the photon. Only when there is absorption is momentum transferred to the material. Interestingly when photons are emitted there is a momentum change as well.

share|cite|improve this answer

answered 3 hours ago

PhysicsDave

61726

• 
  
  
  

  
  

  
  
  
  
  
  

  
  I think that explanation clearly contradicts the conservation of momentum.
  – physicsguy19
  3 hours ago
  

  
  
  
  

add a comment | 

up vote
0
down vote

When a photon is reflected there is no energy lost. There is a brief time when the photon interacts with the electron, momentum is deposited and then transferred right back to the photon. Only when there is absorption is momentum transferred to the material. Interestingly when photons are emitted there is a momentum change as well.

share|cite|improve this answer

answered 3 hours ago

PhysicsDave

61726

• 
  
  
  

  
  

  
  
  
  
  
  

  
  I think that explanation clearly contradicts the conservation of momentum.
  – physicsguy19
  3 hours ago
  

  
  
  
  

add a comment | 

up vote
0
down vote

up vote
0
down vote

When a photon is reflected there is no energy lost. There is a brief time when the photon interacts with the electron, momentum is deposited and then transferred right back to the photon. Only when there is absorption is momentum transferred to the material. Interestingly when photons are emitted there is a momentum change as well.

share|cite|improve this answer

answered 3 hours ago

PhysicsDave

61726

When a photon is reflected there is no energy lost. There is a brief time when the photon interacts with the electron, momentum is deposited and then transferred right back to the photon. Only when there is absorption is momentum transferred to the material. Interestingly when photons are emitted there is a momentum change as well.

share|cite|improve this answer

answered 3 hours ago

PhysicsDave

61726

share|cite|improve this answer

share|cite|improve this answer

answered 3 hours ago

PhysicsDave

61726

answered 3 hours ago

PhysicsDave

61726

answered 3 hours ago

PhysicsDave

61726

61726

• 
  
  
  

  
  

  
  
  
  
  
  

  
  I think that explanation clearly contradicts the conservation of momentum.
  – physicsguy19
  3 hours ago
  

  
  
  
  

add a comment | 

• 
  
  
  

  
  

  
  
  
  
  
  

  
  I think that explanation clearly contradicts the conservation of momentum.
  – physicsguy19
  3 hours ago
  

  
  
  
  

I think that explanation clearly contradicts the conservation of momentum.
– physicsguy19
3 hours ago

I think that explanation clearly contradicts the conservation of momentum.
– physicsguy19
3 hours ago

add a comment | 

 

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