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On weight of photon

by dev70
Tags: photon, weight
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dev70
#1
Jan11-13, 12:51 PM
P: 58
hi pf, my friends asked me a strange question to me today,
"if in a vacuum chamber if light is somehow focused on a suitable weighing machine then will there be any deflection?
if yes does that mean light has mass? "

sounds strange, isnt? that why i choose to ask here?
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mfb
#2
Jan11-13, 01:25 PM
Mentor
P: 11,589
Focused in which way? Coming from above, reflecting light upwards? This will transfer momentum, and the scale will change.
Focused to have a high and constant energy density close to some (hypothetical) very sensitive scale? The scale should see an attraction.
Something else?
dev70
#3
Jan11-13, 11:56 PM
P: 58
coming from above, and focused on a sensitive scale..

mfb
#4
Jan12-13, 03:13 AM
Mentor
P: 11,589
On weight of photon

Light has a momentum, and momentum times intensity is a pressure. Pressure times surface area (of your scale, or focus) is a force, and the scale can measure this.
The Nichols radiometer is not a scale, but it operates with light pressure.
dev70
#5
Jan12-13, 03:41 AM
P: 58
wow..beautiful explanation. i think i got it... i need some time for review..thanks by the way
dev70
#6
Jan12-13, 03:43 AM
P: 58
i guess it doesnt have anything to do with mass of photon?
mfb
#7
Jan12-13, 03:57 AM
Mentor
P: 11,589
A photon has no mass.


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