How can we measure electron speed if light changes it?

[vinayjain]

how come we know the speed of an electron?

Note: As per my understanding we throw a beam of light on a object the amount of time it takes to come back describes the speed and distance of an object, but if we throw a beam of light on an electron that beam will energise the electron and thus it's speed will change.....please reply

 

[Stonebridge]

Indeed. There is always a small, innate, amount of uncertainty in its measured speed.
It cannot be known with certainty at the same time as its position.
(Strictly speaking its the momentum and position which cannot both be known with certainty)
This problem is described by the Heisenberg Uncertainty Principle. It would be interesting and informative to look this up.
Here is a start
http://hyperphysics.phy-astr.gsu.edu/hbase/uncer.html

 

[ZapperZ]

how come we know the speed of an electron?

Note: As per my understanding we throw a beam of light on a object the amount of time it takes to come back describes the speed and distance of an object, but if we throw a beam of light on an electron that beam will energise the electron and thus it's speed will change.....please reply

1. You can measure the kinetic energy of the electron via, say, bending it in a magnetic field. The amount of bend, and your knowledge of the magnetic field strength, gives you the KE.

2. Then, depending on whether it is relativistic or not, you can find the speed of the electron.

3. This is how we know the speeds of electrons in particle accelerators!

Zz.