Find the energy stored in a 60 cm length of a laser beam operating at 4 m W.

Explanation

Solution:
The time taken by the electromagnetic wave to move
through a distance of 60 cm is `t=(60 cm / c)=2 xx (10^-9)s.` The energy contained in the 60 cm length passes through a
cross section of the beam in `2 xx (10^-9)s.`
But the energy passing through any cross section in
`2 xx (10^-9) s is
` U=(4m W) xx (2 xx (10^-9)s)`
`= (4 xx (10^-3)J(s^-1)) xx (2 xx (10^-9) s).`
`= 8 xx (10^-12) J. `
The is the energy contained in 60 cm length.
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