Question

Find the energy stored in a 60 cm length of a laser beam operating at 4 mW.
(1) 8 x 10-12J
(2) 6 x 10-12
(3) 4 x 10-12J
(4) 7 * 10-12​

Answer 1

Given :

• Length of laser beam = 60 cm = 0.6 m
• Power = 4 mW = 4 × 10⁻³ W

To find :

• Energy stored in the laser

Solution :

• We know that, the beam travels with the speed of light = c = 3 × 10⁸
• The time taken by the laser beam to travel through 60 cm is,

        time = distance / speed

     ∴  $t = \frac{0.6}{3 . 10^{8} } seconds$

     ∴ t = 2 × 10⁻⁹ seconds

• Now, the energy stored in the beam will be given by,

        Energy = Power × Time

     ∴ U = (4 × 10⁻³) × (2 × 10⁻⁹)  

     ∴ U = 8 × 10⁻¹² J

Answer : The energy stored in the beam will be 8 × 10⁻¹² Joules.

Hence, option (1) will be correct.

Answer 2

Given:

The length of the beam is 60 cm

The operating power of beam is 4mW

To find:

The energy stored.

Solution:

As it is a beam, so the time taken by the electromagnetic wave is

t= 60/3×10⁸

t= 2×10⁻⁹

Now the energy is

U= 4×10⁻³×2×10⁻⁹

U=8×10⁻¹² J

So the option A is correct.