Light with an energy flux of 25 x 104 Wm-2 falls on a perfectly reflecting surface at normal incidence. If the surface area is 15 cm2, the average force exerted on the surface is:

  • Option 1)

    1.25 x 10-6 N

  • Option 2)

    2.50 x 10-6 N

  • Option 3)

    1.20 x 10-6 N

  • Option 4)

    3.0 x 10-6 N

 

Answers (1)

Average force = \frac{\Delta p}{\Delta t}

Since the surface is perfectly reflecting, \Delta p=2 \frac{\Delta t}{c}

F=2\left( \frac{\Delta E}{c \Delta t} \right )=2\frac{p}{c}

=2\:.\:\frac{25 \times 10 \times 15 \times 10}{3 \times 10^8}N

=2.5 \times 10^{-6}N


Option 1)

1.25 x 10-6 N

This option is incorrect.

Option 2)

2.50 x 10-6 N

This option is correct.

Option 3)

1.20 x 10-6 N

This option is incorrect.

Option 4)

3.0 x 10-6 N

This option is incorrect.

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