HELP! When a parallel beam of light, of intensity or energy flux, I, falls perpendicularly on an absorbing surface of area, A, the energy, E, incident in a time, t, is equal to I At. The momentum, p, transferred to the surface is given by p= E/c= IAt/c where c is the speed of light.

Question

anonymous · Answer

1) Light of intensity 12 Wcm^-2 falls for 30 minutes on a mirror of area 460 cm^2 which reflects 80% of the incident light. Calculate the energy reflected from the mirror in this time. 

2) In sunlight, the force on 2 sail, each of area 2km^2, is of the order of 10N each at a distance fro the sun equal to the Earth's orbital radius. Discuss the viability of using such sails to propel space vehicles. already in orbit, to reach the outer planets of the solar system.

anonymous · Answer

@Compassionate can you help me?

anonymous · Answer

1) As you have stated in the question,energy incident on a surface is given by IAt.
here I=12 Wcm^-2
        A=460 cm^2
and it is said that the light beam is incident for 30 minutes so
      t=30 min=30 x 60 seconds=1800 seconds
now just put the values in the formula to find incident energy.
if 80% energy is reflected then you first need to find the value of incident energy from the process I just told you and then calculate its 80%.

anonymous · Answer

Since the unit for area is m^2, do I have to convert the cm to m?

anonymous · Answer

no need for that since unit for intensity is also given in terms of cm,so it will cancel out eventually.

anonymous · Answer

okay, so after I get the value using the formula IAt, what do I do next?

anonymous · Answer

it is said in the question that 80% energy is reflected,so after you have calculated the incident energy from IAt,then calculate its 80%,i.e. if the incident energy is E,then the reflected energy would be (80/100)xE

anonymous · Answer

Got it! Thank you very much!

anonymous · Answer

Can you help with No. 2?

anonymous · Answer

for ques 2
solar sails are a viable option to propel space vehicles as they provide a constant source of energy and act as a good alternative of fuel but this sail would not be that viable as the area of solar sails is very less.Even this solar sail would constantly propel a space vehicle but it will move very slowly as the energy input will be very low.

anonymous · Answer

Thank you very much for helping! Greatly appreciated :)

anonymous · Answer

your welcome