The intensity at a certain distance from a bright light source is 6.40 W/m2 . Pa

Question

The intensity at a certain distance from a bright light source is 6.40 W/m2 .

Part A

Find the radiation pressures (in pascals) on a totally absorbing surface and a totally reflecting surface.

Answer in the order indicated. Separate the answers with a comma.

Correct

Term 2: Significant Figures Feedback:Your answer 4.26108 = 4.26×108 Pawas either rounded differently or used a different number of significant figures than required for this part.

Part B

Find the radiation pressures (in atmospheres) on a totally absorbing surface and a totally reflecting surface.

Answer in the order indicated. Separate the answers with a comma.

2.15•1016,43.1•1014

Incorrect; Try Again; 9 attempts remaining

Term 2: Not quite. Check through your calculations; you may have made a rounding error or used the wrong number of significant figures.

Explanation / Answer

The pressure from light for a surface that totally absorbs the light (all of the momentum of the light is transferred to the surface and the surface is perpendicular to the light) is given by:

P = E/c

where P is the pressure, E is the energy density of the light ( 6.4 W/m^2) and c is the speed of light

Pabs = 6.4/(3*10^8)

=2.1333 *10^-8 pa

= 2.133 * 10^-8 *9.86923 * 10^-6 atm

Pabs=2.105*10^-13 atm

For a surface that reflects the light completely, in order to conserve momentum (if the light had momentum MO and it was completely reflected, then it will have momentum -MO since the direction of the momentum vector is reversed. This means a total of 2MO must have been transferred to the surface. So the pressure exerted must be multiplied by 2. Therefore

Pref = 2E/c

= 2*6.4 /(3*10^8)

=4.266*10^-8 pa

= 4.266*10^-8 *9.86923 * 10^-6 atm

Pref=4.2108 *10^-13 atm

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