You want to support a sheet of fireproof paper horizontally using only a vertica

Question

You want to support a sheet of fireproof paper horizontally using only a vertical upward beam of light spread uniformly over the sheet. There is no other light on this paper. The sheet measures 22.0cm by 28.0 cm and has a mass of 1.50g. (a) If the paper is black and hence absorbs all the light that hits it, what must the intensity of the light beam? (b) For the light in part (a), what are the amplitudes of its electric and magnetic fields? (c) If the paper is white and hence reflects all the light that hits it, what intensity of light beam is needed to support it? (d) To see if its is physically reasonable to expect to support a sheet of paper this way, calculate the intensity in a typical 0.500-mW laser beam that is 1.00mm in diameter, and compare this value with your answer in part (a).

Explanation / Answer

a)

F = m g = 1.50e-3 * 9.8 = 0.0147 N

P = F/A = 0.0147/(0.22*0.28) = 0.23864 Pa

P = I/c

>>>> I = P c = 0.23864 * 3e8 = 7.16 * 10^7 W/m2

b)

I = E2/(2c)

>>> E = (2cI) = (2*4*3.14e-7*3e8*7.1591e7) = 2.32 * 10^5 V/m

B = E/c = 2.3227e5/3e8 = 7.74e-4 T

c)

P = 2I/c

>>>> I = c P/2 = 0.23864*3e8/2 = 3.58 * 10^7 W/m2

d)

I = 0.500e-3/(3.14*0.5e-3*0.5e-3) = 637 W/m2

Related Questions

Navigate

• Browse (All)
• Browse Y
• Subjects

---

---

Integrity-first tutoring: explanations and feedback only — we do not complete graded work. Learn more.