Question

ieative Problems (SHOW YOUR WORK FOR PARTIAL CREDITS) 1) I 12pts I suppose an object is placed 60㎝ in front of a lems that ha, a fealength of 40 cm. Where is the image located? What are the magnification and the character of the image? L5x10rw.The - l-2) [12pts] A high-energy pulsed laser omits a 1.0 ns long pulse of average power beam is nearly a cylinder 2.2x10 m in radius. A) Determine the energy delivered in each pulse B) the rms value of the electric field Il-3)1Spts] A radio station broadcasts at 80 MHz. How long does it take for this radio signal to travel a distance of 20 - 100 m through space? (c 3.0 x 108 m/s)

Answer 1

1)

Using thin lens formula:

$\frac{1}{d_o}+\frac{1}{d_i}=\frac{1}{f}$

Object distance $d_o=6\,cm$ , focal length of lens $f=4\,cm$

$\frac{1}{6}+\frac{1}{d_i}=\frac{1}{4}$

$d_i=12\,cm$

Image distance from the lens is $d_i=12\,cm$

Magnification $M=-\frac{d_i}{d_o}=-\frac{12}{6}=-2$

Since $d_i$ is positive , image is real and is negative , image is inverted.

2)

i) Energy delivered in each pulse $E={P_{avg}}{t}={1.5*10^{11}}*{1*10^{-9}}=150\,J$

ii) Rms electric field $E_{rms}=\sqrt{\frac{P_{avg}}{c\epsilon_0 A}}=\sqrt{\frac{1.5*10^{11}}{3*10^8*8.85*10^{-12}*\pi (2.2*10^{-3})^2}}=1.93*10^9\,V/m$

3)

Time taken for the radio signal to travel a distance of $d=2*10^7\,m$ is

$t=\frac{d}{c}=\frac{2*10^7}{3*10^8}=0.0667\,s=66.7\,ms$