[physical]

السلام عليكم

A 1.5 mW laser pointer is directed 2.5 m onto a wall where it forms a spot of area 1mm^2. Estimate the power of a lamp emitting spherical waves to replace the laser pointer which produces a similar power per unit area on the wall as the laser pointer.



A 660 Hz sound wave is emitted in all directions from a point source. At waht distance from the source does the amplitude ratio of successive wavefronts drop below 1.1001 ( assume the speed of sound to be 330m/s

الرجاااء المساعدة

[المتفيزق]

first of all, i'm sorry i'll not solve this completely for i 'm afraid it 's a take home exam or some sort of homeworks...

however;
in the laser beam..
you are given the energy flow (power)... and the spot area ...just divide the power by the area when you have the intensity(or power per unit area)

now for the lamp you have the radius( 2.5 m) so that the surface area of the sphere is 4pi r2 ...
from above (the laser beam) you have calculated the intensity and then you equate : intensity of laser= intensity of lamp = power / surface area.

[المتفيزق]

as for the second question...
it is easy also...
just you remember that the intensity is proportional to A2 (i mean the square of the amplitude...)
now you can claim that I1/I2 =1.001
now I1=power/4pi (r1)2 and I2= the same power/ 4 pi (r2)2
but notice here that r2-r1 = lambda (successive fronts) so that you can substitute for this...
now how to evaluate lambda????? just you have 660hz and velocity of light 330m/s and you construct your answer...
i think it is enough for these questions


thanks a lot

[physical]

Thank you very mcuh