I EXAMPLE 6-1I Suppose that the scr

I EXAMPLE 6-1
I Suppose that the screen in Figure 6-8 is 2.00 m from
g the plane of the slits and that the slit spacing is 0.300
I mm. What is the wavelength of radiation if the fourth
L band is located 15.4 mm from the central band?
Substituting into Equation 6-10 gives
0.300 mm X 15.4 mm
2.00 m X 1000 mm/m
5.78 X 10-4 mm or 578 nm

6B-6 Coherent Radiation
In order to produce a diffraction pattern such as that
shown in Figure 6-8a, it is necessary that the electro-
magnetic waves that travel from slits B and c to any
given point on the screen (such as D or E) have sharply
defined phase differences that remain entirely constant
with time; that is, the radiation from slits B and c must
be coherent. The conditions for coherence are that (1)
the two sources of radiation must have identical fre-
quencies (or sets of frequencies) and (2) the phase rela-
tionships between the two beams must remain constant
with time. The necessity for these requirements can be
demonstrated by illuminating the two slits in Figure
6-8a with individual tungsten lamps. Under this circum-
stance, the well-defined light and dark patterns disap-
pear and are replaced by a more or less uniform illumi-
nation of the screen. This behavior is a consequence of
the incoherent character of filament sources (many
other sources of electromagnetic radiation are incoher-
ent as well).
With incoherent sources, light is emitted by indi-
vidual atoms or molecules, and the resulting beam is the
summation of countless individual events, each of
which lasts on the order of 10-8 ร. Thus, a beam of ra-
diation from this type of source is not continuous but in-
stead is composed of a series of wave trains that are a
few meters in length at most. Because the processes that produce trains are random, the phase differences among the trains must also be variable. A wave train from slit B may arrive at a point on the screen in phase with a wave train from c so that constructive interference occurs; an instant later, the trains may be totally out of phase at the same point, and destructive interference occurs. Thus, the radiation at all points on the screen is governed by the random phase variations among the wave trains; uniform illumination, which represents an average for the trains, is the result.
There are sources that produce electromagnetic ra¬diation in the form of trains with essentially infinite length and constant frequency. Examples include radio¬frequency oscillators, microwave sources, and optical lasers. Various mechanical sources, such as a two¬pronged vibrating tapper in a water-containing ripple tank, produce a mechanical analog of coherent radia¬tion. When two coherent sources are substituted for slit A in the experiment shown in Figure 6-8a, a regular dif¬fraction pattern is observed.
Diffraction patterns can be obtained from random sources, such as tungsten filaments, provided that an arrangement similar to that shown in Figure 6-8a is em¬ployed. Here, the very narrow slit A assures that the radi-ation reaching B and c emanates from the same small re¬gion of the source. Under this circumstance, the various wave trains that exit from slits B and c have a constant set of frequencies and phase relationships to one another and are thus coherent. If the slit at A is widened so that a larger part of the source is sampled, the diffraction pat¬tern becotqes less pronounced because the two beams are only partially coherent. If slit A is made sufficiently wide, the incoherence may become great enough to pro¬duce only a constant illumination across the screen.
6B-7 Transmission of Radiation
It is observed experimentally that the rate at which radi¬ation is propagated through a transparent substance is less than its velocity in a vacuum and depends upon the kinds and concentrations of atoms, ions, or molecules in the medium. It follows from these observations that the radiation must interact in some way with the matter. Be¬cause a frequency change is not observed, however, the interaction cannot involve a permanent energy transfer.
The refractive index of a medium is one measure of its interaction with radiation and is defined by

ผลลัพธ์ (ไทย) 3:

ผมยกตัวอย่าง 6-1
ฉันคิดว่าหน้าจอในรูปที่ 6-8 2.00 เมตร จาก
g เครื่องบินของกรีดที่ร่องระยะห่างคือ 0.300
ผมมม อะไรคือความยาวคลื่นของรังสีถ้าวง L 4
ตั้งอยู่ 15.4 มม. จากวงกลาง ? แทนลงในสมการให้
-
0.300 มม. x 15.4 mm
2 m x 1000 mm / m
x 10-4 5.78 มิลลิเมตร หรือ 578 nm


6b-6 กันรังสีเพื่อผลิตเป็นรูปแบบการเลี้ยวเบน เช่นที่แสดงในรูปที่ 6-8a
, ไม่จำเป็นที่ electro -
แม่เหล็กคลื่นที่เดินทางจากช่อง B และ C
ให้ใด ๆจุดบนหน้าจอ ( เช่น D หรือ E ) มีความแตกต่างของเฟสที่กำหนดอย่างรวดเร็ว

ยังคงทั้งหมดคงที่กับเวลา นั่นคือ , รังสีจากช่อง B และ C ต้อง
ได้ติดต่อกัน . เงื่อนไขสำหรับการมองโลกที่ ( 1 )