﻿ GR8677.091
 ขอต้อนรับ ผู้มาเยือน กรุณา ล็อกอิน หรือ สมัครสมาชิก 1 Hour 1 Day 1 Week 1 Month Forever ล็อกอินด้วยชื่อผู้ใช้ รหัสผ่่าน และระยะเวลาใช้งาน Advanced search 41576 Posts in 6275 Topics- by 9813 Members - Latest Member: gofgolf6357
Pages: 1   Go Down
 Author Topic: GR8677.091  (Read 2723 times) 0 Members and 1 Guest are viewing this topic.
conantee
SuperHelper

Offline

Posts: 1400

เราเป็นอย่างไร สังคมเป็นอย่างนั้น

 « on: January 08, 2010, 01:57:57 AM »

91. When a narrow beam of monoenergetic electrons impinges on the surface of a single metal crystal at an angle of degrees with the plane of the surface, first-order reflection is observed. If the spacing of the reflecting crystal planes is known from x-ray measurements to be angstroms, the speed of the electrons is most nearly
(A)
(B)
(C)
(D)
(E)
 Logged
conantee
SuperHelper

Offline

Posts: 1400

เราเป็นอย่างไร สังคมเป็นอย่างนั้น

 « Reply #1 on: October 16, 2010, 10:58:42 AM »

If you don't know Bragg's condition or electron wave, at least you should eliminate 2 or 3 choices. First, (E) is wrong since it is faster than the speed of light. Next, electron in a beam for this kind of experiment should be very fast. (A) is hopelessly small, while (B) is just the speed of car, roughly speaking. Hence, you have only (C) and (D). You are expert familiar with the topic, you can even say that the answer is (D) by order of magnitude.

Suppose you don't know Bragg's condition, you can just use the relation between momentum and wavelength of matter wave. In order to probe the distance of angstroms, you also need roughly wavelength of about 3 angstroms. By De Broglie relation, we have

Hence,