Question

importance of shadow-
id like to understand about shadow, why they say something has to travel in order to form a shadow ?

Answer 1

my notes has :

In one of his experimental investigations, Crookes placed an object within the path of the cathode ray. Because a \(shadow\) appeared on the positive electrode, Crookes concluded that something (either light or small particles) was traveling from the negative to the positive electrode. Crookes hypothesized that the glow was made up of small particles of matter, but the scientific community would need more evidence before they would accept this idea.

Answer 2

In his tube, electrons were emitted at one end and traveled toward the other end. The electrons themselves were not observable, but many would fly past the anode and hit the glass causing it to glow. When he placed something in the electron's path(s), they would be blocked and couldn't strike the glass. That's what they mean by shadow...

Answer 3

oh these electrons from right side hit the inserted object, and left side it would become dark is it

Answer 4

yes.

Answer 5

i get it now... :)

i feel the attached snap is bit misleading as it shows glow all the way from left to right !

Answer 6

i was thinking some electrons passed thru the object also something like that.. n getting mixed up

Answer 7

Yeah, it's a bit misleading, since it 'shows' the electrons themselves as a beam; but they can't be seen in the low concentrations he had in his tube. The only way he 'saw' them was when they caused the end of the tube to glow by striking it; and putting something in their path caused a shadow on the end of the tube...

Answer 8

Why did the end of the tube glow?

Answer 9

omg so the beam is also not true, we cannot see it is it

Answer 10

i mean in real crts, the beam is not visible, onle we see glow on the other end

Answer 11

if it's intense you can see it (lightning)... it has to be pretty intense though..

Answer 12

haha i get it xD

Answer 13

yeah like a CRT beam in old tv's, can't see it, you only see the effect of it striking the phosphor in the screen ...

Answer 14

When they strike atoms in the glass, they knock their orbital electrons into a higher energy level. When the electrons fall back to their original energy level, they emit light. This process, called fluorescence. According to wikipedia. I had never heard of a crookes tube before. Fascinating.

Answer 15

Yep. hence fluorescent light, where a gas is constantly pushed to a plasma state and as it falls back to ground state it gives off light...

Answer 16

since crt is completely vacuum... so we cannot see the actual beam like we see it in fluorescent tube.. u explained very well @Algebraic! thanks a lot !

Answer 17

Just for clarity, when a gas is in plasma state, some of the atoms are ionized When de -ionized (no idea if that' s the right phrase) there is an energy change in the atom, which is released as light. Is the equation for this still E = hf? I have no idea why it wouldn't be ive just never considered the energy change in a completely ionized atom.

Answer 18

yes sorry Sara if i have somewhat hijacked your question but you seem to be answered :)

Answer 19

That's exactly the equation :)

Answer 20

difference between energy levels is proportional to frequency of light (color, for visible light)

Answer 21

So there is no fundamental difference between a free electron joining an ionized atom, and an electron moving from one orbital to a lower. That seems just strange!

Answer 22

its okay :)

Answer 23

u guys plz continue the discussion i hav no problem (:

Answer 24

algebraic has gone missing, What level are you studying at sara? your doing physics I presume

Answer 25

11th grade chemistry

Answer 26

the rays include electrons,so according to me ,there must be a subatomic particle to block the electrons through passing it and thus cast a shadow over the screen.but in order to block the excited electrons a subatomic particle must move with same energy so as to block electrons .