# Why is this laser beam being scattered(and not)?

I was shining a laser beam through a liquid filled test tube(an ester particularly),and I found this phenomenon rather intriguing.Have a look.

Now when I passed the laser straight through the upper part of the test tube, a scattered blob of laser light appeared in front.

But when I put the laser in front of the liquid filled portion, a straight pattern of light( A laser bar if you may) appeared through on the other side.

Now, what is happening here?From what I know about scattering, an intervening medium tends to scatter more light.

• Why is more light being focused in the second experiment rather than the first which has seemingly lesser optical obstruction?
• Why the horizontal bar of laser light? Why is it not vertical?
• If one looks closely enough one can notice a variation in intensity reaching the other end in experiment 2.How is this happening(assuming that this is interference) if there is just one source?

If it is necessary, the test tube was made of borosilicate glass(3 mm thick) and of diameter around 1cm. A ray diagram for the above situation would be most informative. Sorry for the bad picture.Thanks in Advance!!

• You're essentially asking someone to calculate the lensing properties of your object there. What's the conceptual question here? – ACuriousMind Mar 6 '16 at 10:57
• I am merely asking why is the ray being scattered in case 1, while it is focused in case 2, while in my opinion the converse should happen. I have given the specifics just in case anyone asks. – Abhinav Mar 6 '16 at 11:00
• See Cylindrical lens in wikipedia. – Stefan Bischof Mar 8 '16 at 21:20

Do you find that the width of your spot changes as you move the screen towards and away from the tube?

I think that the tube together with the liquid is acting as a cylindrical lens and the second "image" is out of focus as your screen is not in the focal plane of your lens. You will probably never get a perfect focus because of various lens aberrations.

In the first case there is no focusing because your laser light is passing through two very thin pieces of glass with approximately parallel sides and some air.

Later

I do not have a laser or a test tube handy so here is a torch producing an approximately parallel beam which passes through a small bottle with water in it.
The focussing action of the beam can be seen as also the spreading out of the beam further out.

For water with $n\approx \frac 4 3$ the predicted focal plane is roughly the radius of the bottle away from the back end of the bottle.
The photographs seems to indicate agreement between theory and experiment?

• why is the elongated beam in case two not along upside down position? – Abhinav Mar 6 '16 at 15:04
• @Abhinav - Sorry, but that question is not understandable. Please try again, explaining it in detail. What is "upside down position"? Why should the elongated beam be along it? – WhatRoughBeast Mar 6 '16 at 16:51
• Yeah upside down is probably not the best description. What I meant was, if the beam is cylindrical, the probability of it being refracted in any other plane is equally likely(right?),so why the horizontal beam,why not the vertical one? – Abhinav Mar 6 '16 at 16:54
• @Abhinav - There is no curvature along the long axis of the tube, so there is no focusing in that direction. Please look up "cylindrical lens". – WhatRoughBeast Mar 6 '16 at 17:05
• Ok,I did a retake,and one more doubt came in mind. The laser beam is at near normal incidence( That's what I presume,for no other points give a better image),then cylindrical lens or not,the beam should be undeviated nor scattered. Shouldn't it? – Abhinav Mar 8 '16 at 5:46

In the liquid part there is a much lesser drop in the refractive index when the beam hits the glass, hence there is less reflection.

If you have more reflection, the beam gets bounced from side to side multiple times and each time a part of the radiation exits the glass tube. Since you do not have perfect planar orientations, the spot of the exiting light gets broadened.

• I didn't downvote. Cylinder lens is the right concept here and not multiple beam interference. – Stefan Bischof Mar 8 '16 at 21:19
• This is true, cylinder lens is correct for the water part. For the air filled part, however, the question remains why the laser spot is so much blurred, which is (hopefully at least in part) correctly answered in my reply. Please note that I was not talking about interferences, but mainly about reflections on the glass walls and the reduction in amplitude and change in direction going along with it. – M529 Mar 9 '16 at 11:02
• The air filled system can be modelled with a lens system of two thin lenses. – Stefan Bischof Mar 9 '16 at 11:53
• However intensity of one reflection follows the rule of thumb at perpendicular incidence of R=4%. Your explanation requires at least two reflections: 0,16% intensity. Times two from front and back of tube wall. Adding intensity of higher orders does not help further. – Stefan Bischof Mar 9 '16 at 12:01
• That is a very good point, yes. – M529 Mar 9 '16 at 12:27