# Laser or the torchlight does not bend 90 degrees?

1. Jun 9, 2004

### touqra

Consider a situation where a friend have a red laser or a torchlight shining across the room. And you are standing at the side, perpendicular to the light beam. If light traverse in a straight line, why is that you can see the light shining across the room when you know that the light that comes out from the laser or the torchlight does not bend 90 degrees?

2. Jun 9, 2004

### ZapperZ

Staff Emeritus
Hint: do you think you will see the light beam going across if, instead, this was done in a good vacuum (10^-5 torr would be sufficient)?

Zz.

P.S. Eeek! If I knew that you were also going to SPAM this question in other parts of PF, I wouldn't have responded.

3. Jun 9, 2004

### touqra

I'm new here and I don't know where this question should go.

Why is it that you can't see the light beam if it is in a vacuum? Do you mean dust particles in the way of the beam scattered some of the light away from the original straight direction and traverse into our eyes?

Last edited: Jun 9, 2004
4. Jun 9, 2004

### JohnDubYa

Yes, that is correct.

5. Jun 9, 2004

### JohnDubYa

By the same token, how do you find laser beams? You spray smoke throughout the room. Haven't you seen this technique done in movies? The burglar enters the office, and sprays the interior with smoke from a can. The laser light beams magically appear.

6. Jun 9, 2004

### touqra

Ooops my bad. I overlook this.
I have one more question.
If let say the red laser is still shining across the room. But on top of that, another blue laser is also shining, but it is traversing perpendicular to the red laser beam. So, there would be a point, where both laser beam will meet each other. What actually happen at this point of collision?

7. Jun 9, 2004

### Staff: Mentor

Nothing at all. There is no collision.

8. Jun 10, 2004

### touqra

Wouldn't there be a superposition of waves destructive or constructive?

9. Jun 10, 2004

### swansont

In the movies, maybe. Anyone doing that in an optics lab would be rudely introduced to some high-voltage equipment.

10. Jun 12, 2004

### touqra

Wouldn't there be a superposition of light waves, destructive or constructive?

11. Jun 12, 2004

### Integral

Staff Emeritus
Humm.... My first optics/Laser class was done will smoking inside was PC. When we needed a beam finder the PROFFESSOR would light up a cig and start blowing smoke.

12. Jun 12, 2004

### swansont

Ack! That's a class, though. That wouldn't make him too popular with any grad students he might have, had their research been messed up by that. There was one prof I know who wouldn't let a student in who had smoked unless she had showered and changed clothes since - he was worried about anything that might have been collected by her clothes or her hair. (dye laser with frequency doubling)

When we wanted to show off our beams we got ahold of some dry ice or liquid nitrogen and used the condensing vapor.

13. Jun 14, 2004

### bootsam

i shone a red laser through a prism then through the gap on some nail clippers as i reduced the gap manually and onto my wall, the laser dot reduced to a wide straight line as i reduced the clipper gap, brighter in the middle but the line was rotated 90deg to the orientation of the nail clipper slot. why is that? have i just carried out a very inpromptu diffraction experiment?

14. Jun 14, 2004

### swansont

Sounds like it. Did you notice any minima or maxima?