# How do we locate the principal axis?

1. Dec 6, 2004

### JimmyRay

Hi guys we got a lab today and I have to answer a few questions...

I want to make sure my answers are correct...

Convex Lenses
1. How do we locate the principal axis?
We locate the principal axis by directing a beam to the middle of the lens (what is this called? optical center?) perpendicular to the surface. (why does the beam not refract again?)

2. State three rules that describe the path that light takes through a Convex Lens.
If you send a ray perpendicular to the principal axis through the lens it will refract passing through the focal point. If you send a ray passing through the focal point diagonally towards the lens it will become perpendicular. If you send a ray diagonally through the middle of the lens .. it will hit the focal point? lol I dont know the third one....

3. Why is a convex lens called a converging lens?
A convex lens is called a converging lens because if you send rays which are paralel to the principal axis towards the lens they will all refract passing through the focal point and converging towards it.

Now there is another lab on Concave lenses, question 1 and 2 are the same but 3 asks why it is called a diverging lens....... Question 1 will be the same answer, but... I dont really get number 2....

Basically having trouble with the rules... any help would be appreciated.

2. Dec 7, 2004

### James R

A ray which passes through the centre of the lens is undeflected.

3. Dec 7, 2004

### HallsofIvy

Staff Emeritus
"1. How do we locate the principal axis?
We locate the principal axis by directing a beam to the middle of the lens (what is this called? optical center?) perpendicular to the surface. (why does the beam not refract again?)"

The beam does not refract because of the symmetry of the lens. The beam is perpendicular to both faces of the lens.

"2. State three rules that describe the path that light takes through a Convex Lens.
If you send a ray perpendicular to the principal axis through the lens it will refract passing through the focal point. If you send a ray passing through the focal point diagonally towards the lens it will become perpendicular. If you send a ray diagonally through the middle of the lens .. it will hit the focal point? lol I dont know the third one...."
Again, because the lens is symmetric, any beam that goes through the center of the lens is undeflected.

"3. Why is a convex lens called a converging lens?
A convex lens is called a converging lens because if you send rays which are paralel to the principal axis towards the lens they will all refract passing through the focal point and converging towards it."

Yes.

"Now there is another lab on Concave lenses, question 1 and 2 are the same but 3 asks why it is called a diverging lens....... Question 1 will be the same answer, but... I dont really get number 2...."
"If you send a ray perpendicular to the principal axis through the lens it will refract" and diverge away from the focal point (which is now on the same side of the lens as the light is coming from).
"If you send a ray" THAT WOULD PASS THROUGH THE FOCAL POINT ON THE OTHER SIDE OF THE LENS IF THE LENS WERE NOT THERE, "it will become perpendicular".
"A ray which passes through the centre of the lens is undeflected."

4. Dec 7, 2004

### JimmyRay

Thanks guys, I got the three rules eventually, it was due today lol... We're on to equation of a thin lens now, but yeah this was a good check.