1. Locating the image for lenses

2. Defining the Parts
Focal point – where parallel light rays that pass through a lens would come together on the opposite side of the lens. There are two for each lens, one on each side.
Principle axis– a line through the middle of the lens. The bottom of the object always is touching the principle axis.

3. Defining the parts – cont’d
Principle axis
Focal point A
Focal point B
Convex lens – also known as a converging lens because it makes light rays come together

4. How do converging lenses work?
Parallel light rays that enter a lens from one side are refracted. They focus the light on Focal Point B on the other side.

5. Why are there two focal points?
If the rays were to come in from the opposite side, they would focus on the other focal point.

6. So, how do we locate an image?
There are three rays you can draw, but you only have to draw two – the third is optional. All three rays will converge on the same place, and that is where the image will form.
The easiest one to draw goes from the top of the object, through the middle of the lens and out the other side w/out bending.

7. The first ray
From the top of the object through the center of the lens.

8. The second ray Say, “in parallel, out through focal point. A B
Draw the ray from the top of the object, parallel to the principle axis. When it refracts through, it will go out through Focal point B.

9. You have already located your image!!B
Where these two rays meet, shows you where the top of the image is.

10. The third ray (Extra Credit!!)
In through the OTHER focal point, out parallel. A B
See, they still all meet up in the same place.

11. Let’s draw the rays one at a time again.
1. From the top, straight through the middle.
2. In parallel, out through focal point.
3. In through the other focal point, out parallel

12. And where the three meet, there’s the top of your image (since that’s where all the light rays began.

13. So how do we describe this image?
It’s reduced – which means its smaller (just a wee bit) – you would have to show measurements to prove it.
It’s real – which means it can be projected onto a screen – like the overhead projector.
It’s inverted – which means upside down

14. Is it always this easy? No. Here is another example:
Ray 1 straight through the middle.
Ray 2 – in parallel, out through the focal point, but wait!! They won’t meet up!!! Bring back the refracted ray like you did with the mirrors.

15. The refracted rays meet up on the same side as the original object
This image is upright
It is magnified – bigger – that’s how magnifying glasses work.
It is a virtual image – since it’s on the same side as the object, it can’t be put on a screen.

16. What about that extra ray?
Ray 3 – In through the other focal point, out parallel. Don’t worry if it misses the lens, just extend the lens and pretend it’s there.
Now you know why it’s extra credit!!

17. Keep the three rays in mind– all begin at the top of the object.
Ray 1 Straight through the middle of the lens
Ray 2 In parallel, out through the focal point
Ray 3 In through the other focal point, out parallel.
WRITE THE THREE RAYS DOWN NOW!!!

18. Now what about those pesky Concave lenses?
Concave lenses are also called diverging lenses because they spread the light rays out.

19. But if you look closely, these rays seem to have come from the other focal point:

20. When drawing ray diagrams for concave lenses, you need to remember that they spread light rays out.
We still draw the same three rays.
Ray 1: Straight through the middle.

21. Ray 2 – In parallel, out through the focal point, remember where that light was focused???
How would you describe this??

22. What kind of image?
Upright
Reduced
Virtual

23. What about that extra ray?
If you continued this ray, it would have gone through the other focal point
In through the other focal point, out parallel.
Trace back the refracted ray.
This one’s tricky. I’m going to take the others out so you can see it by itself.

24. Altogether
Object
Image