1. Refraction of Light Lenses
Physics 1161: Pre-Lecture 17
Refraction of Light Lenses
sections 26-3 – 26-5 1

2. Light Doesn’t Just Bounce It Also Refracts!
Reflects: Bounces
qi = qr qi qr
Refracts: Bends
n1 sin(q1)= n2 sin(q2) q1 q2 n2 n1

3. Index of Refraction
186,000 mi/s (3 x 108m/s): it’s not just a good idea, it’s the law!
Speed of light in vacuum
Speed of light in medium
Index of refraction so
always!

4. Indices of Refraction

5. Snell’s Law n1 sin(q1)= n2 sin(q2)
When light travels from one medium to another the speed changes v = c/n, but the frequency is constant. So the light bends:
n1 sin(q1)= n2 sin(q2)

6. Shape of a Lens

7. Types of Lenses

8. Lens Terms

9. Three Rays to Locate Image
Ray parallel to axis bends through the focus.
Ray through the focus bends parallel to axis.
Ray through center of lens passes straight through.

10. Characterizing the Image
Images are characterized in the following way
Virtual or Real
Upright or Inverted
Reduced, Enlarged, Same Size

11. Object Beyond 2f
Image is
Real
Inverted
Reduced

12. Object at 2f
Image is
Real
Inverted
Same size

13. Object Between 2f and f
Image is
Real
Inverted
Enlarged

14. Object at F
No Image is Formed!

15. Object Closer than F
Image is
Virtual
Upright
Enlarged

16. Converging Lens Images

17. Concave (Diverging) Lens
Ray parallel to axis refracts as if it comes from the first focus.
Ray which lines up with second focus refracts parallel to axis.
Ray through center of lens doesn’t bend.

18. Image Formed by Concave Lens
Image is always
Virtual
Upright
Reduced

19. Concave Lens Image Distance
As object distance decreases
Image distance decreases
Image size increases

20. Image Characteristics
CONVEX LENS – IMAGE DEPENDS ON OBJECT POSITION
Beyond F: Real; Inverted; Enlarged, Reduced, or Same Size
Closer than F: Virtual, Upright, Enlarged
At F: NO IMAGE
CONCAVE LENS – IMAGE ALWAYS SAME
Virtual
Upright
Reduced

21. Lens Equations convex: f > 0; concave: f < 0do di
convex: f > 0; concave: f < 0
do > 0 if object on left of lens
di > 0 if image on right of lens otherwise di < 0
ho & hi are positive if above principal axis; negative below