1. Reflection and Refraction of Light
Physics 102: Lecture 17
Reflection and Refraction of Light
Today’s Lecture will cover textbook sections 23.3, 8
Physics 102: Lecture 17, Slide 1 1

2. Overview Reflection: qi = qr Refraction: n1 sin(q1)= n2 sin(q2)
Last Time qi qr
qi = qr
Flat Mirror:
image equidistant behind
Spherical Mirrors:
Concave or Convex
Today q1 q2 n2 n1
Next time
Refraction:
n1 sin(q1)= n2 sin(q2)
Flat Lens:
Window
Spherical Lenses:
Concave or Convex
Absorption
Physics 102: Lecture 17, Slide 2

3. Concave Mirror Principal Rays
1) Parallel to principal axis reflects through f.
2) Through f, reflects parallel to principal axis.
3) Through center. #3 #2 O #1 f c
Image is (in this case):
Real or Imaginary
Inverted or Upright
Reduced or Enlarged
Comment that this is “weird” to have a real image.
**Every other ray from object tip which hits mirror will reflect through image tip
Physics 102: Lecture 17, Slide 3

4. Preflight 17.1 Which ray is NOT correct? 1) 2) 3)
p.a. 1) R f 2) 3)
Physics 102: Lecture 17, Slide 4

5. Preflight 17.1 Which ray is NOT correct?
Ray through center should reflect back on self.
p.a. 1) R f 2) 3)
Physics 102: Lecture 17, Slide 5

6. Mirror Equation do O f c do = distance object is from mirror: I di
Positive: object _______ of mirror
Negative: object _______ mirror di
di = distance image is from mirror:
Positive: _______ image (__________ of mirror)
Negative: _______ image (__________ mirror)
f = focal length mirror:
Positive: _________ mirror
Negative: _________ mirror (coming soon)
Physics 102: Lecture 17, Slide 6

7. Mirror Equation do O f c do = distance object is from mirror: I di
Positive: object in front of mirror
Negative: object behind mirror di
di = distance image is from mirror:
Positive: inverted image (in front of mirror)
Negative: upright image (behind mirror)
f = focal length mirror:
Positive: concave mirror
Negative: convex mirror (coming soon)
Physics 102: Lecture 17, Slide 7

8. Preflight 17.3 ACT: Concave Mirror
The image produced by a concave mirror of a real object is:
Always Real
Always Virtual
Sometimes Real, Sometimes Virtual
ACT: Concave Mirror
Where in front of a concave mirror should you place
an object so that the image is virtual?
1) Close to mirror 3) Either close or far
2) Far from mirror 4) Not Possible
Physics 102: Lecture 17, Slide 8

9. Preflight 17.3
The image produced by a concave mirror of a real object is:
Always Real
Always Virtual
Sometimes Real, Sometimes Virtual
Concave mirror: f > 0
Real Object means in front of mirror: do > 0
Mirror Equation:
di can be negative or positive!

10. ACT: Concave Mirror
Where in front of a concave mirror should you place an object so that the image is virtual?
Mirror Equation:
Close to mirror
Far from mirror
Either close or far
Not Possible
Concave mirror: f > 0
Object in front of mirror: do > 0
Virtual image means behind mirror: di < 0
When do < f then di <0 : virtual image.
Physics 102: Lecture 17, Slide 10

11. Magnification Equationdo O do ho
Angle of incidence
ho = height of object:
Positive: di hi
Angle of reflection I
hi = height of image:
Positive:
Negative:
m = magnification:
Positive / Negative: same as for hi
< 1:
> 1: di
Physics 102: Lecture 17, Slide 11

12. Magnification Equationdo O do ho
Angle of incidence
ho = height of object:
Positive: always q di hi
Angle of reflection I
hi = height of image:
Positive: image is upright
Negative: image is inverted
m = magnification:
Positive / Negative: same as for hi
< 1: image is reduced
> 1: image is enlarged di
Physics 102: Lecture 17, Slide 12

13. Solving Equations Preflight 17.2 Example
A candle is placed 6 cm in front of a concave mirror with focal length f=2 cm. Determine the image location.
Preflight 17.2
Compared to the candle, the image will be:
Larger
Smaller
Same Size
p.a. R f
Physics 102: Lecture 17, Slide 13

14. Solving Equations Preflight 17.2 Example
A candle is placed 6 cm in front of a concave mirror with focal length f=2 cm. Determine the image location.
di = + 3 cm (in front of mirror)
Real Image!
Preflight 17.2
Compared to the candle, the image will be:
Larger
Smaller
Same Size
p.a. R f
Physics 102: Lecture 17, Slide 14

15. ACT: Magnification
A 4 inch arrow pointing down is placed in front of a mirror that creates an image with a magnification of –2.
What is the size of the image?
2 inches
4 inches
8 inches
4 inches
What direction will the image arrow point?
Up 2) Down
Physics 102: Lecture 17, Slide 15

16. ACT: Magnification
A 4 inch arrow pointing down is placed in front of a mirror that creates an image with a magnification of –2.
What is the size of the image?
2 inches
4 inches
8 inches
4 inches
Magnitude gives us size.
What direction will the image arrow point?
Up 2) Down
(-) sign tells us it’s inverted from object
Physics 102: Lecture 17, Slide 16

17. 3 Cases for Concave Mirrors
Object
Image
Upright
Enlarged
Virtual
Inside F C F
Object
Image
Inverted
Enlarged
Real
Between C&F C F
Object
Image
Inverted
Reduced
Real
Past C
Physics 102: Lecture 17, Slide 17

18. Convex Mirror Rays
1) Parallel to principal axis reflects ______________.
2) Through f, reflects ______________________.
3) Through center.
Complete the rays! #1 O #2 #3 c
Make this mirror almost flat so that it is like your rearview mirror f
Image is:
Virtual or Real
Upright or Inverted
Reduced or Enlarged
(always true for convex mirrors!)
Physics 102: Lecture 17, Slide 18

19. Convex Mirror Rays 1) Parallel to principal axis reflects through f.
2) Through f, reflects parallel to principal axis.
3) Through center. #1 O #2 #3 I f c
Make this mirror almost flat so that it is like your rearview mirror
Image is:
Virtual (light rays don’t really cross)
Upright (same direction as object)
Reduced (smaller than object)
(always true for convex mirrors!):
Physics 102: Lecture 17, Slide 19

20. Solving Equations Example
A candle is placed 6 cm in front of a convex mirror with focal length f=-3 cm. Determine the image location.
Determine the magnification of the candle.
If the candle is 9 cm tall, how tall does the image candle appear to be?
Physics 102: Lecture 17, Slide 20

21. Solving Equations Example
A candle is placed 6 cm in front of a convex mirror with focal length f=-3 cm. Determine the image location.
Determine the magnification of the candle.
If the candle is 9 cm tall, how tall does the image candle appear to be?
di = - 2 cm (behind mirror)
Virtual Image!
m = + 1/3
hi = + 3 cm
Image is Upright!
Physics 102: Lecture 17, Slide 21

22. Preflight 17.4
Where should you place an object in front of a convex mirror to produce a real image?
Object close to mirror
Object far from mirror
Either close or far
You can’t
Physics 102: Lecture 17, Slide 22

23. Preflight 17.4
Where should you place an object in front of a convex mirror to produce a real image?
Mirror Equation:
Object close to mirror
Object far from mirror
Either close or far
You can’t
di is negative!
f is negative
do is positive
Convex mirror: f < 0
Object in front of mirror: do > 0
Real image means di > 0
Physics 102: Lecture 17, Slide 23

24. Mirror Summary Angle of incidence = Angle of Reflection Principal Rays
Parallel to P.A.: Reflects through focus
Through focus: Reflects parallel to P.A.
Through center: Reflects back on self
|f| = R/2
Physics 102: Lecture 17, Slide 24

25. Light Doesn’t Just Bounce It Also Refracts!
Reflected: Bounces (Mirrors!)
qi = qr qi qr
Refracted: Bends (Lenses!)
n1 sin(q1)= n2 sin(q2) q1 q2 n2 n1
Physics 102: Lecture 17, Slide 25

26. Index of Refraction
186,000 miles/second: 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!
Physics 102: Lecture 17, Slide 26

27. Snell’s Law Preflight 17.6 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)
Preflight 17.6 n1 q1
1) n1 > n2
2) n1 = n2
3) n1 < n2 q2 n2
Compare n1 to n2.
Physics 102: Lecture 17, Slide 27

28. Snell’s Law Preflight 17.6 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)
Preflight 17.6 n1 q1
1) n1 > n2
2) n1 = n2
3) n1 < n2
q1 < q2 q2 n2
sinq1 < sinq2
n1 > n2
Compare n1 to n2.
Physics 102: Lecture 17, Slide 28

29. Snell’s Law Practice Example
Usually, there is both reflection and refraction!
A ray of light traveling through the air (n=1) is incident on water (n=1.33). Part of the beam is reflected at an angle qr = 60. The other part of the beam is refracted. What is q2? 1 r n1 n2
normal
Physics 102: Lecture 17, Slide 29

30. Snell’s Law Practice Example
Usually, there is both reflection and refraction!
A ray of light traveling through the air (n=1) is incident on water (n=1.33). Part of the beam is reflected at an angle qr = 60. The other part of the beam is refracted. What is q2?
q1 =qr =60 1 r
sin(60) = 1.33 sin(q2) n1
q2 = 40.6 degrees n2
normal
Physics 102: Lecture 17, Slide 30

31. Apparent Depth d d
Apparent depth: n2 n1
apparent fish
actual fish 50

32. See you later! Read Sections 23.3 - 23.5, 23.9
Physics 102: Lecture 17, Slide 32