• Textbook sections 26-3 – 26-5, 26-8

Physics 1161: Lecture 17

Reflection & Refraction

R f1)

2)

3)

p.a.

Preflight 17.1

Which ray is NOT correct?

36%

45%

18%

R f1)

2)

3)

p.a.

Preflight 17.1

Ray through center should reflect back on self.

Which ray is NOT correct?

Preflight 17.3The image produced by a concave mirror of a real object is:

1) Always Real

2) Always Virtual

3) Sometimes Real, Sometimes Virtual 39%

30%

30%

Where in front of a concave mirror should you place an object so that the image is virtual?

1 2 3 4

0% 0%0%0%

1. Close to mirror2. Far from mirror 3. Either close or far4. Not Possible

Where in front of a concave mirror should you place an object so that the image is virtual?

1. Close to mirror2. Far from mirror 3. Either close or far4. Not Possible

Solving EquationsA candle is placed 6 cm in front of a concave mirror with focal length f=2 cm. Determine the image location.

R f

p.a.

Preflight 17.2Compared to the candle, the image will be:

• Larger

• Smaller

• Same Size

29%

12%

59%

Solving EquationsA candle is placed 6 cm in front of a concave mirror

with focal length f=2 cm. Determine the image location.1 1 1

6 cm 2 cmid di = + 3 cm (in front of mirror)

Real Image!

R f

p.a.

Preflight 17.2Compared to the candle, the image will be:

• Larger

• Smaller

• Same Size

Physics 1161: Lecture 17, Slide 9

3 Cases for Concave Mirrors

Inside F• C F• Object Image

• C F• ObjectImage

• C F• Object

Image

Between C&F

Past C

Inverted

Enlarged

Real

Upright

Enlarged

Virtual

Inverted

Reduced

Real

Solving EquationsA 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?

Solving EquationsA 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?

-2 cmm

6 cmi

o

d

d m = + 1/3

1/ 39 cm

ih hi = + 3 cm

Image is Upright!

1 1 1

6 cm 3 cmid

di = - 2 cm (behind mirror)

Virtual Image!

Preflight 17.4The image produced by a convex mirror of a real object is ?

1) Always real

2) Always virtual

3) Sometimes real and sometimes virtual

24%

61%

15%

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•

•

1 1 1

o id d f

i i

o o

h dm

h d

Physics 1161: Lecture 17, Slide 14

Indices of Refraction

Snell’s Law

n1

n2

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(1)= n2 sin(2)

1

21) n1 > n2

2) n1 = n2

3) n1 < n2

Preflight 17.6

Compare n1 to n2.

21 %

21 %

58 %

Snell’s Law

n1

n2

When light travels from one medium the speed changes. If the angle of incidence is greater than 0, the light bends. During this process, the frequency remains constant.

n1 sin(1)= n2 sin(2)

1

2

1) n1 > n2

2) n1 = n2

3) n1 < n2

1 < 2

sin1 < sin2

n1 > n2

Preflight 17.6

Compare n1 to n2.

n1

n2

Snell’s Law Practice

n1sin1 n2 sin2

norm

al

2

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 r = 60. The other part of the beam is refracted. What is 2?

1 r

Usually, there is both reflection and refraction!

n1

n2

Snell’s Law Practice

n1sin1 n2 sin2

norm

al

2

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 r = 60. The other part of the beam is refracted. What is 2?

sin(60) = 1.33 sin(2)

2 = 40.6 degrees

1 =r =

1 r

Usually, there is both reflection and refraction!

Which way should you move object so image is real and diminished?

1 2 3

33% 33%33%

1. Closer to the lens2. Farther from the lens3. A converging lens can’t

create a real, diminished image.

F

F

Object

P.A.

Which way should you move object so image is real and diminished?

1 2 3

33% 33%33%

1. Closer to the lens2. Farther from the lens3. A converging lens can’t

create a real, diminished image.

F

F

Object

P.A.

Image Object

ImageObject

Object

Image

3 Cases for Converging Lenses

This could be used as a projector. Small slide on big screen

This is a magnifying glass

This could be used in a camera. Big object on small film

UprightEnlargedVirtual

InvertedEnlargedReal

InvertedReducedReal

Inside F

Past 2F

BetweenF & 2F

1) Rays parallel to principal axis pass through focal point.

2) Rays through center of lens are not refracted.

3) Rays toward F emerge parallel to principal axis.

Diverging Lens Principal Rays

F

F

Object

P.A.

Image is (always true): Real or Imaginary

Upright or Inverted

Reduced or Enlarged

1) Rays parallel to principal axis pass through focal point.

2) Rays through center of lens are not refracted.

3) Rays toward F emerge parallel to principal axis.

Diverging Lens Principal Rays

F

F

Object

P.A.

Image is virtual, upright and reduced.

Image

Which way should you move the object to cause the image to be real?

1 2 3

33% 33%33%

1. Closer to the lens2. Farther from the lens3. Diverging lenses can’t

form real images

F

F

Object

P.A.

Which way should you move the object to cause the image to be real?

1 2 3

33% 33%33%

1. Closer to the lens2. Farther from the lens3. Diverging lenses can’t

form real images

F

F

Object

P.A.