Chapter 23 Quiz - Department of Physics › courses › phy2054 › s08 › lectures ›...

PHY2054: Chapter 23 1

Chapter 23 QuizWhich of the following is not a principal ray?

(1) 1(2) 2(3) 3(4) 4(5) All are principal rays


Concave and Convex Mirrors

2Rf =


Images from Mirrors

fC h qMh p′

≡ = −

1 1 1p q f+ =Location

Size

q > 0 along reflected ray


Mirror Example

1 1 1 133.380 50

qq

+ = ⇒ =

fC

1

2

3

2 cm tall object (h = 2), 80 cm from mirror (p = 80)Mirror: 100 cm radius of curvature concave towards object (R = +100)

( )

133.3 1.6780

1.67 3.33

qMp

h h

= − = − = −

′ = − = −

Roughly agrees with ray diagram (good to check it)


Images Formed by Refraction

1

2

n qhMh n p′

= = −

1 2 2 1n n n np q R

−+ =


Image from Flat Refractive SurfaceR = ∞

1

21n qM

n p= − = +

1 2 2

10n n nq p

p q n+ = ⇒ = −

Same size virtual image!


Example: Image in Pool

1 0.751.333

q p p= − ≅ −

n1 = 1.333n2 = 1.0

2

1

nq pn

= −

So depth of image is about3/4 depth of object


Atmospheric Refraction

Refraction in atmosphere makes sun always appear higher in skyCan see the sun even when it is below the horizon!


Atmospheric Refraction Causes Various Mirages


Approaching Car on Hot Road


Floating Iceberg! Cold Air, Warm Water


“Squashed” Sun Setting Over Ocean


Thin Lenses


Thin Lenses

ff

Converging LensRays parallel to axis refractand pass through focal point

f > 0

ff

Diverging LensRays parallel to axis refract and

appear to emerge from focal pointf < 0


Location and Size of Image (Recall Mirror)

h qMh p′

≡ = −1 1 1p q f+ =


Finding Image from Lens: Example

1 1 1 15cm30 10

qq

+ = ⇒ = +f = 10 cmp = 30 cmh = 4 cm

15 0.530

2cm

qMp

h

= − = − = −

′ = −


Principal Ray Diagrams for Lenses

q > 0


Principal Ray Diagrams for Lenses (cont)

q < 0


Calculating Focal Length for Thin Lenses

( )1 2

1 1 11nf R R

⎛ ⎞= − −⎜ ⎟

⎝ ⎠Lensmaker’s equation

Sign convention

R < 0 R > 0


Calculating Focal Length Example

1 1 10.5 0.110 10f⎛ ⎞= − =⎜ ⎟−⎝ ⎠

1 1 10.5 0.110 10f⎛ ⎞= − =⎜ ⎟−⎝ ⎠

Let n = 1.5, R = 10

1 1 10.5 0.0510f⎛ ⎞= − =⎜ ⎟

∞⎝ ⎠

So f1 = f2/21 has twice the “power” of 2


Quiz on LensesAll the lenses below have either flat sides or radius of curvature R. Rank in descending order the value of 1/f (lens power), i.e. from most positive to most negative

(1) A, C, B, E, D(2) C, A, D, B, E(3) B, A, C, E, D(4) D, E, B, C, A(5) C, A, E, D, B

A B C D E

Flat

( )1 2

1 1 11nf R R

⎛ ⎞= − −⎜ ⎟

⎝ ⎠Each term adds to lens power


Quiz on MirrorsAn upright object is located in front of a convex mirror a distance greater than the focal length. The image formed by the mirror is:

(1) real, inverted, and smaller than the object (2) virtual, inverted, and larger than the object (3) real, inverted, and larger than the object (4) real, erect, and larger than the object (5) virtual, erect, and smaller than the object

1 1 1q f p= − f < 0, so q < 0

M = −q/p < +1

Chapter 23 Quiz - Department of Physics › courses › phy2054 › s08 › lectures ›...

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