Physics 116

Session 22 Review: ray optics

Nov 4, 2011

R. J. Wilkes

Email: ph116@u.washington.edu

•! Exam 2 is Monday! •! All multiple choice, similar to HW problems, same format as Exam 1

•! YOU must bring a standard mark-sense (bubble) sheet

•! Closed book/notes, formula page provided

•! You provide: bubble sheet, pencils, calculator, brain

•! Covers material in Chs. 25 - 27 (not including material from ch. 28 yesterday)

•! Please take alternate seats for the exam (empty seat between students) - Latecomers can sit at tables in front or back

•! Today = review and ”practice exam” = examples similar to exam questions

•! Formula sheet is included in today’s slides and also posted on website

Announcements

3

Today

Lecture Schedule (up to exam 2)

The story so far – stuff you have learned about in 116

(since exam 1):

•!Electromagnetic Waves •!Energy and speed of EM waves

•!Polarization

•!Reflection and plane mirrors •!Ray tracing

•!Spherical mirrors •!Mirror equation

•!Refraction

•!Total internal reflection •!Thin-lens equation

•!Rainbows & related phenomena •!Human vision; the eye

•!Optical Instruments

•! Magnifying glass •! Microscope

•! Telescope •! Camera / projector

•! Formula page

1) The frequency of a microwave signal is 9.76 GHz. What is its wavelength in

meters?

A)! 3.07 ! 10-2 m B)! 2.07 ! 10-2 m

C) 1.07 ! 10-2 m

D) 5.07 ! 10-2 m E) 4.07 ! 10-2 m

2) The rms value of the electric field component of an electromagnetic wave is

27.2 N/C. What is the maximum value of the electric field?

A) 38.5 N/C B) 19.2 N/C

C) 27.2 N/C

D) 47.4 N/C E) 52.8 N/C

Answer: A

Answer: A

Answer: A ! = c /f

3) A 60.0 W light bulb radiates light uniformly in all different

directions. What is the average intensity of the light bulb at a distance of 0.400 m from the bulb?

A) 14.9 W/m2

B) 37.2 W/m2

C) 27.4 W/m2 D) 11.9 W/m2

E) 29.8 W/m2

oops, the problem actually posted was:

I =watts

m2

!

"#$

%&=

60W

4'R2!

"#$

%&

Answer: E

I ==60W

4!R2

"

#$%

&'= c

B2

µ0

"

#$

%

&'( B =

60W( )(4! )10*7T * m / A

4! 0.7m( )23)10

8m / s

=60kg * m2

/ s3( )(4! )10

*7kg / s

2A( )"

#%& m / A( )

4! 0.49m2( )3)10

8m / s

=60 )10

*7( )0.49( )3)10

8kg

2/ A

2s

4( )"#

%& = 2.02 ) 10

-7 T

Notice: since 60W is the time-

averaged power radiated by the bulb, the B value we find

here is the RMS B, not the maximum B value

4) If the magnification of a mirror is negative, which of the

following is correct?

A)! The image is upright and the mirror is convex.

B) The image is inverted and the mirror is convex. C) The image is inverted and the mirror is concave.

D) All of the previous answers can be correct. E) None of the previous answers is correct.

Answer: C negative M means inverted

Image in convex mirror is always upright

5) An object is located 3.8 m in front of a plane mirror. The

image formed by the mirror appears to be

A)! 1.9 m in front of the mirror. B) on the mirror's surface.

C) 1.9 m behind the mirror's surface.

D) 3.8 m in front of the mirror. E) 3.8 m behind the mirror's surface.

6) An object is 12 cm in front of a concave mirror, and the image is 3.0 cm in front of the mirror. What is the focal length of the

mirror? A) 0.25 cm

B) 15 cm

C) 4.0 cm D) 2.4 cm

E) 1.3 cm

Answer: E

f=infinity for plane mirror

di = -do so equal distance behind

Answer: D

7) At what angle must a fish underwater look in order to see a

fisherman at the shore? (Note: n for water is 1.33.)

A)! 41.2° B) 48.8°

C) 60.0°

D) 36.9° E) 90.0°

Answer: B

8) A ray of light consisting of blue light (wavelength 480 nm) and red

light (wavelength 670 nm) is incident on a thick piece of glass at 80°. What is the angular separation between the refracted red and

refracted blue beams while they are in the glass? (The respective indices of refraction for the blue light and the red light are 1.4636 and

1.4561.)

A)! 0.277°

B) 0.455° C) 0.330°

D) 0.155°

E) 0.341°

sin!2=

n1

n2

sin!1

sin!2"BLUE

=1

1.4636 sin80! = 0.6729 #!

2"BLUE= 42.2887!

sin!2"RED

=1

1.4561sin80! = 0.6763#!

2"RED= 42.5577!

$! = 0.27

(roundoff may make your answer a bit different - choose the closest answer)

Answer: A

In class today I had the numbers

reversed, sorry

9) A nearsighted person has a far point that is 4.2 m from his eyes. What focal length

lenses in diopters he must use in his contacts to allow him to focus on distant objects?

A)! 0.48 diopters

B) -0.48 diopters

C) -0.36 diopters

D) -0.24 diopters

E) 0.24 diopters

We want a lens that makes an object at infinity appear to be

4.2 m away, so do=infinity, dI = - 4.2m

(minus sign because image is on same side of lens as object)

10) The total length of a telescope is 2.20 m and the focal length of

the eyepiece is 1.9 cm. What is the focal length of the objective?

A) 41 cm

B) 45 cm

C) 111 cm

D) 218 cm

E) 16 cm

Answer: D

See you Monday!

Please take alternate seats for the exam (empty

seat between students)

Latecomers can sit at tables in front or back