Physics 112 / Exam 3 / Summer 2014
  1. How far are you from your image when you stand 0.75 m in front of a vertical plane mirror?

A)0.75 m

B)1.5 m

C)3.0 m

D)3.75m

E)4.5m

Solution:

Distance to the image in the plane mirror is equal to the distance to the object (). So, the distance between the image and object is twice the distance to the object:

  1. The image formed by a plane mirror of a real object is always

A)Real, erect, and smaller than the object

B)Real, inverted, and the same size as the object

C)Real, erect, and the same size as the object

D)Virtual, inverted, and larger than the object

E)Virtual, erect and the same size as the object

Solution:

The image formed by a plane mirror of a real object is always: virtual (behind the mirror), erect (upright) and the same size as the object.

  1. The critical angle for a substance is measured at 53.7°. Light enters from air at the angle 45.0° to the normal. At what angle will it continue?

A)34.7°

B)45.0°

C)53.7°

D)58.2°

E)It will not continue, but be totally reflected.

Solution:

For air index of refraction is approximately 1.

1)

2)

  1. A concave mirror with a radius of 20 cm creates a real image 30 cm from the mirror. What is the object distance?

A) 30cm

B) 20 cm

C) 15 cm

D) 7.5 cm

E) 5.0 cm

Solution:

  1. When a person stands 40 cm in front of a cosmetic mirror (concave mirror), the erect image is twice the size of the object. What is the focal length of the mirror?

A)27 cm

B)40 cm

C)80 cm

D)100 cm

E)160 cm

Solution:

  1. An object is placed 40 cm in front of a 20 cm focal length converging lens. How far is the image of this object from the lens?

A)40 cm

B)30 cm

C)20 cm

D)13 cm

E)8 cm

Solution:

  1. Radio waves are diffracted by large objects such as buildings, whereas light is not noticeably diffracted. Why is this?

A)Radio waves are unpolarized, whereas light is plane polarized

B)Radio waves are polarized, whereas light is plane unpolarized

C)Radio waves are coherent and light is usually not coherent

D)The wavelength of light is much greater than the wavelength of radio waves

E)The wavelength of light is much smaller than the wavelength of radio waves

Solution:

Radio waves are diffracted by large objects such as buildings because the wavelength of radio waves is substantially larger than buildings.

Light is not noticeably diffracted by large objects such as buildings because the wavelength of light is much smaller than these objects.

  1. A beam of light (f = 5.0 × 1014 Hz) enters a piece of glass (n = 1.5). What is the frequency of the light while it is in the glass?

A)5.0 × 1014 Hz

B)7.5 × 1014 Hz

C)3.3 × 1014 Hz

D)1.0 × 1014 Hz

E)2.5 × 1014 Hz

Solution:

When light is entering glass its frequency is not changing.

  1. In a Young's double slit experiment, if the separation between the two slits is 0.050 mm and the distance from the slits to a screen is 2.5 m, find the spacing between the first-order and second-order bright fringes for light with wavelength of 600 nm.

A)1.5 cm

B)3.0 cm

C)4.5 cm

D)6.0 cm

E)7.5 cm

Solution:

For Young's double slit experiment: . For small angles:

  1. In a single slit diffraction experiment, if the width of the slit increases, what happens to the width of the central maximum on a screen?

A)It increases.

B)It decreases.

C)It remains the same.

D)It depends from the distance to the screen.

E)There is not enough information to determine.

Solution:

.

If the width, D of the slit increases, than decreases, so the width of the central maximum on a screen decreases.

  1. In order to obtain a good single slit diffraction pattern, the slit width could be:

A)/100

B)/10

C)

D)10

E)100

Solution:

If then, and one can observe several fringes on the screen.

  1. Two stars 15 light-years away are barely resolved by a 55-cm (mirror diameter) telescope. How far apart are the stars? Assume and that the resolution is limited by diffraction. (1 light-year = 9.46x1015 m)

A)1.3x107m

B)1.4x108m

C)1.5x109m

D)1.6x1010 m

E)1.7x1011 m

Solution:

  1. A diffraction grating has 5000 lines per cm. The angle between the central maximum and the fourth order maximum is 60°. What is the wavelength of the light?

A)138 nm

B)183 nm

C)367 nm

D)433 nm

E)637 nm

Solution:

  1. What is the minimum thickness of a nonreflecting film coating (n = 1.30) on a glass lens (n = 1.50) for wavelength 500 nm?

A)250 nm

B)192 nm

C)167 nm

D)96.2 nm

E)57.3 nm

Solution:

. This means that there is phase shift on both sides of the film, and for destructive interference (nonreflecting film) we have the following equation:

;

  1. An ideal polarizer is placed in a beam of unpolarized light and the intensity of the transmitted light isI. A second ideal polarizer is placed in the beam with its preferred direction rotated 40° to that of the first polarizer. What is the intensity of the beam after it has passed through both polarizers?

A)0.77I

B)0.64I

C)0.59I

D)0.41I

E)0.36I

Solution:

  1. A beam of unpolarized light in air strikes a flat piece of glass at an angle of 57.3°. If the reflected beam is completely polarized, what is the index of refraction of the glass?

A)1.50

B)1.52

C)1.54

D)1.56

E)1.58

Solution:

;

  1. A near-sighted person has a far point of 20 cm. What lens (in diopters) will allow this person to see distant objects clearly? Assume that the lens is 2.0 cm from the eye (typical for eyeglasses)

A)+ 5.6 D

B)- 5.6 D

C)+ 0.056 D

D)- 0.056 D

E)-5.0 D

Solution:

;

  1. A magnifying glass with a focal length of 8.5 cm is used to read print placed at a distance 7.5 cm. Calculate the angular magnification.

A)2.3x

B)3.3x

C)4.3x

D)5.3x

E)6.3x

Solution:

  1. A person is designing a 10X telescope. If the telescope is limited to a length of 20 cm, what is the approximate focal length of the objective?

A)16 cm

B)17 cm

C)18 cm

D)19 cm

E)20 cm

Solution:

;

  1. What power lens is needed to correct for farsightedness where the uncorrected near point is 75 cm?

A)+ 2.7 D

B)- 2.7 D

C)+ 5.3 D

D)- 5.3 D

E)+6.0 D

Solution:

;

Answer Sheet

41
B) 1.5 m / 51
D) 10
42
E) Virtual, erect and the same size as the object / 52
E) 1.7x1011 m
43
A) 34.7° / 53
D) 433 nm
44
C) 15 cm / 54
D) 96.2 nm
45
C) 80 cm / 55
C) 0.59I
46
A) 40 cm / 56
D) 1.56
47
E) The wavelength of light is much smaller than the wavelength of radio waves. / 57
B) - 5.6 D
48
A) 5.0 × 1014 Hz / 58
B) 3.3x
49
B) 3.0 cm / 59
C) 18 cm
50
B) It decreases. / 60
A) + 2.7 D

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