Phy 1112K, Spring, 2004, Final Name:, Number

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1 Phy K, Spring, 004, Final Name:, Number Multiple Choice ( point each) c. The primary coil of a transformer has 00 turns and its secondary coil has 400 turns. If the ac current in the secondary coil is A, what is the current in its primary coil? a. A b. 8 A c. / A d. 4 A c. The transverse nature of electromagnetic waves is apparent because a. they are electric in nature. b. they are magnetic in nature. c. they can be polarized. d. they are electromagnetic in nature. d 3. A person approaches a plane mirror at a speed of 4 m/s. How fast does he approach his image? a. 4 m/s b. 5 m/s c. 6 m/s d. 8 m/s b 4. How tall must a plane mirror be in order for you to be able to see your full image in it? a. the same height as you. b. half of your height. c. 3/4 of your height. d. twice your height. b 5. When the reflected light is totally polarized, the angle between the reflected ray and the refracted ray is a. 80 o b. 90 o c. 45 o d. 60 o b 6. Which of the following is a correct statement? For a real object, a. a concave mirror always produces a real image. b. a convex mirror always produces a virtual image. c. a concave mirror always produces a virtual image. d. a convex mirror always produces a real image. c 7. Which of the following is a correct statement? For a real object, a. a converging lens always produces a real image. b. a converging lens always produces a virtual image. c. a diverging lens always produces a virtual image. d. a diverging lens always produces a real image. b 8. d 9. When light passes from air to glass, it a. bends toward the normal without changing speed. b. bends toward the normal and slows down. c. bends toward the normal and speeds up. d. bends away from the normal and slows down. The lens in a person's eye is too thick. The condition that this person has is called a. farsightedness b. astigmatism c. chromatic aberration d. nearsightedness

2 a 0. In the two-slit experiment, for the condition of bright fringes, the value of m = +4 corresponds to a path difference of a. 4λ b. 3λ c. λ d. λ d. If the distance between the slits and the screen is increased in Young's two-slit experiment, which one of the following statements is true of the interference pattern? a. The distance between the minima stays the same. b. The distance between the minima decreases. c. The distance between the maxima stays the same. d. The distance between the maxima increases. b. When light travels from a smaller index of refraction towards a larger index of refraction, there is no phase change upon reflection at the boundary. a. true b. false c. it depends b 3. Which one of the following phenomena shows the wave nature of light? a. reflection b. interference c. photoelectric effect d. all of the above c 4. Which one of the following phenomena shows the particle nature of light? a. reflection b. interference c. photoelectric effect d. all of the above b 5. Modern physics considers that a. physical quantities are continuous. b. physical quantities are discrete. c. physical quantities can precisely measured simultaneously. c 6. Which one of the following is true regarding the energy of the photon? a. It depends upon the intensity. b. It depends upon its amplitude. c. It depends upon its frequency. d. It depends upon its velocity. a 7. The wavelength of a light beam is doubled. Which one of the following is correct for the momentum of photons for that light beam? a. It is halved. b. It stays the same. c. It is doubled. d. It is quadrupled. d 8. In the photoelectric experiment, the work function depends on a. the frequency of the incident light. b. the wavelength of the incident light. c. the intensity of the incident light. d. The target material. b 9. If the position of an electron is measured very precisely there is an uncertainty in measuring it a. kinetic energy. b. momentum. c. potential energy. d. None of the other answers is correct.

3 b 0. What happens to the frequency of the scattered X-rays photon in the Compton scattering experiment? a. Stays the same. b. It decreases c. Not enough information. d. It increases Problems (60 points). A step-up transformer has 5 turns on the primary coil and 500 turns on the secondary coil. If this transformer is to produce an output of 4800 V with a -ma current, what input current and voltage are needed? (7 points) I = N s 3 p Is 0 A)( 500 N = ( ) ( p 5 ) = 04. A V = V N p p s( ) V)( 5 N = (4800 s 500 ) = 40 V. Unpolarized light passes through two polarizers whose transmission axes are at an angle of 30 o with respect to each other. What fraction of the incident intensity is transmitted? (7 points) I = I0 I = Icos θ = I0 cos θ I = cos = I0 3. Draw a ray diagram for each of the following cases. (8 points) (a) An object is placed 6 cm in front a converging mirror of focal length 4 cm. (b) An object is placed 6 cm in front a converging lens of focal length 4 cm. (c) An object is placed 6 cm in front a diverging mirror of focal length 4 cm. (d) An object is placed 6 cm in front a diverging lens of focal length 4 cm. (See examples in the book) 4. A converging lens (f =.0 cm) is located 30.0 cm to the left of a diverging lens (f = -6 cm). A postage stamp is placed 36 cm to the left of the converging lens (see figure). (a) Locate the final image of the stamp relative to the diverging lens. (5 points) (b) Find the overall magnification. ( point) (c) Is the final image real or virtual? ( point) (d) With respect to the original object, is the final image upright or inverted? ( point) (e) With respect to the original object, is the final image larger or smaller? ( point) f = cm f = - 6 cm 36 cm 30 cm

4 f = cm 36 cm First, find the image distance for the first lens: f = + cm d 0 = +36 cm d i = [f x d 0 ] / [d 0 -f] = ( cm)(36 cm) / (36 - ) cm = ( x 36 / 4) cm = +8 cm The image is 8 cm to the right of the first lens. From the diagram it is obvious that the image is inverted, real and reduced in size. This first image is the object for the diverging lens which is now located cm from the first image position. X-Position of final image f = - 6 cm F X cm That is, d 0 = + cm and f = -6 cm. d i = [f x d 0 ] / [d 0 - f] = (-6 cm)( cm) / ( - -6) cm = (-6 x / 8) cm = -4 cm d i is negative so image is to the left of the diverging lens. This can be seen in the ray diagram. The final image is also inverted and virtual. (b) Overall magnification M T = M x M = (-d i / d 0 ) x (-d i / d 0 ) = (- 8 / 36) x (- -4 / ) = (-3 / ) x ( / 9) = - /6 (c) Final image is virtual. (d) Final image is inverted because the total magnification is negative. (e) Final image is reduced in size by a factor of A college professor can see objects clearly only if they are between 70 and 500 cm from his eyes. His optometrist prescribes bifocals that enable him to see distant objects clearly through the top half of the lenses and read students papers at a distance of 5 cm through the lower half. What are the powers of the top and bottom lenses? (7 points)

5 Top: d o =, d i = 500 cm = 5.0 m (image on object side). P = f = d + o d = i m B ottom : d o = 5 cm = 0.5 m, P = 0.5 m m = +.6 D. = 0.0 D. 6. Monochromatic light passes through two slits separated by mm. If the angle to the third maximum above the central is 3.5 o, what is the wavelength of the light? (7 points) λ λ 3λ θ = = = sin m ( 3) d d d 3 λ = d sin θ = ( m)sin 35. = 478 nm 3 3 d i = 70 cm = 0.70 m (image on object side). 7. A film of oil with index of refraction.5 is spread on water whose index of refraction is.33. Calculate the minimum thickness of the film for which green light of wavelength 50 nm is reflected constructively. (7 points) There is a half wavelength phase change for ray as ray is reflected at the air-gasoline interface. So t = mλ f is the condition for destructive interference between rays and. And t = (m+/)λ f is the condition for constructive interference between rays and. For constructive interference, the minimum thickness is for m = 0 So t = /λ f or t = λ f / 4 = (50 nm /.5)/4 = nm / 4 = nm

6 8. The maximum wavelength that can give photoelectrons from a platinum surface is 90 nm. When light of wavelength 40 nm shines on the surface what is the maximum speed of the ejected electrons.(mass of electron = 9.x0-3 kg) (8 points) 40( nm ev) W0 = = 653. ev 90( nm) 40( nm ev) E = hc/ λ = = 886. ev 90( nm) ( KE) max = E W0 = 886. ev 653. ev = 33. ev 3 ( KE) max = mvmax = 9. 0 kg v max =. 33 ev kg 9 j ev 5 = 9. 0 m/ s