1 of 7 CTEM 1 Which has higher frequency A Red light B Violet blue light C Same frequency D Depends on how the light is made Answer Red light has longer wavelength 700 nm Violet blue light has shorter wavelength 400 nm and therefore higher frequency f f constant c so if is smaller f must be higher CTEM 2 An EM plane wave is described by K K E E 0 sin k z t x B B0 sin k z t y E x 3 2 1 c z B y Consider 3 antennas labeled 1 2 and 3 Antenna 1 is on the z axis Antenna 2 is in the xz plane above 1 Antenna 3 is is off the z axis at the location shown Rank the antennas by average signal strength received from largest to smallest A 1 2 3 B 1 2 3 C 1 2 3 Answer 1 2 3 Phys1120 Concept Tests M Dubson University of Colorado at Boulder 2 of 7 CTEM 3 A point source of radiation emits power Po isotropically in all directions uniformly A detector of area ad is located a distance R away from the source What is the power p received by the detector A R detector ad Po ad 4 R 2 B Po a 2d R2 C Po ad R Po D Po ad R 2 E None of these Answer power received power area receiver area intensity receiver area Po ad 4 R 2 CTEM 4 Two radio dishes shaped like bowls are receiving signals from a radio station which is sending out radio waves in all directions with power P Dish 2 is twice as far away as Dish 1 but has twice the diameter Which dish receives more power Dish 2 is not in the shadow of Dish 1 A Dish 1 B Dish 2 C Both receive the same power Dish 1 Dish 2 Answer Both receive the same power Dish 2 has four times the area 2 times the diameter But dish 2 is twice as far away so it receive the power per area as dish 1 Phys1120 Concept Tests M Dubson University of Colorado at Boulder 3 of 7 Remember intensity power per area is proportional to 1 R2 so if R doubles R2 increases by a factor of 4 The power received is given by the formula in the previous concept test Po a d For dish 2 ad detector area is 4 times as large as dish 1 4 R 2 Notice that the dotted lines in the diagram above so that dish 1 and dish 2 both cover the same fraction of a big sphere centered on the transmitting tower So dish 1 and dish 2 both receive the same fraction of the total energy coming from the tower CTEM 5 A parabolic dish focuses the EM radiation from a source into a beam of constant diameter D D c R The intensity of the light in the beam falls with distance R as 1 1 A I 2 B I R R C I 1 constant R0 Answer I D Something else 1 constant R0 Phys1120 Concept Tests M Dubson University of Colorado at Boulder 4 of 7 CTEM 6 Light from a source is partially focused so that it diverges from the source uniformly over a cone of angle 200 c R 200 The intensity of the light in the beam falls with distance R as 1 1 A I 2 B I N N 2 R R 1 1 C I N N 2 D I 0 constant D Something else R R Answer I 1 The cone is a constant fraction of a sphere R2 CTEM 7 A polarized beam of light passes through three ideal polaroid filters The filters in order 1st 2nd 3rd are tilted at 0o 45o and 90o with respect to the incoming beam s axis Does any light get through the all the filters and come out the other side A Some light gets through E B No light gets through c Phys1120 Concept Tests M Dubson University of Colorado at Boulder 5 of 7 Answer some light gets through If the 2nd filter were not present then no light would get thru After the first filter the E field is unchanged E1 Eo After the 2nd filter the E field is E2 Eo cos 45o After the third filter the E field is E3 E2 cos 45o Eo cos 45o cos 45o Eo 2 The intensity of the light intensity I S is proportional to the E2 so the final intensity is Io 4 CTEM 8 An unpolarized beam of light passes through 2 polaroid filters oriented at 45o with respect to each other The intensity of the original beam is Io What is the intensity of the light coming through both filters Hints cos45o 1 2 The average of cos2 average over all values of is cos2 1 2 A 1 2 Io Io B 1 4 Io c C 1 8 Io D 1 16 Io E None Answer 1 4 Io After the first filter the intensity is I1 Io 1 2 Reason If the original beam were polarized then after passing through one filter the intensity would be I1 Io cos2 but here the original beam is unpolarized so all angles are in the beam The intensity after one filter is then I1 Io cos2 average Io 1 2 After the second filter the intensity is I2 I1 cos245 I1 1 2 Io 1 2 1 2 Io 1 4 CTEM 9 Under cover of night a Girl scout signals her friends on a distant hill by alternately placing slabs of red or green Jell O over her flashlight This signal is most accurately described as A Frequency modulation B Amplitude modulation Answer When the Girl scout signals by changing the color of the light keeping the brightness fixed this would be frequency modulation because different frequencies of visible light correspond to different colors Phys1120 Concept Tests M Dubson University of Colorado at Boulder 6 of 7 If the scout signals by alternately dimming and brightening the bulb then that would be amplitude modulation The brightness of the light is a measure of the amplitude of the electric field in the E M waves from the light bright less bright dim AM FM CTEM 10 A row of positive charges is stationary on the ground A person with a gauss meter which measures the magnetic field is running to the right along the row of charges at the same height as the charges and in front of them in the diagram below Does the person measure a non zero B field A Yes B No v Answer Yes In the frame of reference of the moving person the positive charges are moving to the left From the person point of view the positive charges constitute a current to the left Currents make B fields What is the direction of the B field which the moving observer measures A Up B Down C Forward direction D backward direction E no …
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