Intensity and polarisationEdexcel A-Level Physics: Subtopic test

1. 1
   A small lamp in a large dark hall behaves as a point source. It radiates 12 W of light uniformly in all directions, and none of the light is absorbed by the air.

   (a)

   What is the intensity of the light at a distance of 2.0 m from the lamp?

   [1 mark]

   • A0.48 W m⁻²
   • B0.24 W m⁻²
   • C0.060 W m⁻²
   • D0.95 W m⁻²

   (b)

   The intensity at 2.0 m is 0.24 W m⁻². What is the intensity at 6.0 m from the lamp?

   [1 mark]

   • A0.027 W m⁻²
   • B0.080 W m⁻²
   • C0.72 W m⁻²
   • D0.12 W m⁻²

   (c)

   State what is meant by the intensity of a wave and explain why the intensity of the light falls as the distance from the lamp increases.

   [2 marks]

   Total for question 1: 4 marks

2. 2
   An engineer tests solar panels outdoors. At midday the intensity of sunlight, measured perpendicular to the surface of the panels, is 1000 W m⁻². The first panel has an area of 1.6 m² and an efficiency of 18%, meaning that 18% of the incident power is converted to electrical power.

   (a)

   Which gives the SI base units of intensity?

   [1 mark]

   • Akg m² s⁻³
   • Bkg m s⁻²
   • Ckg s⁻³
   • Dkg m⁻² s⁻³

   (b)

   What is the electrical power output of the first panel?

   [1 mark]

   • A1600 W
   • B180 W
   • C8900 W
   • D288 W

   (c)

   A second panel has an efficiency of 15% and must also deliver 288 W in the same sunlight. Calculate the area it needs.

   [2 marks]

   Total for question 2: 4 marks

3. 3
   A fisherman wears polarising sunglasses. Light reflected from the surface of a calm lake is partially plane polarised with its oscillations mainly horizontal. The lenses of the sunglasses have a vertical transmission axis.

   (a)

   Explain what is meant by plane polarisation and why sound waves cannot be plane polarised.

   [3 marks]

   (b)

   Explain why the sunglasses reduce the glare from the lake, and why the glare increases when the fisherman tilts his head sideways through 90°.

   [4 marks]

   Total for question 3: 7 marks

4. 4
   A radio engineer models a transmitter as a point source radiating 50 W of radio waves uniformly in all directions, with no absorption by the air. The transmitter uses a single vertical rod aerial. A receiving rod aerial with an effective area of 0.020 m² faces the transmitter and is connected to a meter that shows the strength of the received signal.

   (a)

   Calculate the intensity of the radio waves at a distance of 500 m from the transmitter and the power received by the aerial there. Then deduce the power received at a distance of 1500 m and state two assumptions you have made.

   [6 marks]

   (b)

   The engineer rotates the receiving aerial slowly through 360° about the line joining it to the transmitter and watches the meter. Describe and explain what she observes, and explain what this shows about radio waves. State what would be observed if a microphone were rotated in the same way in front of a loudspeaker.

   [6 marks]

   Total for question 4: 12 marks

End of questions