Intensity and refractionEdexcel International A Level Physics: Subtopic test

1. 1
   A small lamp radiates light uniformly in all directions with a total power of 40 W. Treat the lamp as a point source and assume that no light is absorbed or scattered by the air.

   (a)

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

   [1 mark]

   • A0.20 W m⁻²
   • B1.6 W m⁻²
   • C0.80 W m⁻²
   • D3.2 W m⁻²

   (b)

   The distance from the lamp is doubled. What happens to the intensity?

   [1 mark]

   • AIt falls to one quarter of its original value
   • BIt falls to one half of its original value
   • CIt falls to one eighth of its original value
   • DIt increases to four times its original value

   (c)

   Calculate the distance from the lamp at which the intensity is 0.20 W m⁻².

   [2 marks]

   Total for question 1: 4 marks

2. 2
   A ray of light in air strikes the flat surface of a glass block at an angle of incidence of 40°. The refractive index of the glass is 1.50 and the speed of light in air is 3.00 × 10⁸ m s⁻¹.

   (a)

   What is the angle of refraction in the glass?

   [1 mark]

   • A27°
   • B40°
   • C75°
   • D25°

   (b)

   What is the speed of light in the glass?

   [1 mark]

   • A4.50 × 10⁸ m s⁻¹
   • B2.00 × 10⁸ m s⁻¹
   • C3.00 × 10⁸ m s⁻¹
   • D1.33 × 10⁸ m s⁻¹

   (c)

   The ray leaves the glass and enters water of refractive index 1.33 at the interface at an angle of incidence of 30°. Calculate the angle of refraction in the water.

   [2 marks]

   Total for question 2: 4 marks

3. 3
   In Core Practical work a student measures the refractive index of a rectangular block of transparent plastic. She uses a ray box to send a narrow ray into the block at different angles of incidence i and measures the angle of refraction r each time. She plots sin i on the y-axis against sin r on the x-axis and obtains a straight line through the origin with gradient 1.49. Take the speed of light in air as 3.00 × 10⁸ m s⁻¹.

   (a)

   Describe how she should carry out the experiment to obtain a suitable set of measurements of i and r, and how she should use them to find the refractive index.

   [3 marks]

   (b)

   Explain why the gradient of her graph gives the refractive index of the plastic, and use it to calculate the speed of light in the plastic and the angle of refraction when the angle of incidence is 55°.

   [4 marks]

   Total for question 3: 7 marks

4. 4
   A lighthouse lamp radiates light uniformly in all directions with a total power of 800 W, which can be treated as a point source. The lamp sits behind a flat glass window with parallel faces and refractive index 1.52. A ship is 6.0 km from the lighthouse and carries a light sensor of area 5.0 × 10⁻⁴ m² facing the lighthouse.

   (a)

   Calculate the intensity of the light at the ship and the power received by the sensor. Explain what would happen to these values if the ship moved to a distance of 12 km, and state one reason why the real values would be smaller than your calculated values.

   [6 marks]

   (b)

   A ray from the lamp strikes the window at an angle of incidence of 35°. Calculate the angle of refraction in the glass and the speed of light in the glass. Explain why the ray leaves the window parallel to its original direction.

   [6 marks]

   Total for question 4: 12 marks

End of questions