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Intensity and refractionEdexcel International A Level Physics: Flashcards

What these 12 flashcards ask

  • Define intensity of radiation.
  • Write the equation for the intensity at distance r from a point source of power P.
  • How does intensity change when the distance from a point source is tripled?
  • Write the equation for refractive index.
  • Why is the refractive index of a material always greater than 1?
  • State Snell's law at the boundary between media 1 and 2.
  • From which line are angles of incidence and refraction measured?
  • What happens to a ray entering a medium of higher refractive index?
  • What happens to the direction of a ray that hits an interface along the normal?
  • Why does a ray emerge parallel to its original direction after a parallel-sided block?
  • In the refractive index experiment, what is plotted and what does the gradient give?
  • What value of refractive index is used for air?

Exam questions on Intensity and refraction

  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.
    Calculate the distance from the lamp at which the intensity is 0.20 W m⁻².2 marks
  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.
    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
  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.
    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
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Written by the Exaim team, led by Shaun Daswani (Head of Upper Secondary, Improve ME Institute; MSc Financial Mathematics, Imperial College London; BSc, UCL) and Jason Daswani (operational lead, Improve ME Institute; LSE).