Light waves

Light waves

1. Refraction: the change in direction of a ray of light.

   • You can investigate refraction in glass using a rectangular glass block. By tracing the rays of light on a piece of paper, you can measure the angle of incidence (i) and refraction (r).
     • 1. Place the glass block on the white paper and trace the outline.
       • 2. Shine a single incident ray onto the block, mark its path and draw the incident ray with a ruler.
         • 3. Mark the path of the ray emerging from the block, draw ray with a ruler.
           • 4. Remove the block, connect the entry and exit points to show the path of light inside the block.
             • 5. Draw a normal at the entry point, measure the angle of incidence (i) and angle of refraction (r ) using a protractor.
               • 6. Repeat for a range of angles of incidence.
   • The ratio between the sine of the angle of incidence and the sine of the angle of refraction is constant. this refraction is called the refraction index of the material and is given the letter n. As it is a ratio, it has no units.
     • A beam of light hits a glass block. The angle of incidence angle of incidence is 55°. The angle of refraction inside the block is 33°. Calculate the refractive index.
       • Step 1: Work out the sine of angle i sin 55 = 0.819
         • Step 2: Work out the sine of angle r sin 33 = 0.545
           • Step 3: Divide sin i by sin r refractive index = sin i ÷ sin r refractive index = 0.819 ÷ 0.545 = 1.50

2. Total internal reflection: the complete reflection of a light ray reaching an interface with a less dense medium when the angle of incidence exceeds the critical angle.

   • Calculate the critical angle for red light incident on a water-air interface. The refractive index of water is 1.33 for this colour of light.

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