Applied Science Measuring the Critical Angle

Method of measuring the Critical Angle.

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This experiment will show what happens to light incident on a glass/air surface when the angle of incidence gradually increases. 


  • Ray bos with single slit 
  • Power supply 
  • Semicircular glass or Perspex block
  • A4 Sheet of plain paper 
  • Protractor 
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1) Place the semicircular block in the centre of a piece of plain paper and draw around it.

2) Mark the mid-point of the straight side (M).

3) Draw a line at M at 90 dregees to the straight side. This is called the normal. 

4) Draw a line at an angle of 30 degrees to the normal. This is the incident ray. 

5) Use a ray box with a single slit to shine a ray of light along this line 

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6) Mark the position of the refracted ray.

7) Measure the angle of refraction. 

8) Slowly move the ray box between angles of incidence from 30 to 60 degrees. 

9) Measure the angle between the ray incidence and the normal inside the block for which the angle of refraction is 90 degrees. This is the critical angle. Note its value. 

10) You can use the formula n= 1/sinC to find the refractive index of the block. 

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