refraction

• the colours that make up white light have different refractive indices so they refract by slightly different amounts
• the ray of light that enters a material is called the incident ray
• the angle of light which enters a material is called the incident angle
• the ray of light which is refracted as it leaves a material is called the refracted ray
• the angle of light which is refracted as it leaves a material is called the refracted angle
• when light pass from a less dense to a more dense material, it slows down and bends (refracts) towards the normal. this means the angle of refraction is always less than the angle of incidence
• when light moves from a more dense to a less dense material, it speeds up and bends away from the normal. This means the angle of refraction is always greater than the angle of incidence
• refraction has two effects


  the speed of the waves changes

  if the ray of light enters the material at an angle to the normal then it will change direction
• an increase in speed means the ray of light will refract away from the normal
• a decrease in speed means the ray of light will refract towards the normal
• the frequency of the wave remains the same and is determined by the source
• absolute refractive indices


  these are the ratios of the sine of the angle in a vacuum, not air, to the sine of the angle in the medium
• the absolute refractive index is always a value greater than or equal to 1
• all angles are measured from the normal
• normal is drawn at 90 degrees
• absolute refractive index of air

  1
• the refractive index of a medium depends on the frequency (colour) of the incident light
• since violet is refracted more than red it follows that the refractive index for violet light must be greater than the refractive index for red light
• critical angle

  the angle of incidence which produces an angle of refraction of 90 degrees
• if the light is totally internally reflected, the angle of reflection is equal to the angle of incidence according to the law of reflection
• if the light is incident at an angle less than the critical angle it will escape from the prism and the angle will be given using Snell's law.