• Light travelling from air into a medium has practically the same refractive index as from a vacuum to that medium, so we ignore the difference.
• Refractive index from air to water is simply called the refractive index of water. Likewise the refractive index from air to glass is simply called the refractive index of glass.The same is true for air to any other medium.
• The refractive index of any medium is always greater than 1. This is because light always bends towards the normal when travelling from a vacuum to any other medium.The refractive indices of some common media are given in Table 13.1.
• The bigger the refractive index, the greater the ‘amount of bend’. • Refractive index has no unit associated with it.
SUBSTANCE REFRACTIVE INDEX Vacuum
1 Air Water Glass Diamond
1.0003 1 1.33 4/3
1.5 3/2 2.4 12/5 Table 13.1: Refractive indices for common media MANDATORY EXPERIMENT 16
Verification of Snell’s Law of Refraction Apparatus
Rectangular prism, 4 pins, protractor, pencil and paper
Procedure Place the prism on the page and trace its outline. Remove the prism and draw a normal through one side of the outline. Draw a line representing the incident ray, as illustrated.
Measure the angle of incidence (i) with the protractor.
Place two pins (A and B) on the incident ray. Reposition the prism as before.The point where the incident ray enters the prism is called the point of incidence.
Point of incidence
Emergent ray Refracted ray
i D Pins C r Normal Incident ray B Pins Glass block
Paper Point of emergence
Fig 13.3
View the pins through the prism as illustrated in Fig 13.3. Position two pins (C and D) in line with the images of A and B as seen through the prism.
Remove prism.The line containing C and D is called the emergent ray. The point where the emergent ray leaves the prism is called the point of emergence.
Draw a line from the point of emergence to the point of incidence, as illustrated. This line represents the refracted ray.
Measure the angle of refraction (r) with the protractor. Repeat the process using different angles of incidence so as to get six or seven readings.
