Activity 2.2 - Investigating Snell’s law

• Understanding that Snell’s Law describes the change in direction of light moving from one medium to another
• Understanding that at some angle, light will totally reflect from the interface when travelling from high to low refractive index

The students should be given the materials and asked to discuss how they might go about studying what happens to the direction of light when a laser passes from air into glass and back into air. The students should examine questions such as:

• How will I determine the direction that light takes through the glass?
• How will I measure and quantify the direction into and through the glass?
• How will the measurement be standardised so that the results from different people can be compared and combined?
• Which parameters of the experiment should I keep constant and which should I alter?
• Once I take my data, how will I tabulate and graph it to investigate the relationship between the direction of light into and through the glass?

Next, the students perform the experiment they have devised and report their results in Worksheet 2.2. If they have standardised their measurements, the data can be entered into a software package such as Excel and projected on a screen for the students to see. A plot of ‘angle of incidence’ versus ‘angle of refraction’ will not yield a straight-line graph and the teacher should then show the students how this changes if the Sine (or Cosine, depending on whether the students have measured the angle from the normal or surface of the glass) of the angles are plotted.

The students can then use the slope of this line to extract the ‘refractive index of glass’ (assuming the refractive index of air is 1) and hence extract Snell’s Law.

Finally the teacher should check if the data contains evidence that at some angle the light did not escape from the block of glass, and whether any student reports this observation. If not then the students should be instructed to check this, and discuss why they failed to discover this phenomenon.

• What property of the light changes to cause the change in angle as it passes from one medium to another?
• Is light reflected from the surface of the glass when the beam travels from air to glass? Does this reflected beam change in intensity as the angle of light onto the surface changes?
• How might we use the total internal reflection of light to transmit a laser beam?