1. Laws of refraction

The 1st law of Refraction states the the incident ray the normal and the refracted ray are all in the same 2 D plane. For our purposes this is not hugely important as we will only use regular surfaces. This law takes into account what happens on less regular (rough) surfaces.

the main law of refraction is the 2nd law, that commonly referre to as Snells law

n is the refractive index of a material

(this is for when the incident ray comes from a Vacuum)

that means that the ratio of the sines of the angle of incidence vs the angle of refraction is a constant for any given transparent material.

Refractive index.

n is the symbol given to refractive index. The refractive index of a material is a constant for that material (at least at a certain temperature)

where c = speed of light (in a vacuum)

& v = speed of light in this particular medium.

As nothing can go faster than the speed of light, c then we see that the refractive index of a material must be greater than 1.

Make a pyrex beaker disappear !

What about salt water, sugar water, other oils ??? does density have an effect on refraction?

Look at something in a glass jar, then put it in a water, does is look as full ?

Mandatory Experiment

the experiment to determine the refractive index of a substance is here

Appropriate calculations.

  1. If light enters a diamond at an angle of incidence of 10o calculate the angle of refraction
  2. If light makes an angle of refraction of 29o calculate the angle of incidence if this happened through oil

page 31 ex 4.1 questions 1-8

Practical examples,

e.g. real and apparent

depth of fish in water.

even underwater due to differing densities of water the only true way to fire a scuba harpoon is directly in line laterally or vertically

the moon sunsets

These two things are often photographed, why ?

When the sun is at its greatest size due to the light refracting through the athmosphere !

Atmospheric refraction raises the sun about 1/2 degree upward at sunrise and sunset. This advances the sunrise yet retards the sunset, adding several minutes of daylight at each end of the day. Image credit: Wikipedia

Watch the moon over the next few years when you see it near the horizon it will look bigger, up in the sky it look smaller, again due to the refracting effect of the light leaving the vacuum of space and coming through our athmosphere.

tankards, thick glass mugs,

Did you notice the homework during this video,

outline your reasons why we could see the outline of the liquid inside the glass when the glass and liquid were inside the water ???

turn on a perpetual tap ! why can you not see the plastic tubing anymore

Refractive index in terms of relative speeds.

where c = speed of light (in a vacuum)

& v = speed of light in this particular medium.

Appropriate calculations.

Using the data contained in the table above, calculate the speed of light through

  1. air
  2. diamond
  3. emerald
  4. water
  5. an eye lens

What is the refractive index of a material through which the speed of light is

  1. 123,456,789 m/s
  2. 2 x 108m/s
  3. 2.998 x 108m/s
  4. 3.23 x 108m/s

is there anything wrong with any of the questions above ? Q4

pg36 ex 4.3

Demonstration using ray box or laser or other suitable method.

This is an equally valid method of measuring the refractive index of a transparent material

On a page, that is laid on some cardboard or LDPE, draw the outline of the block to be tested, leave the block there

Take 2 pins and place them so they make a line that extends and goes through the block

Stick the pins in some card/LDPE but through the page the key

look through the opposite side and line up your eye with the

Go to the other side of the block

Look through the block

align the pins in your line of sight so the 2nd pin is hidden (obscured) by the pin closer to the block.

when you can do this, place a 3rd pin in on the side that you are now on

this pin should obscure the refracted images of the original 2 pins along 1 particular line.

Place in another pin, this time blocking out the direct line of sight

An Interesting alternative method of demonstrating refractive index

Laser, Salt, Water, Transparent tank, Funnel and tuubing, Black Hole object

make up a saturated salt solution

Place BHObject at bottom of tank

half fill tank with water

using the funnel and tubing add the brine UNDER the water in the tank

Position laser at the side slightly angled up to the normal to produce the curved effect

For Experiments on the Refractive Index go here

this takes us onto Total Internal Reflection

2. Verification of Snell’s law of refraction.

go here


  1. Plot sin i Vs sin r
  2. Finding the slope of the graph = sin i / sin r = n = refractive index

Appropriate calculations.

  1. glass block 6.5cm across, how far below the incident ray normal does the light come out of the block if the initial ray was at an angle of 17o