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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 !
http://www.thenakedscientists.com/HTML/content/kitchenscience/exp/making-pyrex-invisible/
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.
- If light enters a diamond at an angle of incidence of 10o calculate the angle of refraction
- 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 !
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
- air
- diamond
- emerald
- water
- an eye lens
What is the refractive index of a material through which the speed of light is
- 123,456,789 m/s
- 2 x 108m/s
- 2.998 x 108m/s
- 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
Diagram
- Plot sin i Vs sin r
- Finding the slope of the graph = sin i / sin r = n = refractive index
Appropriate calculations.
- 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
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