Question

Total Internal Reflection?

Could somebody please explain the entirety of Total Internal Reflection in regards to the CIE O level syllabus? I usually use miniphysics.com for help, but even their notes are too difficult for me to comprehend. Please help! Thanks!

Answer 1

Snell's law helps to understand this.

Explanation:

Snell's law tells us how light behaves when it goes from one medium into another.

In the above, Snell's law tells us that :
`n_1sintheta_1=n_2sintheta_2`

If light is travelling from one medium to a medium with a lower refractive index (or put another way, from one medium to a less optically dense medium) then `n_1>n_2`

By Snell's law then `theta_2>theta_1` (following the maths of Snell's law)

If you increase the angle of incidence, `theta_1` eventually `theta _2` will increase until it is reflected along the boundary. At this point `theta_2` is `90^0` (the critical angle).

Beyond this, and the ray will be reflected - this is total internal reflection.

For example, if medium 1 is glass (`n_1=1.55`) and medium 2 is a vacuum (`n_2=1`)

We can see that `theta_2=90^0` occurs when:

`1.55*sin(theta_1)=1*sin(90^0)`
hence
`sin(theta_1)=1/1.55`
`theta_1=40^0`
Hence when `theta_1>40^0` the ray is reflected (total internal reflection)

If you try `theta_1=41^0` then
`1.55*sin(41)=1*sin(theta_1)`
`1.02=sin(theta_1)`
And `theta_1` cannot be solved. Does this help?!