Refraction and Total Internal Reflection
Refraction for Cambridge IGCSE Physics 0625. Snell law, refractive index, the critical angle, total internal reflection and optical fibres.
What you need to know
- Refraction is the change of direction when light crosses a boundary between two media at an angle, because the speed changes.
- Going into a denser medium light slows down and bends towards the normal. Coming out it speeds up and bends away.
- All angles are measured from the normal, never from the surface.
- The critical angle is the angle of incidence in the denser medium that gives an angle of refraction of 90 degrees.
- Above the critical angle no light refracts out. It all reflects back inside. That is total internal reflection, and it is how optical fibres carry signals.
Equations
- n = sin i / sin r
- n = 1 / sin c
- n = speed of light in a vacuum / speed of light in the medium
- refractive index of glass is about 1.5, water about 1.33
Refraction and Total Internal Reflection questions students ask
What is refraction?
The change of direction of a wave as it crosses a boundary between two media at an angle, caused by a change in its speed.
Which way does light bend when it enters glass?
Towards the normal, because it slows down in the denser medium. Coming back out into air it speeds up and bends away from the normal.
What is refractive index?
The ratio of the sine of the angle of incidence to the sine of the angle of refraction. It also equals the speed of light in a vacuum divided by the speed in the medium.
What is Snell law?
n equals sin i over sin r, where i is the angle in air and r is the angle in the medium, both measured from the normal.
What is the critical angle?
The angle of incidence inside the denser medium that gives an angle of refraction of 90 degrees. It is found from n equals 1 over sin c.
When does total internal reflection happen?
When light travels from a denser medium to a less dense one and the angle of incidence exceeds the critical angle. It cannot happen the other way round.
How do optical fibres use total internal reflection?
Light enters at a shallow angle and strikes the fibre wall above the critical angle at every bounce, so none of it escapes and the signal travels along the fibre.
Does the ray leaving a rectangular glass block stay parallel to the ray entering?
Yes. It is displaced sideways but it emerges parallel to the original direction, because the two surfaces are parallel.
Does the frequency of light change during refraction?
No. Only the speed and the wavelength change. The frequency is fixed by the source.
Mistakes that cost marks
- Measuring the angle from the surface rather than the normal.
- Applying total internal reflection when the light is going from less dense to more dense. It cannot happen that way.
- Saying the light slows down and therefore the frequency drops. The frequency is fixed by the source.
- Drawing the emerging ray not parallel to the incoming ray through a rectangular glass block. It should be parallel.
Every Cambridge and Edexcel paper from 2016 to today, with its mark scheme, no login.
Open the paper libraryLive online classes across Egypt and the Gulf. No online payment.
Request accessOnline classes