Total internal reflection and colour

In a nutshell

Total internal reflection is when waves are completely reflected at the boundary between two mediums, due to refraction. Humans see colour due to specific wavelengths reflected by objects, whilst others are absorbed.

Definitions

Keyword	Definition
Opaque	Not able to see through it - no light can pass through.
Transparent	Able to see completely through it - light can pass through.
Translucent	Able to partially see through it - some light can pass through.

Total internal reflection (TIR)

Total internal reflection occurs at the boundary between two materials. This happens due to refraction.

When a ray goes from a more dense to a less dense material, the refracted ray bends away from the normal. This means the angle of refraction is greater than the angle of incidence.

Physics; Waves and the electromagnetic spectrum; KS4 Year 10; Total internal reflection and colour

If the different between the densities is large enough, the refracted ray will be bent at $90\degree$ or more from the normal. The ray will not leave the first material, and will be reflected back.

Physics; Waves and the electromagnetic spectrum; KS4 Year 10; Total internal reflection and colour

When the angle of refraction is exactly $90\degree$ , the angle of incidence is labelled the critical angle. TIR only takes place when light moves from a more dense material towards a less dense material, and when the angle of incidence is greater than the critical angle.

Physics; Waves and the electromagnetic spectrum; KS4 Year 10; Total internal reflection and colour

Example

Fibre optic cables are made of glass which use TIR to contain the light rays travelling through the cables. They are used in fibre broadband internet, for medical purposes (endoscopes), and decorations.

Higher Tier Only

Different types of glass have different densities. Fibre optic cables are made of multiple types of glass. This means the light refracts at slightly different angles as it travels. This improves the quality and speed of transmissions.

Colour

The colour of an opaque object corresponds to the wavelengths of light it absorbs and reflects. Objects strongly reflect the colour they are made up of, and absorb any other colours.

Example

A green lime will reflect green wavelengths of light but absorbs any other wavelength.

Physics; Waves and the electromagnetic spectrum; KS4 Year 10; Total internal reflection and colour

White objects reflect all wavelengths of light. This is because white light is made up of all wavelengths of light. Black objects absorb all wavelengths of light. Black 'light' is the absence of light.

Most objects are opaque and not a primary colour (not red, green or blue). They reflect either the specific wavelengths of light of the colour of the object, or a mix of primary colour wavelengths that mix to make up the colour.

Objects that aren't opaque, and are transparent or translucent, transmit light. This means that some (or all) of the light will pass through the object.

Colour filters

Colour filters allow only certain wavelengths of light to be transmitted. The wavelengths of light it allows through matches the colour of the filter and absorbs all other wavelengths. This means objects viewed through a colour filter may appear differently.

Example

The green lime viewed through a green filter will look the same as it does through no filter.

Physics; Waves and the electromagnetic spectrum; KS4 Year 10; Total internal reflection and colour

Physics; Waves and the electromagnetic spectrum; KS4 Year 10; Total internal reflection and colour

However, if you viewed it through a red filter, no green wavelengths will transmit through so it will appear black.

Physics; Waves and the electromagnetic spectrum; KS4 Year 10; Total internal reflection and colour

Physics; Waves and the electromagnetic spectrum; KS4 Year 10; Total internal reflection and colour