The Science Behind Light And Red Filters

When white light passes through a red filter, all other colours in the spectrum are absorbed, and only red light passes through. This is because a pure red filter only allows red light to pass through it. This process is known as colour separation by subtraction, as filters isolate individual colours by removing the other colours.

White light through a red filter

When white light passes through a red filter, the filter absorbs or removes all colours except red, which is transmitted. This is because a red filter only allows red light to pass through it.

White light is a mixture of all the colours of the spectrum. Light filters work by subtracting some colours from this white light. The central idea to understand here is that white light comes from the sun and ordinary filament bulbs. Filters do not add colour to the light passing through them.

The transmitted light through a red filter is red in colour and less bright, as the other colours are absorbed by the filter. This is why a red object appears red—when white light shines on it, all the colours that form white light are absorbed except red, which is reflected.

In practice, colour filters are not pure, meaning they will allow a small range of colours through. For example, a red filter will also allow some orange and yellow light to pass through.

How a red filter absorbs light

A red filter is a coloured filter that appears red because it absorbs all colours of light except red. When white light, which is made up of a combination of all colours in the visible light spectrum, passes through a red filter, all colours except red are absorbed by the filter. This means that only red light passes through and reaches our eyes.

Coloured filters are usually constructed using transparent pieces of dyed glass, plastic, or lacquered gelatin. These materials are treated to selectively transmit the desired wavelengths of light while restricting others. The two most common types of filters are absorption filters and interference filters.

Absorption filters selectively transmit some wavelengths of light while absorbing or reflecting unwanted wavelengths. They are commonly made of dyed glass, lacquered gelatin, or synthetic polymers (plastics). An example of an absorption filter is a magenta filter, which transmits red and blue portions of the incident white light spectrum but absorbs most of the green wavelengths.

Interference filters, on the other hand, reflect and interfere with unwanted wavelengths instead of absorbing them. Dichroic filters are a type of interference filter that appears one colour when illuminated with transmitted light and a different colour when illuminated with reflected light. For example, a magenta dichroic filter will reflect green light from its face and transmit magenta light from the other side.

Colour separation by subtraction

When white light passes through a pure red filter, for instance, all the other colours in the spectrum are absorbed, and only red light is transmitted. This is because a pure red filter is designed to allow only red light to pass through it. As a result, the transmitted light appears red and slightly dimmer since some light is removed from the incident beam.

Similarly, a pure blue filter will only permit blue light to pass through. If a pure red and a pure blue filter are used together, no light will be able to pass through at all. This is because each filter is designed to isolate a specific colour by absorbing all the others.

It is important to note that the term "colour separation by subtraction" may not be entirely accurate. This is because the process involves the continuous removal of a fraction of the beam of other colours by the colour-specific absorber. Therefore, it may be more accurately modelled by multiplication by a number less than 1, rather than simple subtraction.

In reality, colour filters are not entirely pure and will typically allow a small range of colours to pass through. The effectiveness of a filter is determined by the narrowness of this range. Filters that select a very narrow range of frequencies are relatively rare. More commonly, filters rely on the way the human eye perceives colour, allowing multiple frequencies to pass through without being detectable to the human eye.

Complementary colours

When it comes to light and colour, artists and scientists alike have long been fascinated by the interplay between the two. One key concept in this interplay is that of complementary colours.

Examples of complementary colours

Which pairs of colours are considered complementary depends on the colour theory being used. Modern colour theory uses either the RGB (red, green, and blue) additive colour model or the CMY (cyan, magenta, and yellow) subtractive colour model. In these theories, the complementary pairs are red–cyan, green–magenta, and blue–yellow.

In the traditional RYB (red, yellow, and blue) colour model, the complementary colour pairs are red–green, yellow–purple, and blue–orange.

The science behind complementary colours

The concept of complementary colours is rooted in the science of light and colour. The colour of an object is determined by the wavelengths of light that it reflects. For example, an object appears red because it reflects red light and absorbs all other colours in the spectrum. When two complementary colours are combined, they absorb all colours in the spectrum, resulting in a grayscale colour.

The role of colour filters

Colour filters play a crucial role in understanding complementary colours. A colour filter allows only its specific colour of light to pass through, absorbing or reflecting all other colours. For instance, a pure red filter will only let red light through, while a pure blue filter will exclusively transmit blue light. If these two filters are used together, no light will be able to pass through.

Applications of complementary colours

In practical terms, complementary colours are used in life rafts and life vests, which are typically orange to provide high contrast and visibility when seen against the blue ocean or from ships or aircraft.

The effect of a red filter on light brightness

A red filter is a coloured glass filter that absorbs light in specific wavelength ranges to varying degrees, allowing other ranges to pass through. When white light passes through a pure red filter, all other colours in the spectrum are absorbed, and only red light passes through. This is because the red filter reflects its own colour and absorbs all other colours.

The transmitted light is red in colour and less bright, as the other colours are absorbed by the filter. Some light is removed from the incident beam. This process is known as colour separation by subtraction. However, in practice, colour filters are not entirely pure and will allow a small range of colours through. The quality of the filter is determined by the size of this range, with higher-quality filters allowing a smaller range of colours through.

Frequently asked questions