Colour Filters & Wavelengths Revision Notes | GCSE AQA Higher Physics

1. Why Do Red Objects Look Black Through Green Filters?

Picture this: you're looking at a bright red rose through green sunglasses, and suddenly it appears black. This isn't an illusion - it's physics in action. The rose hasn't changed colour, but the filter has blocked the light that would normally reach your eyes.

Understanding colour filters unlocks how we see the world around us. From photography to stage lighting, from safety goggles to smartphone screens, filters control which colours we perceive. This knowledge helps explain everything from why traffic lights work to how your camera captures different moods.

The key insight is that colour isn't a property of objects themselves - it's about which wavelengths of light reach our eyes. Objects don't 'have' colour; they reflect certain wavelengths whilst absorbing others. Filters add another layer to this process by blocking specific wavelengths before they even reach the object.

2. The Visible Light Spectrum and Wavelengths

Each colour we see corresponds to a specific range of wavelengths. Red light has the longest wavelengths (around 650-700nm), whilst violet has the shortest (around 400-450nm). Between these extremes, we find orange, yellow, green, blue, and indigo - each with its own narrow band of wavelengths.

Think of wavelength like a fingerprint for each colour. Just as no two people have identical fingerprints, each colour has its unique wavelength signature. This is why we can identify colours so precisely - our eyes are detecting these specific wavelength patterns.

The relationship between wavelength and frequency is crucial here. As wavelength increases, frequency decreases. Red light (long wavelength) has lower frequency than blue light (short wavelength). This relationship follows the wave equation: speed = frequency × wavelength, where the speed of light is constant.

3. How Objects Appear Coloured

Here's where many students get confused: objects don't actually 'contain' colour. A red apple appears red because it absorbs most wavelengths of white light but reflects red wavelengths back to our eyes. The apple is essentially rejecting all colours except red.

White objects reflect all wavelengths equally, which is why they appear white in white light. Black objects absorb almost all wavelengths, reflecting very little light back to our eyes. This absorption converts the light energy into heat energy, which is why black surfaces get hotter in sunlight than white surfaces.

The type of reflection also matters enormously. Smooth surfaces like mirrors produce specular reflection - light rays bounce off in a single, predictable direction. This creates clear, sharp images. Rough surfaces cause diffuse reflection, scattering light rays in many directions. This is why you can see a rough wall from any angle, but you can only see your reflection in a mirror from specific positions.

4. How Colour Filters Work

Colour filters work by selective absorption. A red filter absorbs all wavelengths except red, which it allows to pass through (transmits). Think of it like a bouncer at a club - only certain wavelengths are allowed entry, whilst others are turned away.

This creates fascinating effects when viewing coloured objects through filters. A red object viewed through a red filter appears red because the object reflects red light, and the filter transmits red light. However, a blue object viewed through a red filter appears black because the object reflects blue light, but the red filter absorbs blue light, so no light reaches your eyes.

The key principle is that you can only see light that both the object reflects AND the filter transmits. If either the object doesn't reflect a particular wavelength, or the filter doesn't transmit it, that wavelength won't reach your eyes. This explains why photographers use different coloured filters to enhance or suppress certain colours in their images.

Worked example: Predicting Object Appearance Through Filters

A yellow banana is viewed through a blue filter. Predict its appearance and explain why.

1. Yellow banana reflects yellow light (and some green) — Yellow objects typically reflect wavelengths around 570-590nm
2. Blue filter transmits blue light only — Blue filters absorb all wavelengths except blue (around 450-490nm)
3. Yellow light reflected, blue light transmitted = no overlap — The banana reflects yellow but the filter only lets blue through
4. Banana appears black — No light reaches your eyes, so the banana looks black

Answer: The yellow banana appears black through the blue filter because there's no overlap between the yellow light it reflects and the blue light the filter transmits.

5. Real-World Applications and Examples

Colour filters have countless practical applications that affect our daily lives. Photographers use coloured filters to enhance contrast - a red filter makes blue skies appear darker in black and white photography because it absorbs the blue light from the sky. Stage lighting uses coloured gels (filters) to create mood and atmosphere, with warm colours like red and orange creating intimate settings.

Safety applications are equally important. Welders use dark filters to protect their eyes from intense light, whilst yellow-tinted safety glasses enhance contrast in low-light conditions. Traffic lights work because red, amber, and green filters create distinct signals that drivers can distinguish even in bright sunlight.

Modern technology relies heavily on filtering principles. LCD screens use colour filters combined with liquid crystals to create the millions of colours you see on your phone or computer. Tinted sunglasses are different: they reduce glare by absorbing some wavelengths of light, not by selecting colours. Cameras also use tiny colour filters over each pixel sensor to capture full-colour images.

Colour filters are useful whenever you want to isolate certain colours or improve contrast. Stage lighting gels can make objects appear different colours, and camera filters can emphasise particular colours in a scene. These examples stay within the GCSE idea that filters absorb some wavelengths and transmit others.

6. Exam Technique and Common Mistakes

Exam questions about colour filters often test your understanding of the absorption and transmission process. When describing how filters work, always mention both what they absorb AND what they transmit. Saying 'a red filter lets red light through' is incomplete - you must also state that it absorbs all other colours.

Many students confuse reflection and absorption when explaining object colours. Remember: objects appear the colour they reflect, not the colour they absorb. A common mistake is saying 'a red apple absorbs red light' when it actually reflects red light and absorbs other colours.

For calculation questions involving wavelengths, always include units (nanometres for visible light). When comparing wavelengths, remember that red has the longest wavelength and violet the shortest. Questions often test whether you know that longer wavelength means lower frequency.