Visible Light: Absorption, Reflection, and Transmission Revision Notes | GCSE AQA Higher Physics

1. Why Do Objects Have Colour?

Look around you right now. A red apple, green leaves, blue sky - but here's the mystery: light from the Sun is white, containing all colours mixed together. So why don't all objects look white?

The answer lies in what happens when white light hits different materials. Objects don't actually 'have' colour - they interact with light in ways that make us see colour. Think of objects as being picky about which wavelengths of light they keep and which they send back to your eyes.

When white light hits any object, three things can happen to different wavelengths: absorption (the object keeps that light energy), reflection (the light bounces back), or transmission (the light passes through). The combination of these three processes determines everything we see.

2. How Opaque Objects Get Their Colour

Opaque objects are the easiest to understand because light cannot pass through them. When white light hits an opaque surface, each wavelength faces a simple choice: get absorbed or get reflected. There's no middle ground.

Here's the key principle: **the colour of an opaque object is determined by which wavelengths of light are most strongly reflected**. A red rose appears red because it reflects red wavelengths strongly while absorbing most other colours. The absorbed light energy doesn't disappear - it gets converted to heat, which is why dark objects feel warmer in sunlight.

This leads to two special cases that prove the rule. If an object reflects all wavelengths equally, we see white - like fresh snow or white paper. If an object absorbs all wavelengths equally, we see black - like charcoal or a blackboard. These aren't really 'colours' in the same sense as red or blue, but they show how reflection and absorption work together.

**Trap**: Many students think that coloured objects 'contain' their colour, like paint in a tin. Actually, colour is what happens when white light interacts with the object's surface. Change the light source, and you can change the apparent colour of the same object.

3. Transparent vs Translucent Materials

Not all objects are opaque. Some materials allow light to pass through them, and this transmission creates different visual effects. The key distinction is how much the light gets scattered as it passes through.

**Transparent objects** allow light to pass through with minimal scattering. You can see clearly through them because light rays travel in straight lines from objects behind the material to your eyes. Clear glass, clean water, and air are transparent. The light wavelengths that aren't absorbed pass through virtually unchanged in direction.

**Translucent objects** also transmit light, but they scatter it significantly. This scattering means you can't see clear images through them, even though light is getting through. Frosted glass, tissue paper, and thin fabric are translucent. Some wavelengths are still absorbed, which is why translucent objects can have colour.

Both transparent and translucent materials can be coloured. Coloured glass appears green because it absorbs red wavelengths while transmitting green ones. The transmitted light carries the colour information to your eyes, just like reflected light does with opaque objects.

4. The Physics Behind Colour Perception

Understanding why different materials interact differently with light requires thinking about wavelengths. Visible light spans wavelengths from about 400nm (violet) to 700nm (red). Each material has a unique 'fingerprint' of which wavelengths it absorbs strongly and which it reflects or transmits.

This wavelength-dependent behaviour explains some surprising effects. A red object under red light appears bright red because there are plenty of red wavelengths to reflect. But the same object under blue light appears very dark or black because there are no red wavelengths to reflect - the blue light gets absorbed.

The absorption process converts light energy into other forms, usually heat. This is why wearing black clothing makes you hotter in summer - black materials absorb most wavelengths, converting that light energy to thermal energy. White clothing reflects most wavelengths, keeping you cooler.

For transparent coloured materials like stained glass, the same principle applies but with transmission instead of reflection. Blue glass absorbs red and green wavelengths while transmitting blue ones. The transmitted blue light reaches your eyes, making the glass appear blue.

5. Real-World Applications and Examples

These principles of absorption, reflection, and transmission have practical applications everywhere. Photographers use coloured filters that absorb certain wavelengths while transmitting others, allowing them to control which colours appear in their images. A red filter absorbs blue and green light, making red objects appear brighter in black-and-white photos.

Safety clothing often uses fluorescent materials that absorb ultraviolet light and re-emit it as visible light, making the wearer more visible. This is different from simple reflection - the material actually converts invisible UV wavelengths into visible ones through a process called fluorescence.

Understanding transmission helps explain why swimming pools look blue. Water absorbs red wavelengths more strongly than blue ones. In shallow water, this effect is minimal, but in deep water, most red light has been absorbed, leaving mainly blue wavelengths to reach your eyes or reflect back from the pool bottom.

Even plant biology depends on these principles. Leaves appear green because chlorophyll absorbs red and blue wavelengths strongly (using them for photosynthesis) while reflecting green wavelengths. This is why plants look green even though green light is actually the least useful for photosynthesis.

6. Exam Technique and Common Mistakes

Exam questions about colour often test whether you understand the difference between what an object absorbs and what it reflects. Remember: **we see the wavelengths that are reflected or transmitted, not the ones that are absorbed**. This is the most common mistake students make.

When describing why objects appear certain colours, always mention both processes. Don't just say 'a red apple reflects red light' - explain that it also absorbs other wavelengths. Examiners want to see that you understand colour results from differential absorption and reflection.

For questions about transparent or translucent objects, be precise about the difference. Both transmit light, but only transparent objects allow clear vision through them. If you can't see clearly through it, it's translucent, not transparent.

Watch out for questions that change the lighting conditions. If a question asks about a coloured object under different coloured lights, think about which wavelengths are available to be reflected. No available wavelengths means the object appears black, regardless of its normal colour.

Worked example: Worked Method: Visible Light: Absorption, Reflection, and Transmission

Use the relevant formula, substitute values with units, calculate, then state the final answer clearly.

1. Write down the correct physics formula. — Write down the correct physics formula.
2. Substitute known values with units. — Substitute known values with units.
3. Rearrange if needed and calculate carefully. — Rearrange if needed and calculate carefully.
4. Give the final answer with correct unit and sensible precision. — Give the final answer with correct unit and sensible precision.

Answer: Final answer should include unit and appropriate significant figures.