Visible light (AQA GCSE Physics): Revision Notes
Visible light
Visible light is a small part of the electromagnetic spectrum that our eyes can detect. When light hits different surfaces, three main things can happen to it: it can be reflected (bounced back), absorbed (taken in), or transmitted (passed through).
The electromagnetic spectrum includes many types of radiation beyond visible light, such as radio waves, microwaves, infrared, ultraviolet, X-rays, and gamma rays. Visible light represents only a tiny fraction of this entire spectrum, roughly between 400-700 nanometers in wavelength.
Types of reflexion
There are two main types of reflexion that occur when light interacts with different surface textures, each producing distinctly different visual effects.
Specular reflexion
This happens when light hits a smooth surface like a mirror or calm water. When light rays hit these surfaces:
- The reflected rays stay parallel to each other
- The surface irregularities are much smaller than the light's wavelength
- You get a clear, sharp reflexion that you can see easily
Practical Example: Mirror Reflection
When you look at yourself in a bathroom mirror:
- Light from your face travels to the mirror's smooth surface
- The mirror reflects the light rays in a parallel pattern
- These parallel rays reach your eyes, creating a clear, undistorted image
- This is why you can see fine details like individual hairs or skin texture
Diffuse reflexion
This occurs when light hits a rough surface with lots of bumps and irregularities. When this happens:
- The surface irregularities are similar in size to the light's wavelength
- The reflected light rays scatter in many different directions
- The reflected rays are not parallel anymore
- This is also called scattering
Practical Example: Reading a Book
When you read a page in a book:
- Light hits the rough paper surface
- The microscopic bumps and fibres scatter light in all directions
- This scattered light reaches your eyes from multiple angles
- You can read the text from different positions because light is being diffusely reflected towards you regardless of your viewing angle
The colour spectrum
White light is actually made up of many different colours mixed together. Understanding how white light separates into its component colours helps explain many optical phenomena we observe daily.
Critical Concept: White Light Composition
White light is not a single colour but a mixture of all visible colours. This fundamental principle explains why we can see rainbows, why prisms create colourful displays, and how colour philtres work.
You can see this when white light passes through a prism:
- The prism splits white light into different colours through refraction
- Each colour has a different wavelength
- Red has the longest wavelength
- Violet has the shortest wavelength
- The colours appear in order: red, orange, yellow, green, blue, indigo, violet
How we see different colours
The colour of an object depends on which wavelengths of light it absorbs and which it reflects. This selective interaction with different wavelengths is what gives objects their characteristic colours.
- A blue object reflects blue light but absorbs other colours
- A black object absorbs all wavelengths of visible light
- A white object reflects all wavelengths of visible light
When white light hits an opaque surface, some wavelengths get absorbed and others get reflected back to your eyes.
Worked Example: Why Grass Appears Green
Step 1: White light (containing all colours) hits grass Step 2: The grass absorbs red, blue, and other wavelengths Step 3: Green wavelengths are reflected back Step 4: Your eyes detect the reflected green light Result: You perceive the grass as green
Philtres and transmission
Philtres are materials that let some colours of light pass through while absorbing others. They work by selectively blocking certain wavelengths while allowing others to transmit through the material.
- A green philtre lets green light through but absorbs other colours
- Transparent materials (like clear glass) let all light pass through
- Translucent materials (like frosted glass) let some light through but scatter it
- Opaque materials don't let any light pass through
Philtres are commonly used in photography, stage lighting, and scientific instruments to isolate specific wavelengths of light. The colour you see when looking through a philtre is the colour it transmits, not the colour it absorbs.
Key Points to Remember:
- Smooth surfaces give specular reflexion (parallel rays), rough surfaces give diffuse reflexion (scattered rays)
- White light contains all colours of the spectrum with different wavelengths
- Objects appear coloured because they absorb some wavelengths and reflect others back to our eyes
- Philtres work by absorbing certain colours and transmitting others
- Red light has the longest wavelength, violet light has the shortest wavelength