GCSE Physics AQA - Chapter P14: Light Notes

Reflection of Light

Ever wondered why you can see yourself perfectly in a bathroom mirror but only catch blurry glimpses in a puddle? It's all about how light waves bounce off different surfaces.

When light hits a flat surface like a mirror, it follows a simple rule: the angle of reflection equals the angle of incidence. Think of it like bouncing a ball off a wall - throw it at an angle, and it bounces back at the same angle. The normal is an imaginary line perpendicular to the surface that helps us measure these angles.

Specular reflection happens with smooth surfaces like mirrors, where all the light rays bounce off in the same direction, creating clear images. Diffuse reflection occurs with rough surfaces like paper, scattering light in all directions - which is why you can't see your reflection in a textbook!

Key Point: Virtual images (like your reflection) appear to be behind the mirror but can't be projected onto a screen, unlike real images that actually focus light rays.

Refraction of Light

You've probably noticed how a straw looks bent in a glass of water - that's refraction in action, and it happens because light travels at different speeds through different materials.

When light moves from air into glass (or water), it slows down and bends towards the normal. When it goes from glass back to air, it speeds up and bends away from the normal. This happens because glass is optically more dense than air.

The key rule is simple: light bends towards the normal when entering a denser medium, and away from the normal when entering a less dense medium. This bending creates optical illusions and makes objects appear in different positions than they actually are.

Remember: The amount of bending depends on how much the light's speed changes between the two materials.

Light and Colour

White light isn't actually white - it's every colour mixed together! When you see a red apple or blue sky, you're seeing how different materials interact with the full spectrum of light wavelengths.

Colour filters work by absorbing certain wavelengths and letting others pass through. The primary colours of light are red, green, and blue, which can combine to create any other colour. Objects appear coloured because their pigments absorb some wavelengths and reflect others back to your eyes.

Transparent objects let all light pass through clearly, translucent objects let light through but scatter it (like frosted glass), and opaque objects block light completely. A white surface reflects all colours equally, while a black surface absorbs them all.

Fun Fact: You see objects as certain colours because they're actually rejecting those wavelengths - a red apple absorbs all colours except red!

Lenses and Image Formation

Lenses are curved pieces of glass that bend light to create images, and they're everywhere - in your eyes, cameras, microscopes, and glasses.

Convex lenses (thicker in the middle) make parallel light rays converge to a focus point. They can create both real images (that appear on screens) and virtual images (magnified views like through a magnifying glass). Concave lenses (thinner in the middle) make light rays diverge and are used to correct short-sightedness.

Magnification is calculated as image height divided by object height. If the result is greater than 1, the image is bigger than the object. The position and size of the image depends on how far the object is from the lens compared to the focal length.

Cameras use convex lenses to focus real images onto film or digital sensors. For distant objects, the lens needs to be exactly one focal length from the film, but for closer objects, you need to move the lens further away.

Quick Tip: Draw ray diagrams using two simple rules - one ray goes parallel to the axis then through the focus, another goes straight through the lens centre.