Light Energy — Reflection and Refraction

Shine a torch at a mirror in a dark room and you see a beam hit the surface and snap back at an angle — the reflected beam lights up a different part of the wall. Now put a glass of water in the beam's path: the light bends as it enters the water and bends again as it exits, landing somewhere else entirely. Two different behaviours — reflection and refraction — and both are caused by one simple rule about how light moves.

The ray model treats light as straight-line rays. Each ray represents the direction the light is travelling. This model works well for most everyday optics problems.

The normal is an imaginary line drawn perpendicular (at 90°) to a surface at the exact point where a ray hits. All angles in optics are measured from the normal — NOT from the surface itself.

Law of reflection: angle of incidence (i) = angle of reflection (r)

This applies to all smooth (specular) surfaces — mirrors, still water, polished metal. Rough surfaces scatter light in many directions (diffuse reflection) — this is why most surfaces don't act as mirrors but you can still see them (the scattered light reaches your eyes).

Plane mirror image properties: The image in a flat (plane) mirror is same size as the object, the same distance behind the mirror as the object is in front, and laterally inverted (left-right flipped — your right hand appears as the left hand in a mirror). The image is virtual — it appears to be behind the mirror but cannot be projected onto a screen.

Refraction occurs when light crosses from one medium into another with a different optical density. The speed of light changes, causing the ray to bend.

• Into a denser medium (e.g. air → water, air → glass): light slows down and bends toward the normal. The angle of refraction is smaller than the angle of incidence.
• Into a less dense medium (e.g. water → air): light speeds up and bends away from the normal. The angle of refraction is larger than the angle of incidence.

Apparent depth — because light bends away from the normal as it exits water, objects underwater appear closer to the surface than they really are. A 2 m pool appears to be about 1.5 m deep. Divers, lifeguards and spearfishers must allow for this. Spearfishing requires aiming below where the fish appears to be.

Both reflection and refraction underpin enormous amounts of modern technology.

Rainbows — sunlight enters a water droplet, refracts (bends), reflects off the back of the droplet, and refracts again as it exits. Different colours (wavelengths) of light refract by slightly different amounts — red bends least, violet bends most. This separates white sunlight into the spectrum of colours. Each colour exits the droplet at a slightly different angle, creating the arc.

Lenses in glasses and cameras — a lens uses the curved shape of the glass to refract light systematically, bringing it to a focus. Converging lenses (thicker in the middle) focus parallel rays to a point — used to correct long-sightedness. Diverging lenses (thinner in the middle) spread rays — used to correct short-sightedness.

Optical fibres and the NBN — optical fibres carry light (data signals) along thin glass or plastic strands using total internal reflection. When light inside a dense medium (glass) hits the surface at a steep enough angle, it reflects back entirely instead of exiting. This allows light signals to travel around bends without losing significant energy. Australia's NBN (National Broadband Network) uses optical fibre to deliver high-speed internet — light carries the data.

Safety mirrors (convex) at intersections — use reflection to give a wide field of view; the curved shape allows drivers to see around corners. Found at blind intersections in Australian shopping centres and car parks.