Sec 4 Combined Science (Physics) Light notes

Draw a ray hitting a flat mirror and one construction line makes everything work: the normal — a line perpendicular (at 90°) to the surface at the point of incidence. The angle of incidence i sits between the incoming ray and the normal; the angle of reflection r sits between the reflected ray and the normal. Never measure from the mirror surface — that is the classic lost mark.

The law of reflection: angle of incidence = angle of reflection. A ray arriving at i = 35° bounces off at r = 35°, on the opposite side of the normal, in the same plane. Smooth surfaces (mirrors, calm water) reflect in this tidy way; rough surfaces scatter rays at many angles, which is why paper does not mirror your face.

Cross from air into glass or water and light slows down, so the ray kinks towards the normal. Come back out into air, it speeds up and kinks away from the normal. The angle between the refracted ray and the normal is the angle of refraction r — same measuring rule as reflection: always from the normal. Shine the ray straight along the normal (i = 0°) and it passes through without bending.

Everyday evidence: a swimming pool looks shallower than it is, a straw in a glass of water looks bent at the surface — both are refraction bending light from the underwater parts before it reaches your eye.

Measure many pairs of angles for light crossing from air into a medium and one pattern never breaks: sin i ÷ sin r is the same every time. For air-to-glass the constant is about 1.5; for air-to-water about 1.33. That constant is the refractive index n of the medium — and it has a second meaning from speeds:

n = speed of light in vacuum ÷ speed of light in the medium. Light manages 3 × 10⁸ m/s in vacuum but only about 2 × 10⁸ m/s in glass, so n = 3 × 10⁸ ÷ 2 × 10⁸ = 1.5 — the same number the angles give. Denser optical media have higher n: light moves slower there and rays bend more.

A converging (convex) lens is thicker in the middle. Rays from a distant source arrive nearly parallel to the principal axis; the lens refracts them twice (entering and leaving the glass) so they meet at one point — the principal focus (F). The distance from the centre of the lens to that focus is the focal length f. A shorter focal length means a stronger lens: it bends rays more sharply.

The lens works because glass has n ≈ 1.5 — every ray slows and bends towards the normal as it enters the curved surface and away as it leaves, and the curved shape angles each surface so all the rays agree on one meeting point.

Where the object sits relative to F (focus) and 2F decides the image. Beyond 2F: the image is real (rays really meet — it can be caught on a screen), inverted and diminished — that is a camera. At 2F: real, inverted, same size. Between F and 2F: real, inverted, magnified — that is a projector. Inside F: the rays diverge instead, and the eye traces them back to a virtual, upright, magnified image — that is a magnifying glass.