Light

When light hits a surface it can reflect. Draw the normal — a line perpendicular to the surface at the point of incidence. Then:

$\text{angle of incidence} = \text{angle of reflection}$

(both measured from the normal). This is the law of reflection.

A plane mirror produces an image that is:

• Virtual — appears behind the mirror; cannot be projected onto a screen.
• Upright — same way up as the object.
• Same size — magnification = 1.
• Laterally inverted — left and right are swapped.
• Same distance behind the mirror as the object is in front.

Rough surfaces produce diffuse reflection (light scatters in many directions); shiny surfaces give specular reflection (parallel rays stay parallel).

When light passes from one medium to another (e.g. air → glass) its speed changes. If it enters at an angle, this also bends the direction of the light. This is refraction.

• Air → denser medium (glass, water): light SLOWS down and bends TOWARDS the normal.
• Denser → less dense: light SPEEDS UP and bends AWAY from the normal.

The refractive index quantifies how much a material slows light:

$n = \frac{c}{v}$

with c = speed in vacuum (3.0 × 10⁸ m/s), v = speed in the medium. Glass has n ≈ 1.5; water n ≈ 1.33; diamond n ≈ 2.4 (which is why diamonds sparkle so much).

Converging (convex) lenses are thicker in the middle. They bring parallel light rays to a focal point at distance f (the focal length) from the lens. Used in cameras, magnifying glasses, the eye (for focusing on the retina), and reading glasses for far-sighted people.

Diverging (concave) lenses are thinner in the middle. They spread parallel rays as if they came from a virtual focal point on the same side. Used in glasses for short-sighted people, peepholes in doors, and certain telescope designs.

Use ray diagrams to predict the image. For a converging lens, two key rays:

1. A ray parallel to the principal axis passes through the focal point on the other side.
2. A ray through the centre of the lens goes straight on, undeviated.

Where the two rays cross is where the image forms.

White light contains all visible colours (red, orange, yellow, green, blue, indigo, violet — ROYGBIV). When it enters a glass prism, each colour refracts by a slightly different amount because each has a different wavelength, so they spread out — dispersion.

Red light has the longest wavelength (~700 nm) and bends LEAST. Violet has the shortest (~400 nm) and bends MOST. This is why a prism shows the spectrum, and why rainbows form when sunlight passes through water droplets.

Different colours of light have different frequencies: red is low-frequency, violet is high-frequency. The frequency of light is what determines its colour to your eye.