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Light and Optics Explained

From the rainbow formed in a raindrop to the lens in your eye, optics is everywhere. This guide unpacks the behaviour of light: how it reflects off surfaces, bends when it crosses into a new medium, and can be focused or spread by lenses — along with a look at the broader electromagnetic spectrum of which visible light is just one small part.

What Is Light?

Light is a transverse electromagnetic wave — oscillating electric and magnetic fields that travel perpendicular to the direction of propagation. Unlike sound, light needs no medium and travels fastest through a vacuum, at approximately 3 × 10⁸ m/s (often written as c, the speed of light).

The colour of visible light depends on its frequency (and equivalently its wavelength, since v = fλ and the speed in a vacuum is constant). Red light has the lowest frequency (longest wavelength, around 700 nm); violet has the highest (shortest wavelength, around 400 nm). White light is a mixture of all visible frequencies.

The Electromagnetic Spectrum

Visible light occupies only a narrow band of the full electromagnetic (EM) spectrum. Listed in order of increasing frequency (decreasing wavelength):

• Radio waves — TV and radio broadcasts, long-range communication
• Microwaves — mobile phones, radar, microwave ovens
• Infrared — thermal imaging, TV remotes, heat lamps
• Visible light — the only range detectable by the human eye
• Ultraviolet — sterilisation, causes sunburn, fluorescent dyes
• X-rays — medical imaging, airport security scanners
• Gamma rays — cancer treatment, emitted by radioactive nuclei

All EM waves travel at the same speed in a vacuum. They differ only in frequency and wavelength, which determines their energy and how they interact with matter.

Reflection

When light strikes a smooth surface it bounces off according to the law of reflection: the angle of incidence equals the angle of reflection. Both angles are measured from the normal — an imaginary line perpendicular to the surface at the point of contact.

A flat (plane) mirror produces a virtual image that appears to be the same distance behind the mirror as the object is in front, is the same size as the object, and is laterally inverted (left-right flipped). The image is virtual because no light rays actually converge there; they only appear to diverge from that point.

Specular reflection occurs on smooth surfaces (mirrors, still water). Diffuse reflection occurs on rough surfaces where microscopic irregularities scatter light in many directions — this is how most non-shiny everyday objects are visible.

Refraction and Snell's Law

When light passes from one transparent medium into another it changes speed. If it also changes direction, this bending is called refraction. The key rule: light bends toward the normal when it enters a denser medium (slows down) and away from the normal when it enters a less dense medium (speeds up).

A glass block in air is the classic example. A ray entering the top face bends toward the normal on entry (air → glass) and bends away from the normal on exit (glass → air). This is why a straight straw appears kinked in a glass of water.

The relationship between the angles and the media is given by Snell's Law:

n₁ sinθ₁ = n₂ sinθ₂

where n₁ and n₂ are the refractive indices of the two media and θ₁, θ₂ are the angles of incidence and refraction from the normal. The refractive index n of a medium equals c divided by the speed of light in that medium: n = c/v. Glass has a refractive index of about 1.5; water is about 1.33; a vacuum (and approximately air) is 1.0.

Different frequencies refract by slightly different amounts — a property called dispersion. A glass prism splits white light into a visible spectrum (red bends least, violet bends most) because violet light travels more slowly in glass than red light does. This same effect produces rainbows: water droplets act as tiny prisms.

Total Internal Reflection

When light travels from a denser medium to a less dense one (e.g., glass to air) and the angle of incidence exceeds a threshold called the critical angle, no refraction occurs — the ray is entirely reflected back into the denser medium. This is total internal reflection (TIR).

The critical angle c for a medium is found from: sin(c) = 1/n. For glass (n = 1.5), the critical angle is about 42°. TIR is the principle behind optical fibres: light signals bounce along the inside of a glass fibre for kilometres with virtually no loss, enabling high-speed internet and medical endoscopes.

Lenses

A lens refracts light to converge or diverge it. The two main types are:

• Converging (convex) lens: thicker at the centre, bends parallel rays inward to meet at the focal point. Used in magnifying glasses, cameras, and the lens of the human eye.
• Diverging (concave) lens: thinner at the centre, spreads parallel rays outward so they appear to come from a focal point on the same side as the incoming light. Used in spectacles to correct short-sightedness.

The focal length (f) is the distance from the lens to the focal point. A stronger lens has a shorter focal length. The lens equation relates object distance (u), image distance (v), and focal length: 1/f = 1/u + 1/v (using real-is-positive sign convention).

Summary

Light is a transverse EM wave travelling at 3 × 10⁸ m/s in a vacuum, occupying a narrow band of the EM spectrum between infrared and ultraviolet. It reflects off surfaces with the angle of incidence equal to the angle of reflection. It refracts when crossing between media of different optical density, as described by Snell's Law; dispersion splits white light into a spectrum. Total internal reflection occurs beyond the critical angle and underpins optical fibres. Converging lenses focus light to form images; diverging lenses spread it — principles that govern everything from cameras to the human eye.