1. Big picture: what is ray optics?
In ray optics, I imagine light as thin straight lines called rays. Each ray shows the path along which light energy is travelling.
This simple picture helps me draw neat diagrams for reflection (bouncing of light) and refraction (bending of light when it enters another medium). Instead of worrying about the wave nature of light, I just track these rays with a scale and a protractor.
So, whenever I see mirrors, lenses or prisms, I first try to think in terms of rays and how they move, bend or bounce.
1.1. Why this model is useful
Ray optics makes problem solving much easier because:
- I only care about the direction of light, not its detailed wave pattern.
- I can draw clear diagrams showing how images form in mirrors and lenses.
- Most everyday optical devices (torch, spectacles, camera, microscope, telescope) can be understood well using rays.
Whenever the size of obstacles and apertures is much larger than the wavelength of light, the ray picture works very well.
2. Basic terms in ray optics
Some basic words I keep in mind while thinking about ray optics:
- Light ray: A thin straight line drawn in a diagram to show the path of light.
- Light beam: A bunch of rays together (can be parallel, converging or diverging).
- Homogeneous medium: A medium in which properties are the same everywhere, so light travels in a straight line.
- Interface: The boundary between two different media (for example, air and glass).
- Incident ray: Ray that is coming towards a surface or boundary.
- Reflected ray: Ray that bounces back from a surface.
- Refracted ray: Ray that bends and goes into the second medium.
In ray diagrams, I usually draw arrows on the rays to show the direction in which light is travelling.
3. Straight-line travel of light
The main assumption in ray optics is:
In a homogeneous medium, light travels in straight lines.
This matches everyday experience. Shadows have sharp edges, laser beams look like straight lines in air (if some smoke or dust is present), and light from a distant lamp seems to travel along straight paths.
Because of this straight-line travel, I can use simple geometry (angles, triangles, similar triangles) to analyse many situations involving mirrors and lenses.
3.1. Conditions for straight-line path
Ray optics assumes straight-line motion of light when:
- The medium is transparent and uniform (same refractive index everywhere).
- Objects and openings are much larger than the wavelength of light.
- I am not dealing with fine effects like interference or diffraction.
When these conditions are not satisfied, the wave nature of light becomes important and the simple ray picture is not enough.
4. Ray diagrams and how I like to draw them
Ray diagrams are the main working tool in ray optics. A ray diagram is a neat geometric sketch showing how a few important rays travel and where they meet or appear to meet.
To keep my diagrams clear, I usually follow these points:
- Draw the principal rays only (for example, rays passing through special points like focus or centre of curvature).
- Use a sharp pencil and straight lines for rays, with arrowheads to show direction.
- Mark angles with a small arc and label them clearly (like \(i\) for angle of incidence and \(r\) for angle of reflection).
- Show the position and size of the image by where the rays actually meet or appear to meet.
Even though actual light consists of many rays, it is usually enough to draw only two or three well-chosen rays to locate the image correctly.
5. Where ray optics works and where it does not
Ray optics works very well for explaining:
- Reflection from plane and curved mirrors.
- Refraction at plane and curved surfaces.
- Image formation by lenses and simple optical instruments.
However, when I have to talk about fine patterns like interference fringes or diffraction patterns, the simple ray picture is not enough. In those cases, the wave nature of light and wave optics are needed.
So for now, in these notes on ray optics, I will think of light mainly as straight rays and use geometry to understand how light travels and forms images.