Ray Diagrams

Ray diagrams are simple drawings that show how light travels. We use straight lines with arrows to predict where images form in mirrors and lenses.

How to draw rays

• Draw straight rays with a ruler and add arrows to show direction.
• At a surface, draw a normal: a dashed line at 90° to the surface.
• Measure angles to the normal, not to the surface.
• Label important points: mirror line, lens, optical centre (C), principal axis, focal point (F).

Reflection at a plane mirror

Law of reflection: $i = r$ (angle of incidence equals angle of reflection, both measured to the normal).

• Draw the incident ray to the mirror and the normal at the contact point.
• Set the reflected ray so its angle to the normal equals the incident angle.
• To locate the image of a point, extend the reflected ray behind the mirror with a dotted line. The image lies the same distance behind the mirror as the object is in front (virtual, upright, same size).

Refraction at a boundary

When light enters a more optically dense medium (e.g., air to glass), it bends towards the normal. Leaving a denser medium, it bends away from the normal.

• Through a rectangular glass block, the emerging ray is parallel to the incident ray but shifted sideways (lateral displacement).

Lenses

Converging (convex) lenses bring rays together. Diverging (concave) lenses spread rays out. The focal point F is where rays parallel to the principal axis meet (or appear to come from).

• Converging lens rules: (1) Ray parallel to axis → through F. (2) Ray through F → emerges parallel. (3) Ray through C → straight, undeviated.
• Diverging lens rules: (1) Ray parallel to axis → emerges as if from F. (2) Ray through C → straight.
• Real images form where rays meet and can be caught on a screen. Virtual images form where rays appear to come from and cannot be caught on a screen.