Ray Optics: Reflection, Refraction, and Lenses

Normal

An imaginary line drawn at 90° (perpendicular) to the surface where the light ray meets it.

Angle of Incidence

The angle between the incident ray (incoming ray) and the normal.

Angle of Reflection

The angle between the reflected ray and the normal.

Law of Reflection

The angle of incidence is always equal to the angle of reflection (i = r).

Plane Mirror Image Characteristics

A plane mirror produces an image that is the same size as the object, appears the same distance behind the mirror as the object is in front, is virtual (cannot be projected onto a screen), and is laterally inverted (left-right reversed).

Ray Construction in Plane Mirrors

To draw the image in a plane mirror, reflect two rays from the object using the law of reflection and extend them behind the mirror to locate the virtual image.

Angle of Refraction

The angle between the refracted ray (bent ray) and the normal.

How does Refraction occur

Refraction occurs when light passes from one medium to another (e.g., air to glass), changing speed and direction.

Light Bending in Refraction

Light bends towards the normal when entering a denser medium and away from the normal when entering a less dense medium.

Converging Lens (Convex)

Bends light rays towards each other (focuses them).

Diverging Lens (Concave)

Bends light rays away from each other (spreads them out).

Principal Axis

A straight line passing through the centre of the lens and both focal points.

Principal Focus (Focal Point)

The point where light rays parallel to the principal axis converge (or appear to diverge from).

Focal Length (f)

The distance from the centre of the lens to the principal focus.

Ray Diagrams for Converging Lenses

To locate the image formed by a converging lens, draw at least two of the following rays: a ray parallel to the principal axis, refracted through the focal point on the other side; a ray through the centre of the lens, which passes straight without bending; a ray through the focal point on the object's side, refracted parallel to the principal axis.

Real Image

Formed when rays actually converge. Can be projected onto a screen. Always inverted.

Virtual Image

Formed when rays appear to come from a point. Cannot be projected. Always upright.

Image Size

Enlarged / Same size / Diminished.

Image Orientation

Upright / Inverted.

Image Type

Real / Virtual.

Long-sightedness (Hyperopia)

Person cannot focus on near objects. Caused by the eyeball being too short or lens too weak. Corrected with a converging lens, which brings light rays to a focus sooner.

Short-sightedness (Myopia)

Person cannot focus on distant objects. Caused by the eyeball being too long or lens too strong. Corrected with a diverging lens, which spreads out light rays before they enter the eye.