Optics: Lens Types, Refraction, and Image Formation

Lens

A transparent object (glass or plastic) that refracts light to form an image.

Convex Lens (Converging Lens)

A lens thicker in the center that bends light rays inward to a focal point; can form real or virtual images.

Concave Lens (Diverging Lens)

A lens thinner in the center that spreads light rays outward; always forms virtual, upright, reduced images.

Focal Point

The point where parallel light rays converge (or appear to diverge from) after passing through a lens.

Focal Length (f)

The distance from the center of a lens to its focal point.

Real Image

An image formed by converging light rays; appears on the opposite side of the lens and can be projected onto a screen.

Virtual Image

An image formed by diverging light rays; appears on the same side as the object and cannot be projected.

Ray Diagram

A drawing using principal rays to determine image location, size, and type.

Principal Axis

The straight line passing through the center of the lens.

Optical Center

The center of a lens where light passes through without bending.

Refraction

The bending of light as it passes from one medium to another due to a change in speed.

Index of Refraction (n)

A measure of how much light slows in a medium; n = c/v.

Angle of Incidence (θ₁)

The angle between the incoming ray and the normal.

Angle of Refraction (θ₂)

The angle between the refracted ray and the normal.

Normal Line

A line perpendicular to a surface at the point of contact.

Snell's Law

A formula relating angles and indices: n₁sin(θ₁) = n₂sin(θ₂).

Bending Toward the Normal

Occurs when light enters a medium with a higher index of refraction (slower speed).

Bending Away from the Normal

Occurs when light enters a medium with a lower index of refraction (faster speed).

Total Internal Reflection (TIR)

When light reflects completely within a medium instead of refracting out.

Critical Angle (θc)

The angle of incidence at which the refracted ray is 90°.

Critical Angle Equation

sin(θc) = n₂/n₁ (only when n₁ > n₂).

Atmospheric Refraction

Bending of light in Earth's atmosphere due to varying air densities.

Dispersion

Separation of light into colors because different wavelengths refract differently.

Fiber Optics

Technology that transmits light using total internal reflection.

Sign Convention (Focal Length)

Positive for convex lenses, negative for concave lenses.

Sign Convention (Image Distance)

Positive for real images, negative for virtual images.

Sign Convention (Magnification)

Positive means upright image, negative means inverted image.