Principles of Optics: Light, Reflection, and Refraction

Fundamentals of Light Propagation

• The light propagates in a straight line in transparent and homogeneous media.
• The speed of light is approximately 300,000 km/s.

Refractive Index

The absolute refractive index of a substance is the ratio between the speed of light in a vacuum and its speed through the substance, represented by the letter n.

Laws of Reflection

• When light reflects off a specular surface, the incident ray, the reflected ray, and the normal lie in the same plane (first law of reflection).
• The angle of reflection equals the angle of incidence (second law of reflection).
• Images formed in flat mirrors are virtual, upright, and the same size as the object.

Spherical Mirrors

A spherical mirror is a portion of a sphere:

• When the inside of the cap acts as a mirror, it is called a concave mirror.
• When the external face acts as a mirror, it is a convex mirror.

Laws of Refraction

• The incident ray, the refracted ray, and the normal lie in the same plane (first law of refraction).
• The relationship between the sines of the angles of incidence and refraction is constant for two specific media, determined by the ratio of their refractive indices (second law of refraction).
• The angle of incidence that results in a 90° angle of refraction is called the critical angle. For incidents exceeding this, light reflects totally; this is total internal reflection.

Lenses and Focal Properties

A lens is a transparent body bounded by two surfaces, at least one of which is curved.

• Converging lenses cause rays to join at a point.
• Diverging lenses cause rays to spread apart.
• The distance between the main focus of a lens and the optical center is the focal length.
• Lens power is the inverse of the focal length, expressed in diopters (when measured in meters).

Light Dispersion and Spectrum

• When white light passes through a triangular glass prism, it breaks down into a color band called the visible spectrum. This is dispersion of white light.
• Light is a form of electromagnetic radiation. The set of all such radiation is the electromagnetic spectrum.