Fundamentals of Light: Reflection, Refraction, and Lenses

Understanding Light: Key Optical Concepts

Index of Refraction

We define the index of refraction (n) of a medium as the ratio between the speed of light in a vacuum (c) and the speed of light in that medium (v):

n = c / v

Laws of Reflection of Light

When light rays strike a surface, they undergo a rebound that follows these laws:

1. The incident ray, the normal to the surface, and the reflected ray all lie in the same plane.
2. The angle of incidence (i) is equal to the angle of reflection (r), and they are on opposite sides of the normal: i = r.

Laws of Refraction of Light

When light passes from one transparent medium to another where the speed of light changes, its direction also changes. This phenomenon is known as the refraction of light, governed by these laws:

1. The incident ray, the normal to the surface, and the refracted ray all lie in the same plane.
2. The ratio between the sines of the angles of incidence and refraction is a constant value, equal to the ratio of the speed at which light propagates in the first medium to the speed with which it spreads in the second medium.

Total Internal Reflection

Total internal reflection is a phenomenon utilized in the conduction of light beams within optical fibers. Optical fibers consist of clear plastic or glass filaments (the core) coated with a substance of a lower refractive index (the cladding). This difference in refractive indices makes total internal reflection possible.

A beam of light entering one end of the fiber undergoes repeated total reflections on the inner wall of the filament, continuing to reflect until it exits the other end.

Critical Angle

The critical angle is the angle of incidence at which the angle of refraction becomes 90 degrees. For the water-air separation surface, the critical angle is approximately 48.6°.

Focus and Focal Lengths

Image Focus

The image focus is the point where all rays, or their extensions, that arrive parallel to the optical axis converge after passing through an optical system.

Object Focus

The object focus is the point from which all rays, or their extensions, originate such that they exit parallel to the optical axis after passing through an optical system.

Object Focal Distance

The object focal distance is the distance between the object focus and the optical center of the system.

Image Focal Length

The image focal length is the distance between the image focus and the optical center of the system.

Operation of a Lens (Magnifying Glass)

A magnifying glass is a converging lens with a short focal length, placed between the eye and the object being observed to increase the size of the image formed on the retina. When the object is placed between the focus and the optical center, it produces a virtual, upright, and magnified image, though the magnification is not substantial.

Spherical Lenses

A spherical lens is the simplest optical system composed of two spherical dioptric surfaces, although one surface may be flat. Lenses can be either convergent or divergent.

• Convergent Lenses: These are thicker in the middle and thinner at the edges. They converge rays parallel to the principal axis to a single point.
• Divergent Lenses: These are thicker at the edges than in the center. They cause rays parallel to the principal axis to spread out (diverge).