Optics: Reflection and Refraction

Learning Objectives

State the Law of Reflection.

Distinguish between real and virtual images.

Explain refraction and define the index of refraction.

Use Snell's Law to describe how light bends when passing between two media.

The Behavior of Light

Optics is the branch of physics that studies the behavior and properties of light. Geometrical optics treats light as rays traveling in straight lines and focuses on two primary phenomena: reflection and refraction.

Reflection

Reflection occurs when light bounces off a surface.

The Law of Reflection: The angle of incidence equals the angle of reflection (θi = θr). The angles are measured relative to the normal, a line perpendicular to the surface at the point of incidence.

Images formed by Mirrors:

Plane (flat) mirrors produce virtual images that are upright and the same size as the object, located behind the mirror.

Concave mirrors (curved inward) can form real images (if the object is outside the focal point) or virtual images (if the object is inside the focal point). Real images are inverted and can be projected onto a screen.

Convex mirrors (curved outward) always produce virtual, upright, and reduced images.

Real vs. Virtual Images:

Real Image: Formed where light rays actually converge. It can be projected onto a screen.

Virtual Image: Formed where light rays appear to diverge from. It cannot be projected onto a screen.

Refraction

Refraction is the bending of light as it passes from one medium to another. This happens because the speed of light changes in different media.

Index of Refraction (n): A dimensionless number that describes how fast light travels through a material. It is defined as the ratio of the speed of light in a vacuum (c) to the speed of light in the medium (v): n = c/v.

A higher index of refraction means light travels slower in that medium. Light bends towards the normal when entering a denser medium (higher n) and away from the normal when entering a less dense medium (lower n).

Snell's Law quantitatively describes refraction:

n₁sin(θ₁) = n₂sin(θ₂)

n₁ and θ₁ are the index of refraction and the angle of incidence in the first medium.

n₂ and θ₂ are the index of refraction and the angle of refraction in the second medium.

A consequence of refraction is total internal reflection, which occurs when light traveling from a denser medium to a less dense medium strikes the boundary at an angle greater than the critical angle, causing all the light to be reflected back into the denser medium. This is the principle behind fiber optics.

Key Terms