PHY 171 - Lecture 6 - Light - Reflection and Plane/Spherical Mirrors

Oct 25, 2026

Uploaded by gewufjwa

Lecture 6 focuses on how light’s wave and particle models lead into geometric optics. It introduces the ray model, where light travels in straight-line paths, and explains its success in describing reflection, refraction, and image formation. The lecture then develops reflection from plane and rough surfaces, including the normal ray and laws of reflection, and contrasts specular (regular) versus diffuse reflection. Finally, it details virtual image formation in plane mirrors and key characteristics, then extends to concave and convex spherical mirrors, defining the center of curvature and principal axis for locating image positions.

LanguageEnglish

CategoryTechnology

FormatPDF

Pages6

Reading Time15 min

AccessFree to download

Chapter 35: Geometric Optics

Ray Model of Light

Reflection

Laws of Reflection

Diffuse Reflection

Formation of Images in a Plane Mirror

Characteristics of Plane Mirror Images

Spherical Mirrors

PHY 171 Lecture 6 (January 18, 2012) Light Throughout most of the next 2 weeks, we will be concerned with the wave properties of light, and phenomena based on them (interference & diffraction) . Light also can be modeled as if it is made of particles (e.g., for the photoelectric effect)– those of you in Physics and Engineering will see this in PHY 270. But, there is yet another way of studying light that has consequences for the real world (building mirrors and lenses). This is what we will study first, and we will do it in a more experiential sitting than we have been having so far. Chapter 35: Geometric Optics Ray model oflight Ray model assumes that light travels in straight-line paths called light rays. • A light ray is an idealization used to represent an extremely narrow beam of light. • Ray model successful in accounting for reﬂection, refraction, and the formation of images by mirrors and lenses. Demo: Ray model We looked at a demonstration of the straight-line path of a laser beam illuminated by scattering off of gas particles sprayed from a can. In this lecture, we will learn about reflection from plane and curved surfaces. In particular, we will learn about the paths oflight reflected from plane mirrors and convex and concave mirrors. We will also learn about the images formed in these mirrors, and whether these images are real or virtual (the difference is explained below), upright or inverted, enlarged or diminished in size, and formed behind the mirror or in front. Reflection We saw a demo in Blackboard Optics. The normal ray is a ray at right angles to the reflecting surface. If light is incident perpendicular to the mirror surface along the normal ray, it will be reflected back along the same path (as shown in the figure) . Laws of Reflection Normal Incident Ray Reflected Ray Ray 1. The incident ray, the normal ray at the point of incidence, and the reflected ray all lie in the same plane. 2. The angle of incidence is equal to the angle of reflection. Diffuse Reflection When light is incident on a rough surface, it is reflected in all directions. This is known as diffuse reflection. Note that the laws of reflection still hold at each small section of the surface. It is only because each small section of the surface is angled differently for a rough surface that for a parallel beam of light, the reflected rays go offindifferent directions (as seen in the figure) . To distinguish it from diffuse reflection, the reflection from regular (mirrored) surfaces is sometimes called “regular” or “specular” reflection. Formation of Images in a Plane Mirror A bottle is placed in front of a plane mirror. Rays of light from the bottle to the mirror are reflected as shown, in accordance with the laws of reflection stated above. They are then viewed at the position indicated by the eye. Our eyes interpret any rays entering it as having traveled in a straight-line path. So, rays from the object that are reflected from the mirror and enter the eye are traced back by the eye along a straight line, and appear to come from a point behind the mirror. This point behind the mirror is the location of the image. The image formed by a plane mirror is called a virtual image because the light rays do not actually pass through the image position – they only appear to be coming from the image position because the brain interprets any light entering our eyes as having come in a straight line path. The designation “virtual image” distinguishes the image from a real image formed by curved mirrors and lenses in which the light actually passes through the image position —therefore, if you put a screen at the location of a real image, the image can be viewed on the screen (you have experienced the power of projecting a real image many times before, of course—in a cinema theater) . Note that instead of drawing a bottle every time, we will simply be drawing an arrow to indicate the obj

FAQs

What does the ray model assume about how light travels?

It assumes light travels in straight-line paths called light rays, representing an extremely narrow beam.

What are the laws of reflection?

The incident ray, normal ray, and reflected ray lie in the same plane, and the angle of incidence equals the angle of reflection.

Why is the image in a plane mirror considered virtual?

Because light rays reflected from the mirror do not actually pass through the image location; they only appear to originate there when traced backward.

Oct 25, 2026

Uploaded by gewufjwa