Major Concepts of Physics PHY 102 – Lecture #  Syracuse University Lecture #11 Reflection. Geometrical optics Monday, February 29 th Spring 2016 Prof. Martin Forstner Room 211, Physics Bldg., Major Concepts of Physics PHY102

Major Concepts of Physics PHY 102 – Lecture #  Syracuse University 1. The ray approximation (essential for Geometrical Optics) 2. Geometrical optics 3. Reflection 4. Applications 5. Announcements Lecture objectives

Major Concepts of Physics PHY 102 – Lecture #  Syracuse University Alexander Graham Bell – 1870s

Major Concepts of Physics PHY 102 – Lecture #  Syracuse University The light flash of a firefly Oxygen reacts with luciferin. The reaction is catalyzed by the enzyme luciferase.

Major Concepts of Physics PHY102 – Lecture #  Syracuse University Concentric circular ripples Circular waves

The direction of travel of the electromagnetic wave (propagation direction) is the ray direction. Major Concepts of Physics PHY 102 – Lecture #  Syracuse University The Ray approximation – Essential for Geometrical Optics

Major Concepts of Physics PHY 102 – Lecture #  Syracuse University Conceptual example

The RA says: If light travels in a uniform medium, its rays are straight lines. The wave-like properties can be ignored. Effectively, it assumes that the wavelength λ is zero. Major Concepts of Physics PHY 102 – Lecture #  Syracuse University The Ray Approximation 8

The opposite case is >> d. The result then is that the opening behaves like a point source of light. The light spreads out from the opening. The RA is not valid. Major Concepts of Physics PHY 102 – Lecture #  Syracuse University When the RA is Invalid

Let d be the dimensions of any object or opening, or obstacle, encountered. If << d, then the RA is valid. Ex.: Red light ( = 0.7 microns=700 nm) is incident on a barrier. It has an opening of diameter d = 1 cm. Then, << d. So, the RA is valid. Let d be the dimensions of any object or opening, or obstacle, encountered. If << d, then the RA is valid. Ex.: Red light ( = 0.7 microns=700 nm) is incident on a barrier. It has an opening of diameter d = 1 cm. Then, << d. So, the RA is valid. Major Concepts of Physics PHY 102 – Lecture #  Syracuse University Validity of the RA

ray approximation indicate the propagation of light by arrows pointing in the direction of propagation and perpendicular to the wave fronts valid approximation if << the size of any hole or objects doesn’t take into account interference or diffraction Major Concepts of Physics PHY102 – Lecture #  Syracuse University Ray diagrams

This is the study of the paths that the light rays take, when the RA is valid. In contrast, physical optics studies phenomena that develop from the fact that light is a wave. Geometrical Optics is summarized by two laws: The Laws of Reflection and Refraction Major Concepts of Physics PHY102 – Lecture #  Syracuse University Geometrical Optics

When light is incident on a boundary with a new material, there are three possibilities: It is absorbed. 1. For an ideal black body, this is the only process that takes place. 2. It is transmitted into the new material. We consider this later. 3. It is reflected. In this case, part of the light energy remains in the original material. Generally, all three processes occur. Major Concepts of Physics PHY102 – Lecture #  Syracuse University Reflection

We see all objects via light reflected from that object, unless the object is itself a source of light. Two Types of Reflection 1) Mirror (smooth surface) 2) Diffuse (rough surface) Major Concepts of Physics PHY102 – Lecture #  Syracuse University Reflection

Major Concepts of Physics PHY 102 – Lecture #  Syracuse University Big deal: the quality of surface! Specular reflectionDiffuse reflection

When the surface of reflection is smooth, 1) takes place. If it is rough, 2) takes place. What determines smoothness? A smooth surface has surface variations that are much less than. For a rough surface, the variations are larger than. Because is small for light, most surfaces are rough. Major Concepts of Physics PHY102 – Lecture #  Syracuse University Criteria

In diffuse reflection, the light is reflected in all directions. In mirror reflection, a simple law holds: Let  i be the angle of incidence. Always, angles are measured from the direction of the surface normal. The normal is an imaginary line, perpendicular to the surface, at the point of contact of the ray. Major Concepts of Physics PHY102 – Lecture #  Syracuse University Laws for Diffuse and Mirror Reflection Laws for Diffuse and Mirror Reflection

Statement of the Law of Reflection Let  r be the direction of the reflected ray. Then,  inc =  refl. The angle of reflection is equal to the angle of incidence. Plane of the Reflected Ray The reflected ray is in the same plane as the incident ray and the normal. Major Concepts of Physics PHY102 – Lecture #  Syracuse University Statement for the Law of Reflection Statement for the Law of Reflection

light waves can be reflected at a surface between two media Law of Reflection  inc =  refl Major Concepts of Physics PHY 102 – Lecture #  Syracuse UniversityReflection

Major Concepts of Physics PHY 102 – Lecture #  Syracuse UniversityReflection

Major Concepts of Physics PHY102 – Lecture #  Syracuse University Flat mirror forms virtual image Virtual image is similar to the object The Virtual Image The Virtual Image

Draw these reflected rays. They reach the eye of an observer. The rays appear to come from a point behind the mirror. This point is the image-point. All the rays appear to diverge from the image-point. The rays of light do not actually come from the image-point. So, we say that the image is a virtual image. From simple geometry, it is easy to prove the following: The object-size and the image-size are the same. The distances to the mirror of the image and the object are the same. Major Concepts of Physics PHY102 – Lecture #  Syracuse University The Virtual Image The Virtual Image

Great discoveries are based on simple principles! Fig in the textbook 23

Major Concepts of Physics PHY 102 – Lecture #  Syracuse University 1.Reading 1.Reading: Chapter 23 rd, Sections 23.1, 23.2, pp Conceptual examples Homework. 2. Homework. Homework #4 is due on this week’s lab. 3. This week’s workshop: 3. This week’s workshop: Reflection and MirrorsAnnouncements