Ray-Optics Notes (PDF) - 2024-11-04

Ray-optics: A model of light; reflection law; example on imaging an object by a flat mirror Kai Shum/BC/CUNY 2024-11-04  Background Please note when we describe the kinematics of any object, we treat it as just a point in a space; when we consi...

Ray-optics: A model of light; reflection law; example on imaging an object by a flat mirror Kai Shum/BC/CUNY 2024-11-04  Background Please note when we describe the kinematics of any object, we treat it as just a point in a space; when we consider the dynamics/energetics of an object, we add an inertial mass parameter associating to the object. We know this is a simple model. In our real every day’s life, the object can be a person, a car, or train, etc.……much more complicated… Similarly, for us to treat light and study how light travels in space, another word, to understand how we can see each other when meet with ambient light, and how you can see yourself in a mirror every morning, we must introduce a simple model for a light beam → Ray model of a light beam  A ray model of a light beam 2. It has no size (very thin) and lies in this slide plane in a same medium, say in air or in vacuum 3. It has well-defined direction goes to infinitely far UNLESS it meets an interface formed by a different material. 1. It (energy) comes from a source, For an example, a laser that emits light  One light beam is not enough to see the point-source 2. It has no size and lies in this slide plane in a same medium. Your eyes (or detector) must detect light to see the object which emits light can not determine where this point is located along the ray 1. It comes from a source (say, a point source)  Two light beams (rays) are needed to see the point- source! 2. It has no size and lies in this slide plane in a same medium. Ray-1 Ray-2 Your eyes (or detector) can now determine where this point is exactly located. 1. It comes from a source (say, a point source)  Reflection law This ray is called incident light ray NN* interface is basically a mirror. with incident angle of Q N You use it everyday, it has a few mm think glass in front and the back of the glass is coated with a metallic layer, such as Al layer A → Light can not pass it! Incident angle Q C C* XXX NO! B C Dot-dashed CC* line is a principal axis, Big question: where light ray is going perpendicular to the NN*, passing through to go, A? B? or C? or the point where incident light ray meets once it meets the interface NN* the NN* interface. N* The reflection law (physics) tells where the incident light ray Thin metal, such as Al Very thin! to go in next slide.  Reflection law This ray is called incident light ray with incident angle of Q N Q C C* Q* Reflection law states: This ray is called reflected ray 1) Reflected light beam should be in with reflection angle of Q* a plane formed the NN* and CC* 2) Q* = Q N* 3) Light source/sensors (detectors) must be very near the principal axis  RO -Ex.1: finding reflected beams (Ray-optics) Reflected ray N Ray 1 from an object 2* 2 (incident ray) is incident on the mirror with incident angle of Q1, and 3 Ray 2 from the same object 1 (incident ray) is parallel to the CC* axis and also, incident on the mirror. Q1 Find: C1 C1* 1) Incident angle for the ray 2, Q2=0 Q1* 2) Reflection angle for the ray 2, Q2* = 0 3* 3) Draw the reflected ray (2*) on the diagram 1* (Reflected ray ) Examples: Q1 = 35o Q2 = 0 N* Q3 = 30o Next question: where do you see the point that emits light rays 1, 2, and 3?? Imaging an object by a flat mirror