Lecture 15 Reflection & Refraction

Instructor: Kazumi Tolich Lecture 15

¨ 18.1 The ray model of light ¤ Source of light rays ¤ Ray diagrams ¤ Seeing objects ¤ Shadows

¨ 18.2 Reflection ¤ Diffuse reflection ¤ The plane mirror

¨ 18.3 Refraction ¤ Examples of refraction ¤ Total internal reflection ¤ Fiber optics 18.1 The ray model of light

¨ A light ray is a line in the direction along the light energy flow. ¤ Light rays travel in straight lines forever unless it interacts with matter. ¤ Light rays can cross without interacting with each other. ¤ Each point of an object is a source of light rays. ¤ The eye sees by focusing a bundle of rays. 18.1 Sources of light rays

¨ Self-luminous objects (or sources) directly create light rays. ¤ A ray source emits a single ray. ¤ A point source emits light rays in all direction. ¤ An extended source emits light from an extended surface. ¤ A parallel-ray source produces a bundle of parallel rays.

¨ Reflective objects reflect rays originating from self-luminous objects. 18.1 Ray diagrams

¨ A ray diagram is a diagram that shows a few light rays in order to simplify the situation. 18.1 Seeing objects / Demo

¨ In order for our eye to see an object, rays from that object must enter the eye.

¨ Diffuse reflection is the process of reflecting incident light in all directions.

¨ Scattering is a process in which single rays are broken into many weaker rays that leave in all directions.

¨ Demo: Laser and chalk dust 18.1 Shadows

¨ An opaque object can intercept rays, creating a shadow behind it.

¨ A point source creates a completely dark shadow with a sharp edge.

¨ An extended source often creates a true shadow that no light reaches, surrounded by a fuzzy region of increasing brightness. 18.2 Reflection

¨ Specular reflection is the reflection from a smooth surface.

¨ The incident and reflected rays and the normal lie on the same plane, and the law of reflection states:

� = � 18.2 Diffuse reflection

¨ Reflection from a rough surface is diffuse reflection.

¨ Light reflected off a dry street is a diffuse reflection, but light reflected off a wet street becomes more specular, and it makes difficult to see what is on the road. 18.2 The plane mirrors

¨ A plane mirror is a flat mirror.

¨ The virtual image of P is at Point P¢. ¨ The image distance � is equal to the object distance �:

� = � Quiz: 18.2-1 & 18.2-2

11 18.3 Refraction

¨ Refraction is a change in direction of light as it transmits from one medium to another.

¨ Light travels from one point to another along the path that takes least time.

¨ Angle of refraction is the angle of transmitted ray, �. ¨ The angles of incidence and refraction, and indices of refraction of two media are related by Snell’s Law:

� sin � = � sin �

Lifeguard analog Quiz: 18.3-1 Total internal reflection / Demo

¨ If � < �, total internal reflection (TIR) occurs when the incident angle is greater than the critical angle, �, and no refracted ray exists.

� sin � = �

¨ Demo: TIR Quiz: 18.3-2 & 18.3-3