Light sources, light propagation, and shadows Some ancient people thought that humans emitted special invisible rays from their eyes. –If this were true, humans should be able to see in total darkness, which we cannot. To see something, we need a source of light and an object off which the light bounces (reflects) and then reaches the eyes of the observer. © 2014 Pearson Education, Inc.

Ray diagrams Represent the travel of light from one location to another with a light ray, drawn as a straight line and an arrow. Diagrams that include light rays are called ray diagrams. © 2014 Pearson Education, Inc.

Ray model of light Testing experiments show that model 1 is inconsistent with experimental evidence. Model 2 is supported: –Each point on an extended light source emits light in many different directions. –This light can be represented by multiple rays diverging from that point. © 2014 Pearson Education, Inc.

Tip © 2014 Pearson Education, Inc.

Shadows and semi-shadows A sharp shadow is called an umbra. –A shadow is a region behind the object where no light reaches. A semi-shadow is called a penumbra. –A semi-shadow is a region where some light reaches and some does not. It appears as a fuzzy shadow. © 2014 Pearson Education, Inc.

Pinhole camera Cardboard with a small hole in it is the foundation of the pinhole camera, also called a camera obscura. –It consists of a lightproof box with a very small hole in one wall and a photographic plate or film inside the box on the opposite wall. –Before the invention of modern cameras that use lenses, pinhole cameras were used to make photographs. © 2014 Pearson Education, Inc.

Reflection of light Light from a laser pointer shines on a mirror. © 2014 Pearson Education, Inc.

Reflection of light Incident light: light striking the mirror Normal line: a line perpendicular to the surface where the incident light hits the mirror Angle of incidence: the angle between the incident beam and the normal line Angle of reflection: the angle between the reflected beam and the normal line © 2014 Pearson Education, Inc.

Reflection of light Experiments yield the following data: © 2014 Pearson Education, Inc.

Reflection of light The angle of reflection = the angle of incidence. © 2014 Pearson Education, Inc.

Tip © 2014 Pearson Education, Inc.

Law of reflection © 2014 Pearson Education, Inc.

Specular and diffuse reflection © 2014 Pearson Education, Inc.

Red eye effect When a camera flash illuminates the open iris, light reflects from the red blood vessels in the retina on the back of the eye. Some of this reflected light passes back out of the pupil and makes the pupil appear red. © 2014 Pearson Education, Inc.

Refraction of light At the shore of a lake, you see sunlight reflecting off the water's surface. –You also see rocks and sea plants under the surface. –To see them, light must have entered the water, reflected off the rocks and plants, returned to the water surface, and then traveled from the surface to your eyes. © 2014 Pearson Education, Inc.

Refraction of light We can develop a mathematical relationship between the angle of incidence and the angle of refraction. © 2014 Pearson Education, Inc.

Angle of refraction Refraction is the change in the path of light when light travels from one medium to another. n 1 to 2 depends on the two materials through which the light is traveling. © 2014 Pearson Education, Inc.

Snell's law © 2014 Pearson Education, Inc.

Tip © 2014 Pearson Education, Inc.

Refractive indexes © 2014 Pearson Education, Inc.

Tip © 2014 Pearson Education, Inc.

Total internal reflection At the critical angle of incidence, the refraction angle is 90 o. The refracted ray travels along the water-air interface. © 2014 Pearson Education, Inc.

Total internal reflection © 2014 Pearson Education, Inc.

Tip © 2014 Pearson Education, Inc.

Skills for analyzing reflective and refractive processes When you solve problems involving light, use ray diagrams to help in your reasoning and quantitative work. The diagrams will help you evaluate the final answer. © 2014 Pearson Education, Inc.

Skills for analyzing reflective and refractive processes When problem solving: –Draw a ray diagram showing all relevant paths the light travels. Consider the light rays originating from the object and eventually reaching the observer. You can represent the observer by an eye. –Use the sketch and ray diagram to help construct a mathematical description using the law of reflection, or Snell's law for refraction, or the application of Snell's law for total internal reflection. © 2014 Pearson Education, Inc.

Fiber optics We can understand fiber optics by using total internal reflection. –Fiber optic filaments are used in telecommunications to transmit high-speed light-based data and in medicine to see inside the human body during surgery. © 2014 Pearson Education, Inc.

Fiber optics Imagine that you have a long glass block of refractive index 1.56 surrounded by air. Light traveling inside the block hits the top horizontal surface at a 41 o angle. What happens next? © 2014 Pearson Education, Inc.

Prisms The refractive index of prism glass is greater for violet light and smaller for red light. © 2014 Pearson Education, Inc.

Prisms for reflection Prisms reflect almost 100% of the light incident on them, whereas mirrors reflect somewhat less than 100%. Prisms do not tarnish like mirrors. Prisms can invert an image—that is, make it appear upside down. © 2014 Pearson Education, Inc.

Mirages On a hot day, hot air may hover just above the pavement. This hot air is less dense and has a lower index of refraction than the cooler air above it. –When light from the sky passes through air with a gradually changing index of refraction, its path gradually bends, leading us to perceive that the source of light is at a different location than it actually is. © 2014 Pearson Education, Inc.

Mirages © 2014 Pearson Education, Inc.

Color of the sky Due to their sizes, atmospheric particles reflect blue light more efficiently than other colors. © 2014 Pearson Education, Inc.

Particle model of light © 2014 Pearson Education, Inc.

Wave model of light © 2014 Pearson Education, Inc.

Wave model and refraction Imagine a light wave moving in a less optically dense medium 1 and reaching an interface with a denser medium 2 at a nonzero angle of incidence. © 2014 Pearson Education, Inc.

Summary © 2014 Pearson Education, Inc.

Summary © 2014 Pearson Education, Inc.

Summary © 2014 Pearson Education, Inc.

Summary © 2014 Pearson Education, Inc.

Summary © 2014 Pearson Education, Inc.

Summary © 2014 Pearson Education, Inc.