Properties of Light (original) (raw)

Properties of Light Chapter 12 Joseph F. Alward, PhD Department of Physics University of the Pacific

Law of Reflection

	Angle of Reflection = Angle of Incidence Angles are measured with respect to the normal line (the perpendicular line).

Reflection Law Example

Example Problem:

90 - 65 = 25
180 - 120 - 25 = 35
90 - 35 = 55

Billiards and Mirrors

	A ball bouncing off the bank of a pool table behaves like a light ray reflecting off a mirror. To bank the cue ball into another ball (the object ball), imagine you are shooting at the mirror image of the object ball. Billiards and Mirrors This applet allows placement and speed of cue ball.

Specular vs Diffuse Reflection

Diffuse Reflection

Specular and Diffuse Reflection

	Rough surfaces seem smooth for very long-wavelength electromagnetic waves. This parabolic dish reflects long wavelength radio waves to the detector, but reflects most of the shorter wavelength radiation away from the detector.

Specular vs Diffuse Reflection

	The cruiser Aurora, which played an important role in the communist revolution in 1917, is docked on the River Neva at St. Petersburg, Russia. When the water is still, reflection is specular. The image blurs when the water is rough. (Eugene Hecht, Physics)

Specular versus Diffuse Reflection

	What kind of reflections account for the column of light reflected off the water? What would we see on the water if it were perfectly flat, unmoving?

Plane Mirror Geometry

Light from the castle appears to be
coming from a point behind the mirror

Virtual Images in Plane Mirrors

If light energy doesn't flow from the image, the image is "virtual".	Rays seem to come from behind the mirror, but, of course, they don't. It is virtually as if the rays were coming from behind the mirror. "Virtually": the same as if As far as the eye-brain system is concerned, the effect is the same as would occur if the mirror were absent and the chess piece (castle) were actually located at the spot labeled "virtual image".

Image vs Object Distance

	Flat mirror images are as far behind the mirror as is the object. About 4 percent of light is reflected off a windowpane.

The Law of Reflection

	The girl in Edouard Manet's painting, The Bar at the Folies-Bergeres, is standing in front of a large plane mirror. We see reflected in it her back and the face of a man she seems to be talking to. From the law of reflection what if anything, is wrong with this painting? (Eugene Hecht, Physics)

The Law of Reflection

	The Toilet of Venus, by Diego Velasquez. What does Venus see in the mirror? (Eugene Hecht, Physics)

Left-Right Reversal

	The woman's image is as far behind the flat mirror as she is in front of it. Note the that "woman" in the mirror is coming her hair with her left hand.

Left-Right Reversal

Convex and Concave Mirrors

Images in convex mirrors Images in concave mirrors
are always smaller. are always bigger.

Convex Mirrors

Convex Mirrors

The Hubble Mirror

	Images are always smaller in a convex mirror. What type of mirror is the Hubble mirror? What is the person in the mirror pointing at?

Concave Make-Up Mirror

Concave Mirror Focuses Colors

White light is formed where red, blue, and green overlap.

Concave Mirror Focal Point

Rays' paths are determined by the angle law.

Applets

Refraction: The Bending of Wavefronts

Understanding Refraction

Refraction Terminology

	Angles are measured with respect to the line which is perpendicular to the interface. When light travels from one medium--such as air, to another one of higher density--such as water, usually the light ray bends toward the perpendicular line.

Apparent Depth in Water

More Apparent Depth

	Spear-fishing is made more difficult by the bending of light. To spear the fish in the figure, one must aim at a spot in front of the apparent location of the fish.

Refraction at Sunset

The sun actually falls below below the
horizon, i.e., it "sets", a few seconds
before we see it set.

Flattening of the Sun at Sunset

Water on Road Mirage

There's no water on the road; why does it appear so?

Water on Road Mirage

Explaining the Water Mirage

Differential Color Refraction

Note that the shorter wavelengths of light are bent more than the longer; blue more than red.

Applets

Refraction in a Raindrop

Forming a Rainbow

	An observer sees red light coming from droplets of water higher in the sky, while droplets of water lower in the sky send violet light to the eye.

The Shape of the Rainbow

	All of the droplets of water along the arc shown in the figure are equivalent. A rainbow would form a complete circle, not just an arc, if the ground didn't get in the way.

Understanding Rainbow Geometry

Rainbow

	Rainbows always face the observer. As the observer moves, the rainbow moves. One can never get to the "pot of gold" at the end of the rainbow.

Double Rainbows

	The secondary rainbow displays colors in the reverse order. Looking below about 42 degrees, the sky is relatively bright, and acts like a mirror to the sunlight. Above 42 degrees, the water absorbs most of the sunlight, and the sky is darker.

Double Rainbows Explained

Internal Reflection

	All rays reflect internally, but the top three rays reflect only a small percentage internally; most energy leaves the prism. The fourth and fifth rays are reflected 100 % internally.

Internal Reflection and the Critical Angle

	Critical angle is 48 degrees. Any ray which strikes the surface from inside the water at an angle greater than 48 degrees will not escape the water.

The View from Below the Water

	Rays A, B, and C from the bottom of the pond are totally internally reflected. Outside the 96- degree cone, the fish sees only light reflected from the bottom of the pond.

Internal Reflections in Prisms

Prisms in Binoculars

Total Internal Reflection in Diamond

	The critical angle for diamond in air is 24.5 degrees

Internal Reflections in Diamond

	The critical angle for diamond in air is 24.5 degrees; any ray which strikes the surface on the inside at an angle of greater than 24.5 degrees will not escape the diamond.

Optical Fibers

Optical Fiber

Optical Fibers in Medicine

Convex Lenses

Convex Lenses

The Convex Lens
as a Magnifier

Convex Lenses Used as Magnifiers

Concave Lenses

The De-Magnifier

Convex Lens is Inverse
of Concave Lens

Refraction

Surface of water is like a collection of lenses which change shape. Changing air
densities explain why stars twinkle.

Image Formation

Without angle-selectivity, light from all parts of the object overlap all parts of the wall.
Pinhole in box, or a lense directs rays from one part of the object to only one point
on the screen.

In bright light , the eyes' pupils becomes smaller, allowing sharper images to form.

Image Formation with Lenses

	If object is far from the lens (beyond) the focal point, a real inverted image is formed.

Film Projectors

Camera Film Image

Applets

Spherical Aberration

Rays away from the center are not focused at
the focal point.

Chromatic Aberration

Different colors refract by different
amounts.

Photoemission

Important Persons in Early 20th Century Physics

Solvay Conference, 1911	Einstein explained the photoelectric effect by assuming that electromagnetic energy (light) manifests itself as quanta of energy--or, "photons"--of energy hf: f = frequency of light

Duality of Light

	Light acts like a wave when it's moving from one place to another, but when it interacts with matter, it acts like a particle.

Photocells in Garage Door Openers

	Light to photocell is interrupted, and the corresponding drop in photocurrent signals the motor to reverse.

Photocells in Movie Film

	Optical sound track is like a bar-code, but much more detailed. Track modulates the intensity of the light at a frequency which is the same as the sound which was used to produced the track.