Physics: light waves

Properties and Sources of Light Key Question: What are some useful properties of light?

 Light travels almost unimaginably fast and far.  Light carries energy and information.  Light travels in straight lines.  Light bounces and bends when it comes in contact with objects.  Light has color.  Light has different intensities, it can be bright or dim. Properties and Sources of Light

 The process of making light with heat is called incandescence.  Incandescent bulbs generate light when electricity passes through a thin piece of metal wire called a filament. Electric Light  The filament heats up and gives off light.

Light carries energy and power  Light is a form of energy that travels.  The intensity of light is the amount of energy per second falling on a surface.  Most light sources distribute their light equally in all directions, making a spherical pattern.  Because light spreads out in a sphere, the intensity decreases the farther you get from the source.

Light intensity  The intensity of light from a small source follows an inverse square law because its intensity diminishes as the square of the distance.

Light carries information  In some cities, a fiber- optic cable comes directly into homes and apartments carrying telephone, television, and Internet signals.

The speed of light  The speed at which light travels through air is approximately 300 million meters per second.  Light travels almost a million times faster than sound.

The speed of light  The speed of light is so important in physics that it is given its own symbol, a lower case c.  The best accepted experimental measurement for the speed of light in air is 299,792,500 m/sec.  For most purposes, we do not need to be this accurate and may use a value for c of 3 × 10 8 m/s.

Electromagnetic Waves  Radio waves  Microwaves  IR waves  Visible Light  UV waves  X rays  Gamma rays Wavelength Longest shortest Frequency smallest Highest

Flat mirror  Angle of Incidence = angle of reflection  Image is virtual: light does not pass through image

Significance of the focal point All light rays that are parallel to the axis will pass (after extrapolating for convex mirror) through the focal point! For a concave mirror, the light can pass through the focal point.

Properties of the Image If we put an object outside of the center of a concave mirror, we find the image is  Real, in the sense that all light rays pass through the image.  Inverted, in the sense that the direction of the arrow has been changed.  The image is smaller!

Animation for case 1

Animation for case 2

Animation for case 3

Properties of the image If the object is closer to the mirror than the focal point F, the image is  Virtual, it is behind the mirror  Upright, not inverted  Magnified Can be used for shaving!

Summary

Some of the really-cool applications Suppose you put a point source of light at F. All rays will be reflected back parallel to the axis of mirror: a neat way to construct a parallel beam!  Applications: flashlight! Headlight in the car. Conversely, if one has parallel rays, all are reflected to pass through F. So all light energy is focused to one point!

Convex mirror  F, Q (image) and C are (-)  Object (p) is (+)

Ch.15 Reflection and refraction  When light moves through a material it travels in straight lines.  When light rays travel from one material to another, the rays may reflect.  The light that appears to bounce off the surface of an object is shown by a reflected ray.

Ch.15 Reflection and refraction  Objects that are in front of a mirror appear as if they are behind the mirror.  This is because light rays are reflected by the mirror.  Your brain perceives the light as if it always traveled in a straight line.

Ch.15 Reflection and refraction  Another example of refraction of light is the twinkling of a star in the night sky  As starlight travels from space into the Earth’s atmosphere, the rays are refracted.  Since the atmosphere is constantly changing, the amount of refraction also changes.

Ch.15 Reflection and refraction  The light that bends as it crosses a surface into a material refracts and is shown as a refracted ray.

14-4 Color and Vision  When all the colors of the rainbow are combined, we do not see any particular color.  We see light without any color.  We call this combination of all the colors of light "white light".

14-4 Color and Vision  We can think of different colors of light like balls with different kinetic energies.  Blue light has a higher energy than green light, like the balls that make it into the top window.  Red light has the lowest energy, like the balls that can only make it to the lowest window.

How the human eye sees color  The retina in the back of the eye contains photoreceptors.  These receptors release chemical signals.  Chemical signals travel to the brain along the optic nerve. optic nerve

Photoreceptors in the eye  Cones respond to three colors: red, green and blue.  Rods detect intensity of light: black, white, shades of gray.

How we see colors  Which chemical signal gets sent depends on how much energy the light has.  If the brain gets a signal from ONLY green cones, we see green.

14-4 How we see other colors  The three color receptors in the eye allow us to see millions of different colors.  The additive primary colors are red, green, and blue.  We don’t see everything white because the strength of the signal matters.  All the different shades of color we can see are made by changing the proportions of red, green, and blue.

14-4 How we see the color of things When we see an object, the light that reaches our eyes can come from two different processes:  The light can be emitted directly from the object, like a light bulb or glow stick.  The light can come from somewhere else, like the sun, and we see the objects by reflected light.

14-4 How we see the color of things  Colored fabrics and paints get color from a subtractive process.  Chemicals, known as pigments, in the dyes and paints absorb some colors and allow the color you actually see to be reflected.  Magenta, yellow, and cyan are the three subtractive primary colors.

Complementary colors  Add up to make white  Red and cyan  Blue and yellow  Green and magenta  Black and white