Chapter 16

Light waves do not follow the same rules as other forms of waves. Many different models have been used to describe its behavior.

Wave Model of Light: Light is an electromagnetic

wave made up of an oscillating electric and magnetic field.

Light waves have transverse motion.

Frequencies increase and wavelengths decrease as we move from red to purple.

Particle Model of Light Light moves like an invisibly small stream of particles. The particles travel in straight lines until they strike a surface. Intensity (number) of particle strikes increase from red to purple. Particles are not considered matter, but they behave like a

stream of matter particles.

Photon Model Hybrid of wave and particle models. Light is composed of particles called photons, and each

photons contains a certain amount of energy. Color of photon depends upon the amount of energy it

contains (correlates to the frequencies of the wave model)

Even though there are different models of the light waves, their energies seem to be proportional no matter what the model.

Electromagnetic waves are typically arranged on a spectrum according to the wave model.

Lowest to highest frequency:Radio < Microwave < IR < Visible < UV < X Rays < Gamma

Rays

Lowest to highest wavelength:Gamma Rays < X Rays < UV < Visible < IR < Microwave <

Radio

Radio Waves: Uses:

Communication RADAR

Microwaves Uses:

Cooking Communication via

Satellites

Infrared light Uses:

Warmth Night Vision

Equipment Fire Detecting

Equipment

Visible Uses:

Vision Electronic projection

and imaging

Ultraviolet (UV) Uses:

Disinfects tools Medical detection

device Forensics detection

device Dangers:

Can mutate DNA

X-Rays Uses:

Medical Imaging (hard tissue)

Dangers: Can mutate DNA

Gamma Rays Uses:

Cancer treatment Dangers:

Causes radiation sickness.

Will mutate DNA

The speed of light (c) is 3x108 m/s (186,000 mi/s) in a vacuum.

Light slows as it passes through a medium.

Light travels quickest in clear or transparent mediums (A).

Light slows considerably in cloudy or translucent mediums (B). Some energy is absorbed or reflected.

Light will not travel through solid color or opaque mediums (C). Most energy is absorbed or reflected.

Optics is the field of science that deals with the propagation and behavior of light.

Reflection is the bouncing of a wave off of a boundary.

The law of reflection describes how waves reflect: “The angle of incidence equals the angle of reflection.”

Normal – an imaginary line 90o (perpendicular) to a surface.

Angle of Incidence – the angle between the incoming ray and the normal.

Angle of Reflection – the angle between the outgoing ray and the normal.

Entire images can be reflected off of surfaces that are smooth, allowing the angles of reflections to be consistent.

Mirrors are tools used to create images through reflection. There are 3 types

of mirrors: Plane (Flat) Concave Convex

Two types of images can be created with mirrors:

Virtual images form at a point where light rays cannot exist (ex. behind mirror).

All mirrors can produce virtual images.

Real images are formed by many light rays coming together in a specific location (projection)

Only concave mirrors can make real images.

Refraction is the bending of waves as they pass from one medium to another.

Shaped glass tools called lenses are used to bend waves a specific amount through refraction.

There are two types of lenses: Convex Lens

Converge light (beams converge behind it in a central focal point)

Produce real or virtual images. Uses: magnifying glasses, our

eyes Concave Lens

Diverge light (beams spread outward)

This type of lenses can only create a virtual image.

How do we see?1. Light waves reflected from

an object travel toward the eye.

2. The cornea bends and focuses light towards the lens.

3. The lens refracts light and focuses it on the retina.

4. Specialized cells called rods detect light intensity while cones detect color.

5. Signals from the retina travel to the brain from the optic nerve.

Different colors come from white (sun) light. Each of these colors

has its own frequency, wavelength, and energy.

Our eyes can detect the following wavelengths in the electromagnetic spectrum: λ= 400 nm (violet light) – 700 nm (red light). (ROYGBIV)

A prism is a transparent block with a triangular cross-section that can separate white light into all of its colors.

This phenomenon is called dispersion and is caused by the differences in the refractive angles of different colors of light.

Rainbows are formed by natural dispersion through water droplets in the atmosphere.

There are three primary colors of light: red, blue, and green.

Objects that create their own light can produce different colors using the additive method.

These items appear black unless they are producing light.

Objects that cannot produce their own light must reflect light in order for them to have a color.

Pigments absorb specific wavelengths of light. The reflected wavelengths combine to give an object color.

This is called subtractive color.

Objects that are white contain no pigment and reflect all 3 primary colors of light.

Objects that are black have pigments that absorb all 3 primary colors of light and do not reflect any wavelengths.