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Light
The Sun is our major source of light
Light sources can be natural or artificial (man-made)
People and insects do not see the same type of light- people see visible light- insects see ultraviolet light
Electromagnetic Spectrum
• Light is an electromagnetic wave →has wave like properties which can be influenced by electric and magnetic fields.
• the electric and magnetic fields are perpendicular to one another (transverse wave)
• the amplitude of the oscillating fields determines the brightness (intensity) of the light
Electromagnetic Spectrum
• Electromagnetic waves are produced when an electron in an atom vibrates back and forth. • a charge that is changing speed or direction
Light originates from movement of electrons from a higher to a lower orbit → light is energy
Electromagnetic waves travel without a medium or in a vacuum (light waves from the sun)
Electromagnetic Spectrum
• A wave's speed, frequency, and wavelength are related by the equation v = fλ.
• All electromagnetic waves in a vacuum have the same speed, c. Therefore, the frequency, f, and the wavelength, λ, of an electromagnetic wave are related as follows:
Color and the Electromagnetic Spectrum
• The electromagnetic waves that our eyes can detect are known as visible light.
• Since color is related to energy, there is also a direct relation between color, frequency, and wavelength.
White light is the combination of all colors (red-violet)
Electromagnetic Spectrum
• EM waves arranged in order by wavelength and frequency
• Waves with the longer wavelengths have the lower frequencies
• Waves with the shorter wavelengths have the higher frequencies
• As the frequency increases, energy (dangeincreases
Visible Light
When light passes through a prism, the light is separated into different frequencies and wavelengths - Different frequencies bend different amounts
Luminous object- Anything that gives off its own lightIlluminated object- An object that is capable of reflecting light
Visible Light
Materials that transmit light can be classified as:• Transparent- clear image, light passes through
the material easily• Translucent- unclear image, lacks detail• Opaque- light does not pass through
• color of an opaque object is the color it reflects
Visible Light
The color is the result of light that strikes the object and is reflected by it.
White object - All colors (frequencies) are reflected
Black object- No color; the absence of color; all light is absorbed
Color and the Electromagnetic Spectrum
Primary colors
Combining two primary colors results in the secondary colors of light; yellow,
magenta and cyan
In different colors of light these clothes would look different:
Redlight
Shirt looks red
Shorts look black
Bluelight
Shirt looks black
Shorts look blue
Light Movement
• Light travels in straight lines from the source• Can only be reflected in straight lines
Polarization and Scattering of Light• A common incandescent light bulb and the Sun both
produce unpolarized light.• When unpolarized light encounters a filter:
• Some of the light in the unpolarized beam has a vertical polarization and passes right through the polarizer.
• Some of the light has a horizontal polarization and is blocked.
• a filter that transmits light waves with only one direction of polarization.
•
Refraction
The presence of new denser material…. slows light’s progress due to interactions with the electrical properties of atoms
This “light slowing factor” due to the interaction is called the index of refraction
Refraction- light bends or changes direction at the boundary between two media.
Refraction
The index of refraction (n) of a substance is the ratio of the speed of light in a vacuum to the speed of light in the substance
Refraction
Light bends at the point of refractive incidence between different refractive indices
The GREATER the difference between refractive indices, the MORE light will bend
The angle of reflection will be LESS THAN the angle of incidence if light SLOWS DOWN when going from one material to another
The angle of reflection will be GREATER THAN the angle of incidence if light SPEEDS UP when going from one material to another
Snell’s law describes the relationship between the angle of incidence and the angle of refraction.
The degree to which light is bent depends on the medium and the density of the medium.
Snell’s Law
We refer to θ1 as the angle of incidence and θ2 as the angle of refraction.
The Reflection of Light
Specular reflection- light reflecting off a smooth surface
Diffuse reflection- light reflecting off a rough surface
Reflection
Reflection off a flat surface follows a simple rule:Angle IN equals Angle OUT
Plane Mirrors
Image formed by a plane mirror is :• same distance behind mirror• same size (unmagnified)• upright, but reversed right to left• virtual
Real Image ● made from “real” light rays that converge at a
real focal point● can be projected on a screen because light
actually passes through the point where the image appears
● always invertedVirtual Image
● not real because the image cannot be projected
Parabolic Mirrors
Principal axis- line center of the sphere and attaching to the exact center of the mirror
Center of Curvature (C) - the exact center if the imaginary sphere
Radius of Curvature (R) distance from vertex to center of curvature
Parabolic Mirrors
Vertx (A) - geometric center of the mirror
Focal Point (F) midway point between the vertex and the center of curvature
Focal Distance (f) distance from the mirror to the focal point (f=1/2R)
Ray Diagrams
Principal axisFCobject
Two rays, known as principal rays, are used in ray tracing with spherical mirrors:
1. Parallel ray (P ray) reflects through the focal point2. Focal-point ray (F ray) reflects parallel to the principal axis
P-rayF-ray
Image
Concave Mirrors
• Curve inward• Create either a real or virtual image• Reflect light from the inner surface
• eg. inside of spoon• used in telescopes, satellite dishes
For a real object close to the mirror but outside of the center of curvature, the real image is formed between C and f. The image is inverted and smaller than the object.
Concave Mirrors
For a real object at C, the real image is formed at C. The image is inverted and the same size as the object.
Concave Mirrors
For a real object between C and f, a real image is formed outside of C. The image is inverted and larger than the object.
Concave Mirrors
For a real object at f, no image is formed. The reflected rays are parallel and never converge.
Concave Mirrors
For a real object between f and the mirror, a virtual image is formed behind the mirror. The position of the image is found by tracing the reflected rays back behind the mirror to where they meet. The image is upright and larger than the object.
Concave Mirrors
Convex Mirrors• Curve outward• Reduce the image size• Create virtual image• Reflect light from the outer surface
• eg. outside of spoon• Used in rear view mirror, store security• Negative focal length
Curved Mirrors
do - distance object is from the mirrordi - distance image is from the mirror f - distance focal point is from the mirror
Curved Mirrors
• It is important to identify and use the correct sign for each term in the mirror equation. The sign convention for both concave and convex mirrors is summarized in the table below.
Convex Lenses
• Thicker in the center than the edges• Converges (brings together) light rays• Form real images and virtual images
depending on the position of the object
Concave Lenses
• Lenses that are thicker at the edges and thinner at the center
• Diverge light rays• All images are upright and reduced
(that means all images are virtual)
Nearsighted (myopic)• relaxed eyes do not focus at infinity as they should.
Instead, they focus at a finite distance—the far point.• eyeball is too long and images focus in front of the
retina• Fix: concave lens to expand the focal length
Farsighted (hyperopic)• see clearly beyond a certain distance—the near
point—but cannot focus on closer objects • eyeball is too short so images are focused behind the
retina• Fix: convex lens to shorten the focal length
Fiber Optics
There are three main parts to an optical fiber:1. Core - the thin glass center where the light
travels2. Cladding - optical material that surrounds the
core and reflects light back into the core3. Buffer Coating - plastic coating on the outside of
an optical fiber to protect it from damage
Fiber Optics
• Light travels through the core of a fiber optic by continually reflecting off the cladding
• Due to total internal reflection, the cladding does not absorb any of the light allowing the light to travel over great distances
• Some of the light signal will degrade over time due to impurities in the glass