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