Chapter 19 - Light

The Electromagnetic Spectrum

• Electromagnetic radiation can be described in terms of a stream of photons. Each photon is traveling in a wave-like pattern, moving at the speed of light and carrying some amount of energy

• The only difference amongst radio waves, visible light, and gamma-rays is the amount of energy of the photons

  • Radio waves have photons with low energies

  • Microwaves have a little more energy than radio waves

  • Infrared still has more energy

  • Visible, ultraviolet, X-rays, and gamma-rays have photon energies that gradually increase respectively to each other

  • Gamma and Cosmic rays have the highest energy waves

  

Radio Waves

• Emitted by:

  • Astronomical objects

  • Radio station transmitters

• Detected by:

  • Ground based radio telescopes

  • When radio is turned on, it will convert the radio wave energy into sound energy

Microwaves

• Emitted by:

  • Gas clouds collapsing into stars

  • Microwave ovens

  • Radar stations

  • Cell phones

• Detected by:

  • Microwave telescopes

  • Food (heated)

  • Cell phones

  • Radar (systems)

Infrared

• Emitted by:

  • Sun and stars (near)

  • TV remote controls

  • Food warming lights (thermal)

  • Everything at room temperature or above

• Detected by:

  • Infrared cameras

  • TVs, VCRs

  • Your skin

Visible

• Emitted by:

  • The sun and other astronomical objects

  • Laser pointers

  • Light bulbs

• Detected by:

  • Cameras

  • Human eyes

  • Plants (red light0

  • Telescopes

Ultraviolet

• Emitted by:

  • Tanning booths (A)

  • The sun (A)

  • Black light bulbs (B)

  • UV lamps

• Detected by:

  • Space based UV detectors

  • UV cameras

  • Flying insects (flies)

X-ray

• Emitted by:

  • Astronomical objects

  • X-ray machines

  • CAT scan machines

  • Older televisions

  • Radioactive minerals

  • Airport luggage scanners

• Detected by:

  • Space based X-ray detectors

  • X-ray film

  • CCD detectors

Gamma Rays

• Emitted by:

  • Black holes

  • Active galaxies

  • Pulsars

  • Diffuse emission

  • Supernova

  • Gamma-ray bursts

  • Many other unidentified

Cosmic Rays

• Cosmic rays come from deep space and can pass through the Earth

• MOST DEADLY!

The Speed of Light

• Light travels as a wave at a speed of 186,000 miles per second

• The speed of light is:

  c=3.0×10^8m/s

• Light waves travel faster than sound waves!

  • In one second, a beam of light could travel around the Earth 7.5 times

• The distance light travels in one year is called a light year

• Closest star is about 4 light years away. In other words, if you sent a beam of light to that star, it would take about 4 years to get there

  • It takes the sun about 7.5 minutes for light to reach the Earth

• We are technically looking back in time when we look at the stars!

• Light is a transverse wave, and this can be proven with polarized lenses

  

• If light travels through an object, is is transparent

• If some light passes through but not all and a light shadow is present, it is translucent

• If light is blocked by an object and a dark shadow is cast, it is opaque

Colors

• The color an object appears at depends on the colors of light that it reflects

• The primary colors of light are red, green, and blue

• White light can be split up to make separate colors

  

• The separation of light into colors arranged according to their frequency, by interaction with a prism or diffraction grating

  

Refraction

• When light travels from one medium to another different medium, it changes speed

• The more dense it is, the slower it travels

  • Because of this change in speed, the light ray will also change its direction or bend

• The bending of light as it travels from one medium to another is called refraction

• Refraction can be explained in terms of the wave model of light, not the particle model

• The speed of light in a vacuum or for calculations, air is c (3.0×10^8m/s)

• Inside of other mediums, such as air, glass or water, the speed of light is different and less than c

The Law of Refraction

• The index of refraction for a substance is the ratio of the speed of light in a vacuum to the speed of light in that substance

  

• When light passes from a medium with a smaller index of refraction to one with a larger index of refraction (like from air to glass), the ray bends toward the normal

• When light passes from a medium with a larger index of refraction to one with a smaller index of refraction (like from glass to air), the ray bends away from the normal

• If n₁<n₂, then θ₁>θ₂

  

• Objects can appear to be in different positions due to refraction

• Snell’s Law determines the angle of refraction

  

  Sample Problem

  A light ray of wavelength 589 nm (produced by a sodium lamp) traveling through air strikes a smooth, flat slab or crown glass at an angle of 30.0° to the normal. Find the angle of refraction of θ_r

  n_isinθ_i=n_rsinθ_r

  θ_r=sin^-1[n_i/n_r(sinθ_r)]

  θ_r=sin^-1[1.00/1.52(sin30.0°)]

  θ_r=19.2°

Total Internal Reflection

• Total internal reflection can occur when light moves along a path from a medium with a higher index of refraction to one with a lower index of refraction (more dense to less dense)

• At the critical angle, refracted light makes an angle of 90° with the normal

• Above the critical angle, total internal reflection occurs and light is completely reflected within a substance

• Snell’s law can be used to find the critical angle:

  

  •    On the formula sheet, it is:

    θ_c=sin^-1(n_r/n_i)

• Total internal reflection occurs only for light traveling from a more dense medium to a less dense medium

• When light passes from a medium of larger refractive index to one of smaller refractive index, the refracted ray bends away from the normal

  

Dispersion

• Dispersion is the process of separating polychromatic light into its component wavelengths

  • White light passed through a prism produces a visible spectrum through dispersion

    

Lenses

• Formed by two curved boundaries between transparent media

• Lenses often have spherical surfaces. The curved surfaces are parts of a large sphere(s) of radius r

• Every lens shaped like a circle has a diametet, D, and a focal length, f

• The focal point is determined by many factors including the curvature of the lens, the type and thickness of the medium used for the lens, etc.

  

• Use the acronym LOST to refer to the image!

  L=location

  O=orientation

  S=size

  T=image size

• Real images form where light ways really meet

• Virtual ones form where light rays only appear to meet

• Real images can be projected onto a screen

• Virtual images cannot be projected onto a screen

• Real images usually appear upside down

• Virtual images usually appear right side up

• For lenses, real images lie behind the lens

  

• d_o is positive for objects to the left of the lens, negative for objects to the right of the lens (virtual objects)

• d_i is positive for images to the right of the lens, negative for images to the left of the lens (virtual images)

• f is positive for converging lenses, negative for diverging lenses

  

Mirrors

• The Law of Reflection states the angle of incidence equals the angle of reflection

  

• Flat (or plane) mirrors form virtual images that are the same distance from the mirror’s surface as the object is

• The image formed by rays that appear to come from the image point behind the mirror—but never really do—is called a virtual image.

  • Real images form where the light rays really meet

  • Virtual images form where the light rays only appear to meet

  • Real images can be projected onto a screen

  • Virtual images cannot be projected onto a screen

  • Real images usually appear upside down

  • Virtual images usually appear right side up

  • For mirrors, real images form in front

  • For mirrors, real images form behind them

  • For lenses, real images lie behind the lens

  • For lenses, virtual images lie in front

• A concave spherical mirror is a mirror which reflects light from it’s inner surface (caved in)

• Concave mirrors can be used to form real images or virtual ones

  • A real image is an image formed when rays of light actually pass through a point on the image. Real images can be projected onto a screen

  

• For the equations:

  • “c” stands for the center of the circle created by the curved mirror

  • “f” stands for the focal length of the mirror

    • (it’s different for each mirror and depends on its curvature)

    • (it’s halfway between the circle’s center and mirror)

  

• When doing these calculations, we will refer to the LOST of the image

  • L = location (d_i)

  • O = orientation (h_i)

    • h_i > 0, it’s upright

    • h_i < 0, it’s upside down

  • S = size (h_i)

    • Absolute value of the h_i

  • T = image type (d_i)

    • d_i > 0, it’s in front of the mirror and real

    • d_i < 0, it’s behind the mirror and the virtual

  

  • f = focal length

  • d_i = image distance

  • d_o = object distance

  • h_i = image height

  • h_o = object height

  • m = magnification

• Ray diagrams can be used for checking values calculated from the mirror and magnification equations for concave spherical mirrors

• Concave mirrors can produce both either real or virtual images