Understanding Visible Light

Light

• Light is a form of visible energy detected by the eyes.
• It travels in waves of different lengths.
• Light is a form of radiant energy.
• Formed from chemical, electrical, and nuclear energy.
• Travels at approximately 300,000 km per second
• Light passes through transparent objects
• Light sort of passes through translucent objects like frosted glass.
• Light does not pass through opaque objects like a brick wall

Visible Light

• Electromagnetic radiation with a wavelength visible to the human eye, ranging from 400 to 700 nm.
• In science, "light" refers to the entire electromagnetic spectrum.
• In optics, "visible light" is electromagnetic radiation with approximate wavelengths of 300 nm to 1400 nm.
• Light consists of elementary particles called photons.
• The speed of light in a vacuum is exactly 299,792,458 m/s, or about 186,282.397 miles per second.
• Light speed varies depending on what medium it passes through.
• Velocity includes magnitude and direction

Photon and Electromagnetic Radiation

• Photons are energy waves produced by the oscillation or acceleration of an electric charge.
• Electromagnetic waves possess both electric and magnetic properties.
• Electromagnetic spectrum includes gamma rays, hard and soft X-rays, UV radiation, visible light and simple lights.

Electromagnetic Spectrum

• The electromagnetic spectrum is the entire range of electromagnetic radiation with various forms of light.
• It spans from long radio waves to short gamma rays.
• It is used in applications like radio, microwave communication, X-ray imaging, and gamma-ray detection.
• It is used to have a better understanding of the behavior and interactions of light with matter.

Electromagnetic Spectrum and Visible Spectrum

• Electromagnetic spectrum is radiant energy, including: radio waves, microwaves, infrared light, visible light, ultraviolet light, x-rays, and gamma rays.
• Visible light constitutes the only fraction of the electromagnetic spectrum perceptible to humans
• Visible spectrum is the portion that affects human sight, ranging from 400 to 700 mu.

Wavelength

• Wavelength is the distance between two peaks in a light wave.
• It is used to measure the intensity of light.
• Common unit of measurement of light intensity is in terms of millimicrons (mu)
• It is the distance between two troughs or two crests.
• Crests are the high points where the electrical field is the highest.
• Troughs are the low points where the electrical fields are the lowest.
• A wave's amplitude is how much energy the wave has.

Frequency

• The number of complete waves per unit of time.
• It is used in measuring the speed of light.
• The number of wavelengths that pass through a given point in one second. -The speed of the wave equals the frequency times the wavelength.
• The higher the frequency, the shorter the wavelength.
• Longer wavelengths have less energy, and the lower the frequency the longer the wavelength.

Invisible Radiation

• Shorter than 400 MU
• Ultraviolet rays
• X-rays
• Gamma rays
• Cosmic rays
• Longer than 700 MU
• Infrared rays
• Radio waves
• Hertzian waves
• Long electrical oscillations

Invisible Radiation in Police Photography

• Ultraviolet rays provide better contrast while photographing wounds and injuries
• They cause latent fingerprints and bodily fluids to fluoresce, revealing clues invisible to the naked eye.
• X-rays are used to capture images of wounds and injuries, especially for injuries affecting bones and teeth.
• They reveal fractures and foreign objects embedded in wounds.
• Gamma rays are used in recording/photographing internal organs and objects hidden by an opaque medium.
• Infrared rays are used in night vision photography to capture images in low light conditions
• Can penetrate fog and smokescreens, so police can photograph scenes in difficult conditions.

Colors Of Light Found In Visible Spectrum

• Primary Colors of Light (in nm)
• Red has a wavelength of 700
• Blue has a wavelength of 450
• Green has a wavelength of 550
• Complementary Colors of Light
• Magenta's shortest wavelength is 400
• Cyan is 500
• Yellow is 590
• Neutral Colors of Light
• Gray
• White
• Black

Color Mixing Principle

• Additive Color: When all three colors are combined equally, the result is white light.
• Subtractive Color: Subtracting colors by absorbing certain wavelengths of light, the result becomes darker.

Color Wheel

• Abstract, illustrative organization of color hues around a circle
• Shows relationships between primary, secondary, and complementary colors.
• Mechanical device with array of colors arranged as petals/gradients around an axis.
• Shows the primary, secondary, and tertiary colors.
• Shows relationships between complementary colors and analogous colors
• Complementary colors are across from each other on the wheel, like blue and orange.
• Analogous colors are next to each other, like yellow, green, and blue.
• Black/white are not colors; White light is the presence of all color, and black is the absence of reflected light/color.

Mediums of Light

• Transparent objects slow light but allow it to pass freely
• They transmit 90% or more of the incident light.
• Translucent objects allow light to pass through, but the outline of the source is not clearly visible.
• They transmit 50% or less of the incident light.
• Opaque objects divert/absorb light, not allowing it to pass through.

Sources of Light

• Natural Sources originate from nature, such as the sun, moon, stars, lightning, and fire.
• Artificial Sources are man-made, divided into Continuous Radiation and Short Duration.

Kinds of Artificial Light Source

• Photoflood Lamp: reflectorized light or spotlight
• Flash Bulb: chemical lamp that generates lights by the rapid combustion of metal in oxygen
• Electronic Flash: produces light by instantaneous electrical discharge between electrodes in a gas-filled bulb
• Fluorescent Lamps are tube lamps wherein the walls are coated with material capable of fluorescing
• Infrared lamps are special type pf artificial light that is capable of producing infrared radiation
• Ultraviolet lamps are gas lamps burning disulfide vapor in oxygen or in nitric oxide.

Types of Photographic Light Rays

• Visible light is perceived by the human eye.
• Ultraviolet light is to photograph question documents suspected of being falsified or forged.
• Can be used to detect overwritten, erased, or hidden writing.
• Infrared light is useful for photographing/reducing unseen objects in dimly lit/dark areas.
• Also used for recording charred patterns, overwriting, or internal injuries.
• X-Rays can be used to photograph internal organs.

Photographic Rays/ Types of Radiation

• Radio Waves

• Have the longest wavelengths in the electromagnetic spectrum. German physicist Heinrich Hertz discovered radio waves in 1888 and initially called them Hertzian waves.

• Typically utilized by antennas of appropriate size with wavelengths ranging from hundreds of meters to about one millimeter. Used for transmission of data, via modulation.

• Ex: Television, mobile phones, MRI, wireless networking and amateur radio

• Infrared radiation/light

  • Infrared light has wavelengths greater than 700 millimicrons with a range of 700-800 millimicrons.
  • It is not a color, but an invisible ray detected by the skin as heat.
  • Invisible light is important in law enforcement photography because it permits result to be obtained which are not possible with only visible light present.

• Visible light

  • Strike the human eye and create some kind of sensation.

• Different objects are usually mixtures of light of various wavelengths and not a special color. -Wavelength of the visible light is from 400 to 700 millimicrons which produce white light

• Ultra-violet light

  • Light is divided into the near and far ultra-violet and ranges from about (200)-(400) millimicrons wavelength. The position of the ultra-violet region falls between X-ray and visible wavelength. Since ultra-violet radiation can kill bacteria, it is sometimes used to sterilize foods and water.

• Police scientists may use ultraviolet light ot detect bloodstains, forged documents, and faked oil paintings

• X-rays were discovered by German physicist, Wilhelm Roentgen in 1895.

  • Electromagnetic radiation similar to light, but of shorter wavelength and can penetrate solids.
  • X-rays can fog photographic film.
  • Hard X-rays have shorter wavelengths than soft X-rays.
  • They are used for seeing through some things and not others.

• Gamma rays

  • After hard X-rays, are the most energetic photons with no defined lower wavelength limit.
  • Useful to astronomers in the study of high-energy objects or regions and find a use with physicists thanks to their penetrative ability and their production from radioisotopes.
    • Can pierce materials and kill cells.

Behavior of Light

• The bouncing of light from matter.
  • When it strikes a surface and rebounds it is called incident light.
  • The angle at which it strikes is called the incident angle
• If an object does not emit its own light, it must reflect light in order to be seen.
  • Reflection involves two rays: an incoming/incident ray and an outgoing/reflected ray
  • Normal: A line perpendicular to a flat surface
  • Incident Ray: A ray of light that hits the surface
  • Reflected Ray: The ray of light that hits the surface
• Angle between the incident ray and the normal is identical in measurement as the angle between the reflected ray and the normal and obeys Snell's Law
  • Snell's Law holds that the angle of incidence equals the angle of reflection
• Specular Reflection: Reflection from mirror-like surface
• Diffuse reflection: If the surface is rough, the rays of light are reflected in many directions.

Kinds of Reflection

• Diffuse Reflection: when light shines on an ordinary surface such cloth, it bounces off the surface in every direction. The angle of incidence and reflection are unequal.
• Specular Reflection: when a bright ray of light strikes shiny, smooth surface like metal, it bounces off straight back in the opposite direction. The angles of incidence and reflection are the same.

The Law of Reflection

• Light behaves predictably.
• A ray of light approaching and reflecting off a flat mirror follows a predictable law, the law of reflection
• Law of Reflection- the incoming ray is equal to the outgoing ray

Transmission

• A. TRANSPARENT- object permits light to pass through
• EX. glass or lens
• B. TRANSLUCENT – object scatters light to pass through
• C. OPAQUE – object does not permit light to pass through

Reflected Light vs Transmitted Light

• Reflected Light
• If object is opaque, then vibrations are not passed from atom to atom.
• Electrons vibrate on the material's surface for short periods of time then reemit energy as a reflected light wave
• Transmitted Light
• If object is transparent, electron vibrations are passed to neighboring atoms and reemitted on the opposite side of the object.

Absorption

• Happens when light falls upon an object and it neither transmits nor reflected

Refraction

• A ray of light traveling through a substance of different density is refracted, or bent.
• Refraction of light occurs when the light ray travels between air and then going through water, changing medium.
• Depending on the new medium the light will travel faster or slower
• Light rays:
• Slow down about 25% with water, resulting in a denser medium
• Slow down about 35% with glass, resulting in a denser medium.
• When light enters a denser medium the ray bends toward the normal
• Light enters a rarer medium, it is bent away from the normal

Explore the nature of light as a form of visible energy, its wave properties, and its speed. Learn about the electromagnetic spectrum and how light interacts with different types of materials like transparent, translucent, and opaque objects.