Properties of Light Light is a transverse wave. Light consists of oscillating electric and magnetic fields. Light always travels at ONE speed: 300,000,000 meters/second. Light is an electromagnetic wave. It can travel in a vacuum.

Moving electric field creates magnetic field. Moving magnetic field creates electric field.

The Electromagnetic Spectrum

Transparent Materials When light hits a transparent material, the light is absorbed and released.

The time it takes each atom to absorb and re-emit the light causes a delay. The real speed does NOT slow down. The apparent speed of light slows down.

Fermat’s Law determines the bending. The apparent speed of light in a “thick” substance is slower than in a “thinner” one. Light bends to minimize the time through the thick substance. The apparent slowing down of light is called the Index of Refraction. The higher the index of refraction the slower the light appears to go.

If the frequency of light is close or the same as the resonance frequency of the atoms, the light is absorbed and held for a long time. The energy is dissipated as heat as the atom continues to collide with it’s neighbors.

Opaque Materials Most materials are opaque. Light is blocked. Metals: Free electrons reflect light rather than pass it on. That’s why many metals look shiny. Opaque materials absorb light but do not re-emit it. The energy is just what the atom is looking for so it is absorbed.

Earth’s Atmosphere The atmosphere passes most light. Yellow light is an example. The atmosphere blocks other types of light, infrared is an example. The human eye is most sensitive to light frequencies most common in the sun!

Shadows Light likes to travel in straight lines (rays). Shadows result when light is blocked by an opaque object. Shadows consist of umbra and penumbra. Penumbra caused by other light sources “filling in.”

Eclipses

The Human Eye Rods see B&W Cones see Color

Colors Newton discovered that light is made up of various colors. Each color refracts (bends) a different amount.

Reflection of Light Your eyes see the color being reflected. All other colors are being absorbed.

If the color of light is at the resonance frequency of the electrons, it is absorbed. If not, it is reflected. If the light of some colors is absorbed, where does the energy go? HEAT!! The absorbed energy raises the average kinetic energy of the molecules in the object. The temperature goes up.

Selective Transmission Transmission of color involves letting some frequencies travel through the object while all others are absorbed.

A blue piece of glass looks blue because the dyes and pigments in the glass absorb all colors except blue. Transmission of Light

Red, Green, and Blue are the primary colors. Mixing these colors in various combinations gives us all the colors.

Complementary Colors The primary colors for paints and pigments are different. These colors reflect light, they are not producing light themselves. For reflected light such as paint, the primary colors are Cyan, Magenta, and Yellow. These are called the Complementary Colors.

You can add a primary color to a complementary color and get white. Red + Cyan = White Blue + Yellow = White Green + Magenta = White

Reflection Light bouncing off of an object is called reflection. Fermat’s Principle of Least Time: Light always takes the fastest path.

There are many ways for light to get from Point A to Point B.

Which path is the fastest?

Law of Reflection: The Angle of Incidence equals the Angle of Reflection.

Reflections are all around us. Light bouncing off of a smooth surface is called a specular reflection. Light bouncing off of a rough surface is called a diffuse reflection.

Specular Reflection Specular reflection is a reflection from a highly polished surface. Reflections from a mirror or shiny metal are specular.

A plane, or flat mirror does not distort the image. Remember, you only see light that hits your eyes! An image in a mirror appears an equal distance behind the mirror as it exists in front of the mirror. The image in the mirror is not real, it is called a virtual image.

What is a “rough” surface? It depends on the wavelength. A radio telescope is specular for long radio waves but extremely diffuse for very short light waves. Diffuse Reflection

Is the moon a specular or diffuse reflection? How do we know the answer just by looking?

A curved mirror can focus light energy at a the mirror’s focal point. The same mirror can take rays that originate from a point and turn them into plane waves.

Archimedes invented a device that used a curved mirror to focus the sun on enemy ships, setting them on fire.

Curved mirrors can create special effects. Look at a spoon, it is concave on one side, convex on the other.

Refraction Refraction is the bending of light as it travels between two different substances.

Refraction as Illusion

Optical Illusions

Bending of light analogy.

Light rays bending like the lawn mower wheels.

Snell’s Law

Refraction and Dispersion In real life, different frequencies (colors) of light bend different amounts. Blue bends more than red. This leads to dispersion.

Rainbows Rainbows are caused by the dispersion of light in drops of water. Rainbows form complete circles, the ground blocks our view.

Dispersion from a Single Raindrop

A rainbow is the cumulative effect of dispersion from many raindrops.

The rainbow you see is the edge of a three dimensional cone.

The ground interrupts the circle turning it into an arc. From the air, the full circular rainbow can be seen.

Total Internal Reflection This is how fiber optic cable works!

Lenses Lenses are based on refraction (bending) of light. Convex (Converging) lenses: Thick in the middle. Concave (Diverging) lenses: Thin in the middle.

Eyeglasses correct the distortions of the eye’s lenses.

Why is the Sky Blue? Molecules in the sky like to scatter light (Rayleigh scattering). Light is absorbed and then re-emitted in all directions.

The air is mostly Nitrogen and Oxygen. These molecules like to scatter blue and violet light the most. The sky is really more violet! But are eyes are extremely sensitive to blue so we see the sky as blue. Nitrogen Oxygen

Why are the Sunsets Red? The setting sun sends it’s light through a thicker chunk of air. That air, closer to the ground, is denser and dirtier. Most of the blue and violet light are scattered away before the light gets to you.

Why are Clouds White? A cloud contains water droplets and particles of many different sizes. Each size likes to scatter light of different colors. The end result is that all colors are scattered and the result is white light.

Why is Water Blue-Green? Although the water can appear blue by reflecting the sky, that is not the real reason for water’s color. Water absorbs infrared and reds and Rayleigh scatters blue and green. That blue-green scattering acts a lot like the color of the sky.

Leaves in the spring and summer are green thanks to chlorophyll used for photosynthesis. Chlorophyll reflects green light but absorbs the other colors. In the fall, the chlorophyll breaks down and allows carotenoids, reflecting red, yellow, and orange to dominate.

Polarization Polarization: Transverse waves vibrating in one direction only. A single vibrating electron creates a plane-polarized wave

Herapathite is a crystal that can polarize light. This crystal is used to create polarizing plastics.

Glare from reflections are often polarized ← Without Polarization With Polarization →