Physics

Behavior of Light

The only thing we really see is light.

Light has been studied for thousands of years.

 

Ancient Greeks though light was a particle that entered the eye to create vision, or that vision resulted from streamers or filaments emitted by the eye.

Early Concepts of Light

One Greek believed that light was a wave, Empedocles. Dutch scientist Christian Huygens also believed light was a wave.

The particle theory was supported with the fact that light seemed to move in straight lines, and did not spread out. Huygens showed that it does spread out.

In 1905, Einstein published his theory explaining photo-electric effect. This theory said light consisted of particles or photons. These are little bundles of mass-less energy.

 

It is now accepted that light has a dual nature: wave and particle.

People had been trying to measure the speed of light since before the time of Galileo.

 

Danish scientist Olaus Roemer tried to calculate the speed of light using Jupiter and its moon Io.

The Speed of Light

Christian Huygens who used Roemer’s calculations to figure the distance from Jupiter’s moon Io, to earth found that the moon is 300,000,000 km farther away from earth at a different time of year.

Huygens did not realize the significance of his numbers.

In 1880, Albert Michelson used mirrors, and a telescope to calculate the time it took light to reflect across a certain distance. He later won the Nobel peace prize in 1907 for his work. He calculated the speed of light to 299,970 km/s.

We now know the speed of light is 300,000 km/s.

 

It takes 8 minutes to travel to earth from the sun.

 

The light from the next closest star, Alpha Centuri, takes 4 years to reach us.

 

The distance light travels in one year is a light-year.

Light is energy that is emitted by accelerating electric charges.

 

This energy travels in a wave that is partly magnetic and partly electric.

 

This type of wave is an electromagnetic wave.

 

Light is an extremely small portion of the electromagnetic spectrum.

Electromagnetic Waves

The electromagnetic spectrum includes radio, microwaves, x-ray, infrared waves, and more.

 

Our eyes detect a tiny portion of the spectrum, with red being the lowest frequency and violet being the highest frequency we see.

Other electromagnetic waves are detectable by our bodies, but not our eyes.

 

Infrared waves make us feel warm, and ultraviolet waves burn our bodies.

Machines around us can detect other waves on the spectrum, too.

Light is carried in an electromagnetic wave that is generated by vibrating electric charges.

 

When light falls on matter, electrons in the matter are forced to vibrate.

 

How a receiving material responds when light falls on it depends on the frequency of light, and the frequency of electrons in the material.

Light and Transparent Materials

Glass and water both allow light to pass through, that is, they are transparent.

 

The atoms in glass or water have a high vibration frequency. Visible light has a lower frequency, and so the atoms pass on the energy quickly.

This allows for the light to pass quickly and be reemitted from atom to atom. The actual speed of light is slower in these transparent substances.

Glass is transparent to visible light, but not to UV or IR. That is why glass will warm up when these waves are incident upon it.

Most materials absorb light without re-emitting it. No light is allowed through. This means it is opaque.

 

The vibrations from the light are turned into random kinetic energy, like heat.

Opaque Materials

Metals are opaque, but different. The outer layers of electrons are not bound to any particular atom, so they can wander.

This is why metal conducts heat and electricity well.

The light that shines on metal sets these electrons into motion, and the energy does not move from atom to atom, but is reemitted at visible light. It is reflected, and this is why metals look shiny.

The atmosphere is transparent to visible and infrared light, but mostly opaque to ultraviolet. The UV that does get through is responsible for sunburns.

 

Clouds are semitransparent to UV, and UV reflects off of sand and water. You can get sunburn in the shade!!

A thin beam of light is called a ray.

 

Any beam of light that is thicker is a bundle of rays.

 

When some of the rays are stopped by an object while others continue on, a shadow is formed.

 

Sharp shadows are produced by small light sources that are close by, or large sources that are far away.

Shadows

Light waves are transverse, and not longitudinal. We know this because of polarization.

Polarization

Vertically vibrating electrons emit electromagnetic waves that are polarized vertically. Horizontally vibrating electrons emit horizontal waves.

Most light sources do not emit polarized light. The electrons vibrate in random directions.

 

When the light falls on a polarized surface, the light that passes through is polarized.

When polarized filters are at right angles to each other, they allow no light through.

 

This is also like skipping stones on a lake.

When a wave reaches a boundary between two media, some or the entire wave bounces back into the first medium. This is reflection.

Reflection

Metals such as aluminum and silver are good reflectors because little energy is absorbed, most is reflected. Most frequencies of visible light are reemitted.

Light will reflect off of objects at which the angle they hit the object.

 

Incident rays and reflected rays make equal angles with a line perpendicular to the surface, called the normal.

The Law of Reflection

The angle made by the incident ray and the normal is called the angle of incidence.

 

The angle made by the reflected ray and the normal, called the angle of reflection is equal to the angle of incidence.

 

This relationship is called the law of reflection.

Rays of light are reflected in all directions from the surface of a plane (flat) mirror.

 

A virtual image is an image formed through reflection that can be seen by an observer but cannot be projected on a screen because light from the object does not actually come to a focus.

Mirrors

Our eyes cannot tell the difference between an image and a virtual image. This is because the light entering the eye is entering as if there really were an object there.

Objects in mirrors are exactly as far behind a mirror as they are in front of it. They are also the same size, as long as the mirror is flat.

When light is incident of a rough surface, it is reflected in many directions. This is diffuse reflection.

 

Each individual ray obeys the Law of Reflection.

 

What constitutes a rough surface for some rays may be a polished surface for others.

 

Most objects around us are visible due to diffuse reflection.

Diffuse Reflection

An echo is reflected sound.

 

Sound energy reflected is more when a surface is rigid and smooth, and less when the surface is soft and irregular.

 

Sound energy not reflected is absorbed or transmitted.

Reflection of Sound

Sound reflects from all surfaces of a room. Designers take this into account and study the acoustics, or reflective properties of all these surfaces.

When the walls of a room or other space are too reflective they create garbled sounds, or reverberation.

 

When the room is too absorbent, the sounds are dull and lifeless.

 

When designing concert halls or auditoriums, a balance between absorption and reverberation is needed.

Designers do this by making grooves or ripples in the walls of these spaces.

 

If a stage is involved there will usually be reflective devices behind and above the stage to send the sound and light waves out to the audience.

The change in direction of a wave as it crosses the boundary between two media is called refraction.

 

Think of a pencil in a glass of water. The light rays are bent as they enter the water, so the pencil appears broken.

Refraction

Sound waves are refracted when parts of a wave front travel at different speeds.

 

This could occur due to wind or uneven temperatures.

 

Often at night you can hear sounds from much longer distances.

Refraction of Sound

This is not because the night is quieter.

It could be because the night is cooler, causing the sounds to slow down, bending them toward the earth.

This allows them to be carried much farther.

Ponds and swimming pools often appear shallower than they actually are.

 

These effects are due to the refraction of light.

 

As the light changes media, the rays bend. They can often meet before the actual bottom of the pool or pond and your eye perceives it as shallower than it really is.

Refraction of Light

Atmospheric refraction occurs when due to heating or cooling of the atmosphere, waves travel faster or slower. This causes the alteration of images.

Atmospheric Refraction

This can occur when hot air near the ground permits waves to travel faster. This can look like wet pavement on a hot day.

 

This can also be called a mirage.

 

Mirages are not tricks of the mind. They can be captured on film.

 

Atmospheric refraction also allows us to have longer sunsets and see elliptical shaped sun and moon.

Different frequencies of light travel at different speeds in transparent materials.

 

This causes them to refract differently and bend at different angles.

 

This causes the separation of the different colors of light through a prism and is called dispersion. They are arranged according to their frequency.

Dispersion in a Prism

Rainbows are also formed by dispersion.

 

Each drop of water disperses a full spectrum of colors. An observer however is in a position to see only a single color from any one raindrop.

 

If violet light enters your eye from one drop, red light from that same drop falls below your eye.

The Rainbow