Light

Nature of Light

 Light is a wave, but the traveling disturbance is and electric and magnetic field, not a pressure variation like a sound wave.  Sound has to travel through matter, but light does not have to travel through a medium.  Therefore light can travel through a vacuum.  Light waves also carry energy with them; that is how sunlight warms us up when we absorb it. 

In a vacuum, light travels with a speed of about 3´10⁸ m/s, or 300,000,000m/s, i.e. very fast.  At that speed, and one could go all the way around the earth almost 8 times in one second.  It is about 93,000,000 miles to the sun and it takes light about 8 minutes to get from the sun to the earth.  (In matter, like glass or water, light travels slightly slower.)  

Light is a wave that consists of oscillating Electric and Magnetic fields that travel through space.  We characterize a wave by how many cycles or complete oscillations it goes through in each second.  This is called its frequency, f, which has units of cycles/second or Hz.  We characterize light by its wavelength (W.L. or l), which is how far it will travel during on cycle of its oscillation.    The time for one oscillation is called the period, T, and it is equal to 1/f.  As a result the wave speed is the distance traveled in one cycle (the W.L.) divided by the time it took to complete one cycle (T = 1/f).  Therefore the wave speed c = f ´W.L.  The electromagnetic waves we see have wavelengths between 400nm and 700nm.  Since we have f ´ W.L. = wave speed, the frequency varies from 7.5x10¹⁴ Hz (for 400nm) to 4.3x10¹⁴ Hz (for 700nm).   

Reflection, Transmission and Absorption 

When light travels, we often think of it as a ray, i.e. traveling in a straight line as long as the medium or vacuum it is moving through does not change.  However, when a light ray encounters a different medium, three things can happen.

1. It can be reflected, either totally or partially.  We think of it as being scattered when it is reflected.  If the surface is smooth, the angle at which it strikes the medium is the same as the angle of reflection.  If the surface is rough, the reflected ray is not just one ray, but it is scattered in all directions.  The reflection from the smooth surface is called specular reflection and that from a rough surface is called diffuse reflection.  Paper, most paint, clothing and similar objects are “rough” and scatter light in all directions, producing diffuse reflection.  Mirrors and polished metal act like smooth surfaces and produce only a little diffuse reflection, instead they produce mostly specular reflection. 

2. When light strikes a medium it can also be transmitted through the material and exit the other side.  For instance, glass transmits most of the light that strikes it, while reflecting a little.  Materials that transmit light do not interact with the light very strongly.

3. The third possibility is that the light is absorbed.  When this happens the energy from the light is deposited in the material and heats it up.  When materials absorb light it is often because they interact strongly with it, sort of a resonance condition between the light and the atoms or molecules in the material. 

Vision

 Our eyes have cells in the retina that can detect light.  They are grouped into two general categories, rods and cones. 

There is only one type of rod, and it is sensitive to light over most of the visible spectrum, but somewhat less sensitive in the red.  Rods do not distinguish between different colors of light, so they only “see” in black and white.  They are quite sensitive and can detect very low levels of light.

There are three types of cones, each more sensitive to different colors of light.  One is more sensitive to blues, another to greens and the third to reds.  These cones are responsible for color vision.  However, they are not as sensitive as the rods and do not work as well at low light levels.  Therefore, objects seem to be “black and white” at low light levels because we are using our rods..