Light

Light is an electromagnetic wave.

We see things because:Either they EMIT light

What emits light?Light bulbsFlamesThe sunStars

or REFLECT light.

What reflects light?

Everything!

Properties of LightRectilinear PropagationRectilinear PropagationLight travels in straight lines Proof: Distinct shadows are cast Flashlight beams appear to be straight We infer physical orientations from what we see.

ReflectionReflection

Rays of light are reflected from objects at every angle. The reflected rays that enter your

eyes are the ones that allow you to see it.

Speed of Light

experimentsGalileo(1564-1642)

Investigated the speed of light by trying to time how long it took for his assistant’s lamp to be seen from a far off hill.

The problem with his idea was reaction time.

Roemer (1644-1710)

A Dutch scientist who studied the period (42.5 hrs) of the moons of Jupiter.

He noticed that sometimes the period was 14 seconds longer than other times.

He eventually figured out that the period wasn’t longer, but the time it took for his eyes to receive the image of that moon coming around its orbit was longer.

The major finding of this study was that the speed of light was a finite number.

Michelson(1852-1931)

Albert Michelson used a rotating eight sided mirror.

Light reflected from one side travels to the fixed flat mirror, reflects, and can be detected after being reflected from the next side while rotating.

C = 299792458.8m/s

Refraction

Light changes speed and direction when it travels from one medium to another.

When you see something through water, it is displaced (by your eye). Your eye interprets the light as traveling in a straight line, even though the light has bent.

The water looks shallower than it is. Light from the object does not travel in a

straight line to your eye, but your eye thinks it does.

INDEX of REFRACTION:

The ratio of speed of light to speed in another substance.

n = c v

n index of refractionC speed of light in a vacuumV speed of light in another substance

What is the index of refraction of glass if the speed of What is the index of refraction of glass if the speed of light in glass is 1.94 x 10light in glass is 1.94 x 1088m/s?m/s?

n = c v

n = 3.00 x 108

1.94 x 108

n = 1.54

This means that speed of light in glass is 1.54 times slower than in air.

What is the speed of light in diamond if the What is the speed of light in diamond if the index of refraction of diamond is 2.42?index of refraction of diamond is 2.42?

n = c v

2.42 = 3.00 x 108 m/s v

v = 1.24 x 108 m/s

Diffuse ReflectionWhat do mirrors, stainless steel, glass, and chrome have

in common?They are all highly polished, flat surfaces.

Most surfaces are irregular microscopically ex. Paper, chalkboard, skin

Therefore, incident rays will be reflected at disorganized angles.

That’s why you can’t see your reflection in a piece of paper.

Diffraction of Light: Interference

You know that:1.Waves spread out when they go through small

openings.2.Waves from two sources interfere in a pattern

of alternating nodal and antinodal lines.

If a laser is shone through two small slits side by side, it diffracts as it goes through the openings.

The light waves will interfere with one another. It has to be a single source of light so that the

light hitting the two slits is coherent (has the same frequency and in phase).

http://upload.wikimedia.org/wikipedia/commons/a/a9/Doubleslit.gif

Red light with a wavelength of 685 nm is shone through two small slits. An interference pattern is observed on a screen that is 4.2 m away. The

distance between the central maximum and the second order bright spot is 3.2 cm. What was the distance between the two slits?