Chapter 27

Light

The Definition of Light The current scientific definition of Light

is a photon carried on a wave front. This definition incorporates the dual

nature of light, which means that light behaves both like a particle and a wave.

Light is a form of electromagnetic radiation – that means that it does not require a medium to travel through.

All electromagnetic radiation travels at 3 x 108 m/s

Electromagnetic Waves The electromagnetic spectrum is a range of

waves extending from radio waves to gamma waves.

The lowest frequency of light that we can see is red in color – all waves below that are called infrared.

The highest frequency of light that we can see is violet in color and any frequencies that are higher than that are said to be ultraviolet.

Transparent and Opaque

Materials that transmit all of the light which they receive are said to be transparent. They let light through.

Materials which absorb light without reemission are said to be opaque – they do not let light through.

Shadows

A shadow is caused when an object block light from a source.

A shadow has two parts: Umbra – total shadow – no light

received. Penumbra – partial shadow – some

light received, some blocked.

Chapter 28

Color

The Colors of Light Light is defined as the frequencies of

electromagnetic radiation which stimulate the human retina.

Light ranges in color from: Red - lowest frequency/longest wavelength Orange Yellow Green Blue Indigo Violet – highest frequency/shortest

wavelength

White Light and the Color Black

White light contains all of the colors (frequencies) of visible light.

Black is the absence of light. Vision is the process of seeing

reflected light. The color that you see is reflected from an objects – all other wavelengths are absorbed.

Color by Transmission The color of a transparent object

depends upon the color it transmits. The material which selectively

absorbs colors of light is called a pigment.

Absorbed light warms the object – hence darker colors tend to get warm in sunlight and lighter colors tend to remain cooler.

Mixing Light

When red, blue and green light (primary colors) are mixed, new colors are formed (secondary colors).

All three colors form white light. Red and Green – yellow Red and Blue – magenta Green and Blue - Cyan

Mixing Pigments When you mix pigments (paints) you add by

subtraction – that is each pigment you add to your mixture subtracts a color of light.

Primary pigments Magenta Yellow Cyan

Secondary Mixtures Yellow and Cyan – Green Yellow and Magenta – Red Magenta and Cyan - Blue

Why the sky is blue and sunsets are red…

Tiny O and N molecules scatter incoming light. Especially in the high energy spectrum. Our eyes are not very sensitive to violet so we see a blue sky.

In the morning and evening the sunlight has to travel a longer path through the atmosphere which will scatter longer and longer wavelengths of light before reaching us. Hence we see red!

Chapter 29

Reflection and Refraction

Reflection

When a wave reaches a boundary between two media and is turned back – this is reflection Total reflection –boundary with a

more rigid medium Partial reflection – less rigid medium.

Law of Reflection The law of

reflection states that the angle of incidence = angle of reflection

Incident rayReflected ray

Angle of incidence

Angle of reflection

Mirrors

Plane mirrors are flat mirrors which form a virtual image. A virtual image is one which forms on

the opposite side of the mirror. Images are the same size as the

object and the same vertical orientation.

Curved Mirrors: Concave and Convex

Concave Mirror Curves inward Close objects Form a virtual image which is

larger than the object Objects further away form real images

which are cast on a screen Convex Mirror

Curves outward Close objects form a virtual image which is

smaller than the object These mirrors are used to give a wide field

of view

Refraction Refraction is the

bending of light at a boundary.

If the speed in the new media is less than the speed in the old, the wave bends toward the normal

If the speed in the new media is more than the speed in the old, the wave bends away from the normal.

Sidewalk

Grass

Snell’s Law and the Index of Refraction

How much light bends at a boundary is given by Snell’s Law or the Index of refraction:

materialinlightofspeed

vacuumainlightofspeedn

Another way that this law is written is:

n0 sin i = n sin r

Where n0 is the index for the material you are moving out of and n is the index for the material you are going into.

Sample Problem

What is the speed of light in crown glass if its index of refraction is 1.52?

What is the angle of refraction for a wave in crown glass if its angle of incidence is 30°. Assume that it is coming from air into a cube of the glass.

Solution

glassinspeed

smx

materialinlightofspeed

vacuumainlightofspeedn

/10352.1

8

smxsmx

glassinSpeed /1097.152.1

/103 88

n0 sin i = n sin r

1 sin 30° = 1.52 sin r

1 sin 30°/1.52 = sin r

r = 19.73°

Dispersion through a Prism

Dispersion through a prism is a form of refraction.

Atmospheric refraction is what causes a rainbow.

FIRE RAINBOW

When light enters through a vertical side face of such an ice crystal and leaves from the bottom face, it refracts, or bends, in the same way that light passes through a prism. If a cirrus's crystals are aligned just right, the whole cloud lights up in a spectrum of colors