part 1 – properties of light

Light travels in straight lines:

Laser

Light travels VERY FAST – around 300,000 kilometres per second.

At this speed it can go around the world 8 times in one second.

1) Thunder and lightning start at the same time, but we will see the lightning first.

2) When a starting pistol is fired we see the smoke first and then hear the bang.

We see things because they reflect light into our eyes:

Homework

Luminous and non-luminous objects

A luminous object is one that produces light.

A non-luminous object is one that reflects light.

Luminous objects Reflectors

Shadows

Shadows are places where light is “blocked”:

Rays of light

Reflection from a mirror:

Incident ray

Normal

Reflected ray

Angle of incidence

Angle of reflection

Mirror

The Law of ReflectionThe Law of Reflection

Angle of incidence = Angle of reflectionAngle of incidence = Angle of reflection

In other words, light gets reflected from a surface at ____ _____ angle it hits it.

The same !

!!

Smooth, shiny surfaces have a clear reflection:

Rough, dull surfaces have a diffuse reflection.

Diffuse reflection is when light is scattered in different directions

RedOrangeYellowGreenBlue

IndigoViolet

White light can be split up to make separate colours. These colours can be added together again.

The primary colours of light are red, blue and green:Adding blue and

red makes magenta (purple)

Adding blue and green makes cyan

(light blue)

Adding all three makes white again

Adding red and green makes yellow

The colour an object appears depends on the colours of light it reflects.

For example, a red book only reflects red light:

White

light

Only red light is

reflected

A white hat would reflect all seven colours:

A pair of purple trousers would reflect purple light (and red and blue, as purple is made up of red and

blue):

Purple light

White

light

If we look at a coloured object in coloured light we see something different. For example, consider a football kit:

White

light

Shorts look blue

Shirt looks red

In different colours of light this kit would look different:

Red

lightShirt looks red

Shorts look black

Blue

light

Shirt looks black

Shorts look blue

Be TRANSMITTED laser aimed at water or

glassBe REFLECTED

specular reflection of light by a mirror

diffuse reflection of the light in this room off all the other students

reflection is re-radiation of light by the electrons in the reflecting material

Be ABSORBED Cyan light shining on a

red apple is absorbed by electrons in the apple

Something new!!A light wave shining on

molecules in the air or plastic or other “transparent” materials can be:

SCATTERED Light ray moves over to

the side in all directions rather than forward, backward or being absorbed.

Intensity of the scattered light can depend on wavelength

Speed of light in empty space is c = 3 x 108 meters/sec

RayWaveform

Amplitude(maximum height)The intensity of the light is proportional to the amplitude squared. Large amplitude means bright light. Low amplitude means dim lightW

avel

engt

h

Light consists of electric (and magnetic) fields moving through space at the speed of light

Human eyes are only able to process information from the visible part of the spectrum

Toward longer wavelengths, the spectrum includes infrared light, microwaves, and radio

Toward shorter wavelengths, the spectrum includes ultraviolet light, X-rays, and gamma rays

All of these are forms of electromagnetic radiation

What we see as white light is actually made up of a continuum of components

Traditionally, we break white light into red, orange, yellow, green, blue, indigo, and violet (ROY G BIV)

There is actually a continuous transition of color, each with its own wavelength and frequency

Red light has an approximate wavelength of 7.0 x 10-

7 m and a frequency of 4.3 x 1014 Hz

Violet light has an approximate wavelength of 4.0 x 10-7 m and a frequency of 7.5 x 1014 Hz

Rayleigh Mie Geometric

Blue skyRed SunsetBlue water (from underwater)

The shorter the wavelength, the more light is scattered blue is scattered more than

red. this is why the sky is blue and

sunsets are red.

Think of white light from sun as a mixture of R, G and B

Blue is scattered the most so sky looks blue when we look away from the sun

Earth

Atmosphere

Rays from Sun (not scattered)

White or yellow

•Larger particles scatter red as well as blue and hence look white.

•Dust or smoke•Clouds; •Milk;

•There are many other types of scattering…

Explains scattering around larger droplets such as Corona around the sun or moon, and similar phenomena.