Light Properties

LightLight

1) Properties of light2) Reflection3) Colours

Part 1 – Properties of Part 1 – Properties of LightLight

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.

Light travels much faster than sound. For example:

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

-The Sun- Lamps- Lights- Lasers- Campfires

-The Moon- Mirrors- People- Objects

Shadows

Shadows are places where light is “blocked”:

Rays of light

Out of the DarknessOut of the Darkness-If the light source is small compared to the size of the object, then the shadow will be SHARP

-If the light source is of comparable size to the object, then the shadow will be “fuzzy”. The dark inner region is called the UMBRA; the grey region is called the PENUMBRA.

Properties of Light Properties of Light summarysummary

1) Light travels in straight lines2) Light travels much faster than sound3) We see things because they reflect

light into our eyes4) Shadows are formed when light is

blocked by an object

Part 2 - ReflectionPart 2 - ReflectionReflection 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 Angle of incidence = Angle of reflectionreflection

In other words, light gets reflected from a surface at THE SAME ANGLE it hits it.

The same !

!!

Clear vs. Diffuse ReflectionClear vs. Diffuse Reflection

Smooth, shiny surfaces have a clear reflection:

Rough, dull surfaces have a diffuse reflection.

Diffuse reflection is when light is scattered in different directions

Using mirrorsUsing mirrorsTwo examples:

1) A periscope

2) A car headlight

Flat MirrorsFlat Mirrors

Image is VIRTUAL, UPRIGHT, UNMAGNIFIED

Spherical Mirrors - Spherical Mirrors - ConcaveConcave

Image is REAL, INVERTED, and DEMAGNIFIED !!!

C F

Spherical Mirrors – ConcaveSpherical Mirrors – ConcaveObject Inside the Focal Object Inside the Focal

PointPoint

Image is VIRTUAL, UPRIGHT, and MAGNIFIED

C F

Spherical Mirrors - Spherical Mirrors - ConvexConvex

Image is VIRTUAL, UPRIGHT, and DEMAGNIFIED

C F

ColourColour

White light is not a single colour; it is made up of a mixture of the seven colours of the rainbow.

We can demonstrate this by splitting white light with a prism:

This is how rainbows are formed: sunlight is “split up” by raindrops.

The colours of the rainbow:The colours of the rainbow:

RedOrangeYellowGreenBlue

IndigoVioletRemember: Roy G. Biv !!!

Light from the SunLight from the Sun

The light from the sun contains a grand mixture of electromagnetic waves of varying frequency (i.e. wavelength)

The spectrum of light is characteristic of the TEMPERATURE of the sun!

Wavelengths of LightWavelengths of Light

Red Light – nm

Green Light - nm

Blue Light - nm

Adding coloursAdding coloursWhite 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

Seeing colourSeeing colourThe 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 pants, in addition to being ugly, would reflect purple light

(or red and blue, as purple is made up of red and blue):

Purple light

White

light

Using coloured lightUsing coloured light

If we look at a coloured object in coloured light we see something different. For example, consider the outfit below – I mean, from a physics standpoint, not as a fashion choice:

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

Using filtersUsing filtersFilters can be used to “block” out different colours of

light:

Red Filte

r

Magenta

Filter