Properties of Light

Light travels in straight lines.Light travels ~ 300,000 km/sReflection – bouncing back (mirrors)Refraction – bending due to different

mediums (lenses)

Laser

NormalLine

r i

Incidentray

Reflectedray

Mirror surface

Angles are ALWAYS measured from the NORMAL LINE

• Reflection from a mirror:

Incident ray

Normal

Reflected ray

Angle of incidence Angle of reflection

Mirror

Clear 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

Angle of incidence = Angle of Reflection For each ray

Dotted lines Shows the Apparent

Ray source

Parallel rays

Reflected rays

RealFocus

Focal length = +Forms real, inverted,Reduced or enlarged

Image. Also formsVirtual, erect,

Enlarged images.

Parallel rays

Reflected rays

Virtual focus

Focal length = -Forms only virtual,

erect, reduced images betweenthe virtual focus and the mirror.

Apparent Convergence of rays

2 f(f)

Focal ray

Parallel rayRay thru 2f

Image is:VirtualErectReducedAppears behind the mirror

Parallel rays

Reflected rays

Virtual focus

Focal length = -Forms only virtual,

erect, reduced images betweenthe virtual focus and the mirror.

Dotted lines Shows the ApparentRay focus

2. RefractionRefraction is when waves ____ __ or slow down due to travelling in a different _________. A medium is something that waves will travel through. When a ruler is placed in water it looks like this:

In this case the light rays are slowed down by the water and are _____, causing the ruler to look odd. The two mediums in this example are ______ and _______.

Words – speed up, water, air, bent

NormalLine

NormalLine

i

r i

r

Glassn = 1.5

Airn =1.0

r = angle of refraction

i = angle of incidence

Light travels more slowlyin glass than air and

bends towards the normalwhen entering glass from air

Light moving fromglass to air

increases speedand bends awayfrom the normal

Virtual focusFocus = -

Can form only virtual,Erect and reduced

images

Principlefocus

Parallel rays

Focus = +Can form real images(enlarged or reduced

& inverted) orEnlarged virtual images

Which color refracts most leaving the prism? The least?

Colored light is made from white light by REFRACTION, but we see color because of REFLECTION.

• We see things because they reflect light into our eyes:

Homework

Adding colors• White light can be split up to make separate colors. These

colors can be added together again.

• The primary colors 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 color• The color an object appears depends on the colors of light

it reflects.

For example, a red book only reflects red light:

White

lightOnly red light is

reflected

A white hat would reflect all seven colors:

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