Light

The Facts of Light – Notes

Light Intro

Shadows

Shadow

A book of height 20 cm is held 50 cm from the light and the wall is at a distance of 125 cm to the light. Find the size of the shadow.

ds =

What is Light?

• ELECTROMAGNETIC WAVES.

http://hawkins.pair.com/radiation/ewaves.gif

Electromagnetic Waves

Copywrited by Holt, Rinehart, & Winston

Light is

Produced when:• An electric charge is accelerated• Molecules, atoms or atomic nuclei

make transitions to lower energy states.

Cutnell & Johnson, Wiley Publishing, Physics 5th Ed.

Electromagnetic Spectrum

• What are other types of electromagnetic waves?

Cutnell & Johnson, Wiley Publishing, Physics 5th Ed.

Spectrum

Speed of Light

• Light travels at 3.00 x 108 m/s in a vacuum.

• Call this “c” – for constant speed.

What can cause the speed of light to change?

Passing through a transparent (clear) medium.

http://www.art.com/products/p12817857730/product.htm?RFID=217825&ProductTarget=40470995847&gclid=COnGmZyvkrkCFbE7MgodgAoAzg

ROY G. BIV

EM Spectrum l ( m)Visible Light 400-700 nm

• Red 674 nm• Orange 616 nm• Yellow 580 nm• Green556 nm• Blue 458 nm• Indigo 410 nm• Violet 400 nm

nanometer (nm) = 1.0 x 10-9 mhttp://www.ems.psu.edu/Courses/earth002/03012D.gif

Cutnell & Johnson, Wiley Publishing, Physics 5th Ed.

Transparent

• Transparent materials transmit light – allowing it to pass through undistorted.

• Notice that glass blocks both infrared and ultraviolet light, but is transparent to all the frequencies of visible light!

Visible Light

• Translucent materials transmit light – but distort it, which means objects can’t be seen clearly.

• Opaque materials do not transmit light. They either reflect or absorb the light.

http

://w

ww

.npl

.co.

uk/p

ublic

atio

ns/n

ew

s/o

ptic

alrm

/issu

e16/

tran

slu

cen

t_m

ate

rials

_im

age

04.jp

g

http://sprout.site.ne.jp/photo/archives/images/karasuma.jpg

Light & Materials

Principle of Least Time

• Light travels in straight lines.• Among all possible paths for light

to go from one point to another, light will take the path that requires the shortest time.

This idea is the basis for Laws of Reflection

http://jeremy.lwidof.net/downloads/beam-3.jpg

Light Behavior

What happens when light strikes a new surface (medium)?

1. Bounce off of the new medium - REFLECTION

2. Go through the new medium and bend as it changes speed - REFRACTION

3. Be ABSORBED and changed into heat

http://www.rpi.edu/dept/phys/ScIT/InformationTransfer/reflrefr/rr_content/images/refllaw.gif http://www.kfs.oeaw.ac.at/oeal/corr/10_Brechung2b.gif

http://www.glossary.oilfield.slb.com/files/OGL98151.gif

Light bounces off Homer

Light enters the eye of an observer

Light is produced by a luminous

object

How Can We See Homer?

If light bounces off Homer in only one direction…

…Homer can only be seen from one direction

This observer can see Homer

This observer receives no light from Homer so cannot see him

Why Can We See Homer in All Directions?

When light hits a rough surface, it is scattered in many directions, so…

… objects can be seen from many directions.

Light Scattering

Sealed box with non-reflective coating inside (preferably

black)Very

small hole(pin

hole!)

Translucent screen (e.g.

tracing paper)When the camera is pointed towards a bright object, an image of the object

appears on the screen

What is a Pinhole Camera?

How does a pinhole camera work?

Every point on Homer’s head will scatter light in many

directions, however…

… only one ray of light from each point on Homer’s head is travelling in

precisely the right direction to pass through the pinhole

Also…

… only one ray from each point on Homer’s foot will be

travelling in precisely the right direction to enter the pinhole

Light is scattered as it passes through the

translucent screen, so…

Pinhole Camera

Image is inverted

… we see an image on the screen

Light scattered by screen

Object

Pinhole Camera

Ray diagrams show the path of rays of light from an object.

They can be tricky to draw, so the best thing is to…

CHEAT!!!

Drawing Ray Diagrams for Pinhole Cameras

… so that the rays cross in the pinhole

Step 1: Draw your object

Step 3: Draw in rays from top and bottom of your object

Step 4: Draw in rest of pinhole camera …

Step 2: Draw your inverted image and your translucent screen

Pinhole Camera

4. Light always reflects off an object at the same point that it hits the object.

5. When using a diagram to explain how somebody sees something – show the ray of light entering the eye

2. Always use an arrow head to show which direction the rays of light are travelling in

1. Always use a ruler – light travels in straight lines

3. Rays of light should touch the objects that they are reflecting off.

Rules for Drawing Ray Diagrams

Pinhole Camera

Pinhole Camera

Pinhole Camera

Similar Triangles

Pinhole Camera

Using your pinhole camera, measure the height of an apartment building that is 320 m away.

Ray

• An arrow drawn on a diagram to show the direction of propagation of a set of waves

• A ray is always at 90° to the wavefront.

wavefront

ray

Refraction

• Change in direction of a wave• When waves travel across a boundary

between two different media (substances), their speed changes.

• The change in speed

can result in a change

in direction of

propagation of the

waves. Refraction ht

tp://

scie

nce.

unis

erve

.edu

.au/

scho

ol/c

urric

/sta

ge6/

phys

/com

mun

icat

es/S

tude

nt_s

ectio

n/st

uden

t_ph

ysic

s/ph

ysic

s_pi

cs/a

nim

ated

_ref

ract

ion2

.gif

Skateboarding on Grass

Think about a skateboard example:• If a skateboard is rolling from

pavement to this triangular plot of grass, show its path both into and out of the grass.

• If the grass plot were a piece of glass, and the skateboard path were a ray of light, the light would bend the same way due to refraction.

• Notice that the light bends toward the base of the triangle.

Ray Tracing

• Show how a ray of light will bend as it passes through each piece of glass below: (To simplify, the incident ray is drawn straight to the center of the glass. Bend the ray at that point and draw a straight line out of the glass.)

Lens

• If a piece of glass has just the right shape, it can bend parallel light rays so that they all cross – or appear to have crossed – at a single point – focal point

• Such a piece of glass is called a lens.

http://ebiomedia.com/gall/eyes/images/lensRefractBlack300.gif

Lens

• A lens is simply a combination of pieces of glass.

• The combination at the left is thicker in the middle and converges the light (convex lens).

• The arrangement at the right is thinner in the middle than at the edges; it diverges light (concave lens).

Convex Lens

• Convex lens – is a converging lens as it bends the light inward to a real focal point (F). – focal point can be seen– true if lens is properly shaped

Copywrited by Holt, Rinehart, & Winston

Concave Lens

• Concave lens – is a diverging lens as it spreads the light outward away from a virtual focal point (F’).– Focal point cannot be seen, but can be

found by projecting the rays back toward the light source (use dashed lines)

Copywrited by Holt, Rinehart, & Winston

Focal Length

• The focal length (f) is the distance from the center of the lens to the focal point (F).– measured to real focal point then the

focal length is positive– measured to virtual focal point then the

focal length is negative

Lens Power

• The greater the curvature of the lens the greater the power of the lens – more refraction or bending of light.

• Greater the power, the shorter the focal length.

Reflection

• occurs when the new medium is too different from the original medium for the light ray to pass through

• is the process of returning re-emitted light into the medium from which it came.

1st Law of Reflection:

• the angle of incidence is equal to the angle of reflection.

• Both of these angles are measured from a normal (which means perpendicular) to the surface at the point of incidence.

http://www.museumonline.at/1997/schulen/bg10/physik/refr14.gif