light and shadokwng/spring2011/phy160/lecture materials/light_a… · start from simple - point...

Human sensationFor us to see an object, the object has to send light

Light

j , j gto our eye.

Light

Sound

Smell

TouchTouch

Where light comes from

• Some objects can radiate light by itself.

• Some objects reflect light from other sources.

If an object cannot radiate light itself, and there is no other li ht th bj t t blight source, the object cannot be seen.

Start from simple - Point object

• A point object is an object small as a point (i.e. no size).

• An extended object is composed of

i t bj tmany point objects.

Light from a point objectIf a point source can give out light by itself or if there isIf a point source can give out light by itself, or if there is

other light source:

The point object willThe point object will send out light rays in all directions.

Properties of Light RaysProperties of Light Rays• A light ray is a perfect straight line. (ThisA light ray is a perfect straight line. (This

provides the geometric basis of the theory)• A light ray originates from a point object• A light ray originates from a point object.• A light ray extends indefinitely until it hits

( ) b t lan (opaque) obstacle.• For a point object to be seen, one of its light

rays has to hit the eye.

Conditions for a point object toConditions for a point object to be seen

• Either the point object can radiate light by itself or there are other light sources.g

• There is no opaque obstacle blocking the straight line joining the point object and thestraight line joining the point object and the eye.

An Example

Seeing light ray?

• Light ray is a conceptual entity that we intend to represent it as a straight line in our drawing.U lik h i l i h li li h b• Unlike a physical straight line, a light ray cannot be seen from the sideway.

• When a light ray hits our eye, we will see the object thatWhen a light ray hits our eye, we will see the object that emit the light ray.

Light ray from a point source

Light beam

A beam

Light beam

• A beam is a bundle of light rays.

• A beam can be outlined by extreme

Beam from a flash light

rays.

Beam from a laserBeam from a laser

Light beam

A beam

Beam of Light

Beam Scattered (or Reflected) byBeam Scattered (or Reflected) by Particles

Beam from a flash light

The original beam serves as a light source The particles in theThe original beam serves as a light source. The particles in thebeam will become lighted point object. What we are seeing arethe lighted particles outlining the beamthe lighted particles outlining the beam.

In summaryA i t ith di t li ht• A point source can either radiate light rays from itself, or reflect light rays from other sources.

• A point source send out light rays in all directions.

• For the point source to be seen, one of these plight rays has to reach one of our eyes.

• Light ray travels in straight line.Light ray travels in straight line.

Point source and a screen

Point source and a screen

With a mask in between

E t RExtreme Rays

E tExtreme rays

With a diff t hdifferent shape

hole

Extreme rays

SummaryF i t lFor a point source only:

• Point source send out light rays in all directions.• Light rays travel in straight line.• Extreme rays formed by straight lines joining the point source to the

edge of the hole.• Only rays enclosed by the extreme rays can go through the hole and hitOnly rays enclosed by the extreme rays can go through the hole and hit

the screen.• Rays outside the region bound by the extreme rays will be blocked by

the mask.e as .• Image on screen is a projection of the hole: they have the same

orientation.

Schematic Diagram

T’Extreme rays

T

Point sourceB

B’

Mask Screen

B

Example: Moving point sourceExample: Moving point source towards the mask

Example: Moving point sourceExample: Moving point source towards the mask

Example: Moving point sourceExample: Moving point source towards the mask

Extensive source

Extensive source

Different hole shape

Different hole shape

Summary

• An extensive source is formed by many many point source.

• Every point source project a spot of the same shape and orientation as the hole of the mask.

• The spot size determine how blur is the image on the screen.

• Projection on the screen is an inverted image of the extensive light source.

Inverted imageInverted imageScreen

Mask ??

M

Inverted imageInverted imageScreen

Mask

M

Inverted imageInverted imageScreen

Mask

MMM

Schematic diagramSpot size

Extensive source Mask

Screen

Sh dShadowScreen

P i tPoint source

ShadowF i t lFor a point source only:

• Point source send out light rays in all directions.• Light rays travel in straight line.• Extreme rays formed by straight lines joining the point source to the

edge of the object forming the shadow.• Rays enclosed by the extreme rays will be blocked by the object.Rays enclosed by the extreme rays will be blocked by the object.• Only rays outside the region bound by the extreme rays can pass and

hit the screen.