IGCSE Unit 2 Light

Cambridge IGCSE PhysicsAdapted by Science

DepartmentGreat Neck North High School

WHAT IS LIGHT?Theories of Light, Luminous Objects, Ray Model of Light

What is Light?Wave Theory of Light

For most of the 17th, 18th, and 19th centuries, light was understood to travel through space and the Earth’s atmosphere as a wave.

What is Light?Wave Theory of Light, continued…

By the end of the 19th century, light was understood to be an electromagnetic wave traveling through space and the Earth’s atmosphere at a constant, very high speed, c (300,000,000 m/s or 186,000 mi/s).

What is Light?Particle Theory of Light

In the early 20th century evidence began to accumulate that light traveled as a continuous stream of particles moving at the speed of light, c.

These particles came to be known as “photons” and have no mass like ordinary particles (i.e. protons & electrons)

What is Light?Wave-Particle Theory of Light

By the mid-20th century, this dual nature of light became accepted.

This “wave-particle duality” basically means that light exhibits wave properties at times and particle properties at other times.

In other words… light is weird!!! (you can quote me on that)

Luminous Objects

Non-Luminous Objects

Rays and Beams

Light travels in straight linesRays are represented in diagrams by an

arrow

Beams are a stream of light that consists of many rays

Beams of Light

Arrows must be straight linesUsing a straight edge is often necessary

Do NOT have to be parallel

Beams of Light

Light striking a material surfaceIncident light is either

ReflectedBounces off the surface of the material

RefractedTransmitted through the material

AbsorbedIncreases the internal energy of the material

» Warms it up

REFLECTION OF LIGHT

Light Reflection, Law of Reflection, Protractor Use, Images Formed in Plane Mirrors

Reflection

Bouncing off a boundary or surfaceParticles or matter

AtomsMoleculesBalls

Waves or energyLightSoundHeat

Protractor

Used to measure angles

Zero Edge

Center Mark or Intersect Point

Inner ScaleOuter Scale

How to Use a Protractor Activity

Measuring Angles ActivityMeasuring Angles Homework

Reflection of Light

Plane mirrorMicroscopically smooth and flat surface

that reflects lightSpecular reflection

Reflection of Light

Students will be able to use the law of reflection Law of reflection

angle of incidence = angle of reflectioni = r

Reflection of Light

Images Formed by Plane MirrorsPlane mirror image formation

Reflects light from the object to the observerPlane mirror image characteristics

Virtual imageLaterally inverted (inversed)

Back to frontSame distance behind the mirror as the object

is in frontSame size, shape, and colorSame vertical orientation

Plane Mirror Image Formation

Virtual Images

Virtual images cannot be projected onto a screen The reflected rays appear to come

from the mirror image

Real Images

Real images can be projected onto a screenFormed as light rays actually converge on a

particular regionALWAYS vertically inverted (∴ upside down)Can be: same size or magnified (enlarged or

reduced)

Lateral Image Inversion

Practice Exam Question

Image Location

Line of Sight

Line of Sight

Practice Exam Question

REFRACTION OF LIGHT

Refraction, Dispersion, Total Internal Reflection, & Convex Lenses

Refraction of LightRefraction is the bending of light as it passes

from one transparent material into anotherOccurs for two reasons: (a) because the speed of the

light changes; and (b) because the light enters the new material at an angle to the normal

Angle of incidence (i)

» Angle from the normal line that the light strikes the border between the two substances

Angle of refraction (r)

» Angle from the same normal line that the light travels through the new medium

Refraction of Light

Refraction of Light

Refraction

Refraction of Light

Demonstration of the refraction of lightObject submerged in

water

Refraction of Light

Demonstration of the refraction of lightLaser and transparent

block

Refraction of Light

As light passes through a parallel-sided transparent material, notice the behavior of the ray…

REFRACTION PHENOMENATotal Internal Reflection, Fiber Optics, Lenses, & Dispersion

Refraction Phenomenon:Total Internal Reflection

Internal reflectionWhen light passes through a transparent material and a

fraction of the light is reflected back into that materialThe remaining light is refracted

Total internal reflectionWhen light passes through a transparent material and

the inside surface of the material behaves like a mirror, reflecting all the light back into the material

Angle of incidence > critical angle» Angle of incidence = angle of reflection

Total Internal Reflection

The “Critical Angle”The angle of incidence which causes the angle of refraction

to be 90o

*Can only be reached when light is passing from a slower (optically denser) material to a faster (optically less dense) material

Notice the refracted ray moves along the edge of the material

Total Internal Reflection

total internal reflection

Total Internal Reflection

Total Internal Reflection:Practice Exam Question

Application of Total Internal Reflection: Fiber Optics

Application of Total Internal Reflection: Fiber Optics

Utilizes the phenomenon of total internal reflectionRays of laser light enter one end of the fiber which is optically

slower (denser) than air.The rays strike the edge of the fiber at angles greater than the

critical angle, thus they are totally internally reflected

Application of Total Internal Reflection: Fiber Optics

How are they made?Layer-1 is a protective layer, usually made of some

form of plastic to physically protect the inner glass layers

Layers 2 & 3 are both glass, however, with different optical densities (light speeds)

Application of Total Internal Reflection: Periscopes

There are two ways to make a periscopeReflection with plane mirrorsTotal internal reflection with two triangular blocks of glass

This version uses two plane mirrors

Eye

This uses two blocks of glass

where light rays are incident

greater than the critical angle

Eye

Refraction Phenomenon:Dispersion of Light (Prism Effect)

Dispersion of Light

“White” light Visible light that is composed of the

full range of visible colors (wavelengths)

White light can be dispersed (separated) by most transparent substances Dispersion is separation of white

light into the range of visible colors by passing through a transparent substance

Occurs because different wavelengths of light are refracted at slightly different angles

Practice Exam Question

Do not answer C, requires understanding of the electromagnetic spectrum which will be covered in the waves unit.

Optical Lenses

Curved pieces of glass that refract light for the purpose of forming clear “REAL” images

Thin Converging Lens

“Converging” lenses are “Convex”Causes parallel rays of light (from a distant object)

passing through the lens to refract and cross the principal axis at a fixed point known as the focal point (F or f)

Thin Converging Lens

Key Terms:Principal focus (focal point)

Point where light rays intersect the principal axisRefracted light rays are focused at this point

Focal lengthDistance from the center of the lens to the principal

focus

Thin Converging Lens

Thin Converging Lens

Burning a hole in paper, or starting a fire using a magnifying glass (converging lens)Lens forms a REAL image of the sun at the focal point

Thin Converging Lens

How to geometrically show real image formation by a single converging lens…There are three rays that can be drawn to determine (a)

location, (b) size, and (c)orientation of the imageLine traveling parallel to the principal axis and is refracted so that it

passes through the principal focal point on the other side of the lensLine travels through the principal focal point on the same side of

the lens as the object and is refracted by the lens so that it travels parallel to the principal axis on the other side of the lens

Line travels through the center of the lens (origin) and continues in a straight line on the other side of the lens

Thin Converging Lens

Thin Converging Lens

Thin Converging Lens