Unit 3Light and Optical Systems

Light The form of energy we can seeBBC videoWhat are some natural light sources?

Natural light sources• Sun• Fire• Stars• Glow-worms• Lightning• MagmaWhat are some artificial light sources?

Artificial light sources• Light bulb• Cameras• Stoves• Televisions

Topic 1 : What is Light?

First Basic Principle of LightLight is a form of energy and when it reaches a surface it can be

absorbed and transformed into other forms of energy• Solar cells change light into electricity• Trees change sunlight into chemical energy

Sources of Light

1. Incandescent Sources https://www.youtube.com/watch?v=YnMP1Uj2nz0

An object heated to such a high temperature that it emits visible light

• Light bulbs• Candle flames

2. Fluorescent Sources https://www.youtube.com/watch?v=BNpFoVdI2LY

When invisible ultraviolet light is absorbed by the particles of an object. The object then emits some of the energy as visible light.

• Fluorescent light bulbs

3. Phosphorescent SourcesThe emission of light following exposure to and removal of a source

of radiation.• Inner surface of televisions and computer screens

Main difference between phosphorescent and fluorescent is – particles in a fluorescent source release their light immediately

4. Chemiluminescent SourcesThe emission of light resulting from chemical action and not

involving heat• Glow sticks

5. Bioluminescent SourcesAn organism that relies on chemical reactions inside its body to

produce light https://www.youtube.com/watch?v=9HXXQBz6Vv0

• Jelly fish https://www.youtube.com/watch?v=7kyP0XsF0zM

• Squid surf https://www.youtube.com/watch?v=uqJbUKEPgXc

Take a break

The Ray Model of LightThe ray model of light helps you to predict where shadows will form

and how large they will be• Light travels in straight lines• Objects farther away from the light source cast a smaller shadow

and objects closer to the light cast a larger shadow

The ray model also helps you understand what happens when light reaches different materials

• Light continues in a straight line through transparent materials• Light travels through translucent materials but the light is

scattered and images aren’t seen well• Light does not travel through opaque materials and they cast

shadows

ReflectionProcess when light hits a surface and bounces back off that surface

Analyzing ReflectionDraw the picture in figure 3.15 on page 189

• The ray that comes from the light source and hits the surface is called the incident ray

• The ray that bounces off the surface is called the reflected ray• The line that is at a 90 degree angle to the reflecting surface is

called the normal line• The angle between the normal line and the reflected ray is the

angle of reflection• The angle between the normal line and the incident ray is the

angle of incidence

Topic 2 : Reflection

The Law of ReflectionThe angle of reflection equals the angle of incidence

Ex. If the angle of incidence is 60˚ then the angle of reflection is 60˚

Forming an image

Two important facts about how images form are:

• Light reflects off all parts of an object in all directions. All the rays from the object reflect off the mirror according to the law of reflection. The rays that reach your eye appear to be coming from behind the mirror because your mind knows that light travels in a straight line.

• Images in a mirror appear to be the same size as the reflected object and come from an equal distance from behind the mirror.

• To summarize – Objects appear to be behind the mirror and the objects appear to be equal size and from an equal distance behind the mirror.

Take a break

Curved Mirrors

Convex Mirrors • Convex mirrors bulge out• Images in a convex mirror appear smaller and farther away• Convex lenses are used :

- in stores for security- side mirrors in cars

Concave Mirrors

• Concave mirrors bulge in• Images in concave mirrors appear to be closer than they actually

are• Concave mirrors are used on side mirrors in cars, these are often

labelled – Objects appear closer than they actually are and concave mirrors are also used in shaving / make up mirrors.

Rough Surfaces

• Light reflects off rough surfaces in many different directions • When reading something on paper, the white paper reflects light

off in all directions...however, the black absorbs the light and doesn’t reflect so therefore you pick up the image of the letters in between the reflected light. This creates letters and images on paper.

Using the Reflections

Reflections are used in everyday life in the following ways:

• Reflectors on bikes so people can see you in the dark• Reflectors on clothes so you can be seen in the dark• Lining up a shot while playing pool• Traffic signs so we can see them at night• Mirrors

Review assignment for marks:

Topic 1 Review on page 187• Questions 1 , 3, 4, 6 and the answer the following:- What are the 5 sources of light and give one example of each?

Topic 2 Review on page 199• Questions 1, 2, 3 and 4 (only drawings 1 and 2 – need protractor

for this)

Take a break

The Spectrum• The spectrum is the colors that appear when white light is

refracted.• The seven colors are red, orange, yellow, green, blue, indigo and

violet. The memory aid to remember is ROY G BIV

Seeing Red• An object is the color of the light that it reflects.

Example:A tomato appears red because it reflects red light and absorbs the

other colors.White paper reflects all light so it appears white. All the colors

combined create white light.Black absorbs all light so it appears black.

Topic 6 : The Source of Colors

How we see color• Rods are structures that help detect light and cones are structures

that help detect colors.

Hint...cones start with C and so do colors.

• Rods and cones are located in the retina.

Looking at WavelengthDefinitions of:• Crest – The high parts of a wave• Trough – The low parts of a wave• Wavelength – The distance from crest to crest• Amplitude – The height or depth of a wave from rest position

• Frequency is the rate at which a wave moves up and down. Frequency is measured in hertz or cycles (cycles are the movement of a wave from one crest to the next crest or one trough to the next trough) per second.

• A buzzer at 900 times/second = 900 hertz• A guitar string at 880 times/2 seconds = 440 hertz• A ball that bounces 10 times/50 seconds = 0.2 hertz

Topic 7 : The Wave Model of Light

Wave Model of Light• The wave model of light says that light travels in waves.

• How far light spreads out depends on how long the wavelengths are. Waves with longer wavelengths spread out more.

Take a break

Light Waves in Action• Shorter wavelength colors such as blue and violet hit particles in

the sky and reflect back into space. • Longer wavelength colors such as red and orange miss more of

these particles and therefore are seen by the eye.

Laser Light• Laser light emits light of only one wavelength. • Other light sources emit light of different wavelengths.• Light of one wavelength is called coherent.

• Lasers can be used for:a. Scanners in storesb. To use as a surgical tool c. Measure the speed of cars

The Electromagnetic Spectrum Arrangement of visible light and invisible forms of radiant energy

according to wavelength.

Decreasing Wavelength

Increasing Frequency

Visible Light

Topic 8 : Beyond Light

Radio Waves Infrared Ultra Violet

X Rays Gamma Rays

1. Infrared Radiation Heat Radiation

Uses of Infrared Radiation• Motion detectors• Burglar alarms• Heat lamps• Thermographs

2. Radio WavesA wave useful for carrying sounds or pictures through the air

Uses of Radio Waves• Broadcasting radio and T.V programs• Radar• Microwave ovens for cooking (microwaves are shortest

wavelength and highest frequency of radio waves)

Take a break

3. Ultraviolet RadiationEnergetic form of radiation that causes tanning

Depletion of the Ozone layer• Ozone in the atmosphere protects us from Ultraviolet radiation• UV radiation causes tanning• Too much UV radiation can cause cancer

4. X RaysRadiation that can penetrate skin and muscle

Uses of X Rays• Used in hospitals and dentist offices

5. Gamma RaysGamma rays result from nuclear reactions and can kill cells

Use of Gamma Rays• Used to treat cancer cells with radiation therapy

YOU ARE NOW GOING TO DO:

TOPIC 3,4,6,7,8 REVIEW

YOU CAN DO THIS WITH A PARTNER!!