Unit 3: Light and Optical Instruments

J. PulickeelJanuary 2007

What is light?

• Light is the form of energy that we can see.• Two sources of natural light are the Sun and

fire.

Good Morning Starshine!• The Sun is a star; all stars are sources of light. Light radiates (spreads

out) from the Sun in all directions.

• Radiant Energy is energy emitted as waves or rays in all directions

• Radiant energy does not have to travel through matter. Radiation is the transfer of radiant energy (like light)

• It is our primary source of energy. All forms of energy derive from the sun

Less than 0.0000001% (1/10 000 000) of the sun’s light hits the earth.

Slightly Off Topic…

• Did you know light from the Sun is produced by nuclear fusion of hydrogen particles?

• The Sun is composed of about 70% hydrogen. When the hydrogen particles fuse (combine) they form a particle called helium. During the process, an enormous amount of energy is released as heat and light. The temperature inside the sun is about 16 000 000 °C

Helium

How do we see light sources?

• Light travels from a source to our eyes.

• We can see sources in a dark room.

How do we see things that are not sources of light?

Light bounces off and some reaches our eyes.

If there is no light we cannot see anything!

TWO BASIC PROPERTIES OF LIGHT

• There are TWO basic properties of light

Property No. 1 Light is a form of energy

Property One

Light is a form of Energy

• When light hits a surface, it can be absorbed by the surface. When it is, it can be transformed into several different forms of energy such as:

• Thermal - Sunlight cause a black sweater to get warm

• Electrical - Solar cells change light into electricity

• Chemical - Photosynthesis

Property One

Light is a form of Energy

• The brightness (or intensity) of the light indicates how much energy a surface will receive.

• A surface can absorb more energy if the brightness intensifies

Examples• The pavement in the summer can get very hot, but will feel only warm if the clouds are out.

• Sitting in front of the window on a winter day can feel really nice if its sunny outside!

Intensity (energy hitting the earth) increase Intensity (energy hitting the earth) decrease

TWO BASIC PROPERTIES OF LIGHT

Light travels in a straight lineProperty No. 2

If you stand in front of a movie screen, part of the view will be blocked. This is because light will not bend around you to reach the screen.

Property Two

Light Travels in a Straight Line

• Until light strikes something, it will continue to travel in a straight line.

• If light strikes a clear substance, like water or a window, it will pass through them. These media are transparent.

• Other media let light through diffusely. These media are called translucent. Wax paper is translucent.

• Finally some materials will not allow any light to pass through. These materials are called opaque. Examples include books

Artificial Light Sources

• Unfortunately the Sun does not always provide us with light, so it is necessary for artificial sources of light.

• All light is created by converting a different form of energy into light

• Candles

• Flashlight

Chemical Energy

Electrical Energy

Sources of Artificial Light

Incandescent Light SourcesIncandescent light sources are sources that are heated to such a high temperature that they emit visible light. The visible light emitted by a hot object is called incandescence.

Candles and Light bulbs are examples of incandescent

light sources. In the light bulb, the filament becomes so hot that it glows white.

Electrical Energy Visible Light EnergyThermal Energy

Sources of Artificial Light

Fluorescent SourcesIn this process, high energy invisible ultraviolet light is absorbed by particles. These particles then emit some of this energy as light. The glow is called fluorescence.

Ultraviolet Light Energy Visible Light EnergyEnergy absorbed by particles

Have you ever noticed that white clothes glow when you stand under “black light”? In this process your white shirt or socks absorb the UV light. Your clothes then emit the light, which makes it look like it glows!

Sources of Artificial Light

Fluorescent Light SourcesIn a Fluorescent light source, an electrical current from the lead wires and electrode cause the mercury vapor inside the tube to give off UV radiation.

A phosphor coating on the inside of the tube absorbs the UV energy. This causes the coating to glow, producing light that we can see!

Sources of Artificial Light

Ultraviolet Light Energy Visible Light EnergyEnergy absorbed by phosphor particles

Electrical Energy Energy absorbed by mercury particles

Fluorescent Light Sources

Did you know…

• Ultraviolet light can cause eye damage! Never stare directly at an ultraviolet light source!

Sources of Artificial Light

Phosphorescent Sources (Glow in the Dark…)

• Light energy is absorbed by certain particles that can store energy for a long time. This stored energy is later released as visible light.

• The original light energy could be in the form of UV light or visible light.

• The main difference between Fluorescent sources and Phosphorescent sources is that fluorescent sources release their light energy immediately.

• Phosphorescent particles take longer to emit light, and they continue to glow for a while after the energy source has been removed

Sources of Artificial Light

Chemiluminescent SourcesThis is light released by a chemical reaction. . The reaction produces particles that give off visible light energy. The process is called chemiluminescence.

Glow sticks are an example of chemiluminescent light sources

Chemical Energy Visible light energy

Sources of Natural Light

Bioluminescent SourcesDeep in the ocean there are many creatures that glow! They however can not be incandescent or fluorescent sources. Instead they rely on chemical reactions inside their bodies to provide the energy for light.

They need this because so little light reaches that far below the surface. Some fish use it to attract their prey. Certain fungi, and flies also produce bioluminescence .

Liquid droplets form a string of shining beads to attract sects.

tail of worm

Cost of LightingDifferent sources of light cost. We most commonly use incandescent and fluorescent light sources.

A Watt (W) is a unit of measurement for electrical power. A Kilowatt (kW) is 1000 Watts.

Watts 1000 W = 1kW

Grams 1000 g = 1kg

meters 1000 m = 1km

Cost of Lighting

A Kilowatt Hour (kWhr) is 1000 kilowatts operating for 1 hour.

Electrical energy costs about 9¢ per kWhr. That means if you used 1 KW for 1 hr, you would have to pay 9¢

Cost of Lighting

Incandescent BulbEnergy Cost9¢/kWhr

Power 60 W

Time 10 hrs

1. Convert 60W to kW

60W ÷ 1000 = 0.06 kW

2. Calculate the kilowatt hours by multiplying the kW by the number of hours it was on.

0.06 kW x 10 hrs = 0.6 kWhr

3. Calculate the cost of leaving the light on for 10 hours

0.6 kWhr x 9¢/kWhr = 5.4¢

Fluorescent BulbEnergy Cost9¢/kWhr

Power 12 W

Time 10 hrs

1. Convert 12W to kW

12W ÷ 1000 = 0.012 kW

2. Calculate the kilowatt hours by multiplying the kW by the number of hours it was on.

0.012 kW x 10 hrs = 0.12 kWhr

3. Calculate the cost of leaving the light on for 10 hours

0.12 kWhr x 9¢/kWhr = 1.08¢

You can’t go wrong with Fluorescents!!!

• Fluorescent tubes are more energy efficient than incandescent bulbs. A 12W fluorescent tube can produce as much light as a 60W incandescent bulb!

• A fluorescent light is 1/5 the cost of an incandescent bulb!

… except for a few things

• Fluorescent bulbs are much more expensive to manufacture.

• They are also more expensive to dispose of (the mercury vapor and phosphor coating are toxic!)