18.5 Using Light

Pg. 633- 641

Optical Instruments

• There are three common types of optical instruments:

1. Telescope – helps you see far away objects 2. Microscopes – helps you see nearby objects

3. Cameras

Telescopes

• Distant objects are difficult to see because light from them has spread out by the time they reach your eyes

• Telescopes form enlarged images of distant objects– Use lenses or mirrors to collect and focus light

from distant objects

Types of Telescopes

• There are two types of telescopes1. Refracting Telescopes2. Reflecting Telescopes

Refracting Telescopes

• Refracting Telescopes consists of two convex lenses, one at each end of the tube– the larger lens is called the objective • Gathers the lighting coming from an object and

focuses the rays to form a real image

– The lens close to your eye is called the eyepiece• Magnifies the image so you can see it clearly

Reflecting Telescope• Reflecting Telescopes use a large concave mirror to

gather light – The mirror collects light from distant objects and focuses

the rays to form a real image – A small mirror inside the telescope reflects the image to

the eyepiece • The images you see are upside down

Microscopes

• Microscopes are optical instruments that form enlarge images of tiny objects

• Microscopes use a combination of lenses to produce and magnify an image – Example: Figure 23. • Two convex lenses are used to magnify the specimen • The objective forms a real, enlarged image • The eyepiece enlarges the image even more

Cameras

• Cameras use one or more lenses to focus light and film to record an image

• Light travels from an object, travels to the camera and passes through the lens

• The lens of a camera focuses light to form a real, upside-down image on film in the back of the camera – In most cameras the lens will move closer to or

away from the film until the image is focused

How to Take a Picture

• To take a photo:– Press a button that briefly opens the shutter, a

screen in front of the film– Light passes through the lens to hit the film – The diaphragm is a device with a hole that can be

made smaller or larger• Changing the size of the hole controls how much light

hits the film

Lasers• Laser light consists of light waves that all

have the same wavelength, or color• The light waves from lasers are coherent, or

in step– All of the crests of the waves align with one

another

What is a Laser?• A laser is a device that produces a narrow

beam of coherent light• Light• Amplification

by• Stimulated • Emission

of • Radiation

What is a Laser?

• Light amplification means that light is strengthened

• Stimulated emission means that the atoms emit light when exposed to electromagnetic radiation

Producing Laser Light

• Figure 26. pg 636 – The laser tube contains a mixture of helium and

neon gases – An electric current causes the gas mixture to emit

photons (a packet of light energy)– Mirrors at both ends of the tube reflect photons

back and forth – Photons bump into neon particles – Neon particles emit photons with the same energy

as the one that caused the collision

Producing Laser Light

• The two photons travel together in step with one another

• Process continues until there is a stream of in step photons traveling up and down the tube

• Some light leaks through the partially reflecting mirror. – This light is the laser

Uses of Lasers

• Lasers have many practical uses – Stores use lasers to scan barcodes – Engineers use lasers to make sure surfaces are

level and bridges and tunnels are properly aligned– Lasers are also used to read information on

compact discs, create holograms, and perform surgery

Compact Discs

• Lasers can be used to store and read information

• Compact discs are created by converting data into electrical signals

• Electrical signals are used to control a laser beam, which cuts patterns of pits into blank discs

• Lasers also used when you play compact discs– Laser beams shines on the disc and is reflected

Holography• Videos and books may have pictures that

appear to move • Holograms are three dimensional

photographs created by using the light from a laser beam

• The process of making holograms is called holography

Laser Surgery

• A beam of laser light can be powerful enough to replace a knife – As a laser cuts it seals blood vessels– Reduces the amount of blood a patient loses– Wounds from surgery heal faster– Used to remove skin blemishes or cancerous

growths

Laser Surgery: Vision• A common laser surgery is to correct vision by

reshaping the cornea • Lasers can also be used to repair detached

retinas – Detached retinas can lead to partial or total

blindness because the cones and rods can no longer send signals to the brain

– Lasers weld/burn the retina back into the eyeball

Optical Fibers

• Laser beams can carry signals from one place to another

• Laser beams are sent through optical fibers– Optical fibers are long, thin strands of glass or

plastic that can carry light for long distances without letting the light escape

Optical Fibers

• Optical fibers can carry laser beams long distances because the beam stays total inside the fiber as it travels

• The angle of incidence determines whether or not light passes through the surface

Total Internal Reflection• The complete reflection of light by the inside

surface of a medium is called total internal reflection (Fig. 29 pg. 640)– Each time, the light ray strikes the side of the

optical fiber, the angle of incidence is large – Because the angle is large the light ray is always

completely reflected • NO LIGHT CAN ESCAPE THROUGH THE SIDE OF THE

OPTICAL FIBER!

Medicine

• Optical fibers are commonly used in medicine – Can be placed inside the body and then attached

to a camera or computer– Doctor can see internal organs without

performing major surgery – Often used to repair damage to joints • Allows less damage to the knee• Tissues recover easier

Communications

• Optical fibers have led to great improvements in telephone service, computer networks, and cable television systems– Signals sent over optical fibers are much faster

and clearer than those sent over copper wire – Example: One tiny optical fiber can carry

thousands of phone conversations at the same time