# Light and Optics What is light?

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Light and Optics What is light?
It's a kind of energy called "electromagnetic (EM) radiation" (but this kind of radiation is not harmful, except for an occasional sunburn). There are other kinds of EM radiation too (radio waves, microwaves, x-rays, etc.), but light is the part WE can see, the part that makes the rainbow.

How does light travel? FAST and STRAIGHT.
How FAST? About 186,000 miles per second [300,000 kilometers per second], so light from the sun takes about 8 minutes to go 93 million miles [149 million kilometers] to earth. Does this seem SLOW? Well, if you could DRIVE to the sun at 60 mph [100 kph], it would take you 177 years to get there! In one second, light can go around the earth 7 times!

Light travels VERY FAST – around 300,000 kilometres per second.
At this speed it can go around the world 7 times in one second.

How STRAIGHT. Perfectly straight, until something bends it
How STRAIGHT? Perfectly straight, until something bends it. The straight paths of light are called LIGHT RAYS.

Light travels in straight lines:
Properties of Light Light travels in straight lines: Laser

Light travels much faster than sound. For example:
Thunder and lightning start at the same time, but we will see the lightning first. 2) When a starting pistol is fired we see the smoke first and then hear the bang.

There are three ways to control light.
Block it with something opaque that creates a shadow Reflect it by changing its path with a mirror Bend it Change its direction by making it pass into another transparent material of different density, like glass or water. This is called REFRACTION, and it's how lenses work.

Law of Reflection The incident ray strikes the mirror.
The reflected ray bounces off. The angle of incidence equals the angle of reflection.

Angle of incidence = Angle of reflection
The Law of Reflection Angle of incidence = Angle of reflection In other words, light gets reflected from a surface at the same angle it hits it.

Reflection In specular reflection each incident ray bounces off in a single direction. A surface that is not shiny creates diffuse reflection. In diffuse reflection, a single ray of light scatters into many directions.

We see things because they reflect light into our eyes:
Homework

Reflection Images appear in mirrors because of how light is reflected by mirrors. The incident ray follows the light falling onto the mirror. The reflected ray follows the light bouncing off the mirror.

Refraction Light rays may bend as they cross a boundary from one material to another, like from air to water. This bending of light rays is known as refraction. The light rays from the straw are refracted (or bent) when they cross from water back into air before reaching your eyes.

Words – speed up, water, air, bent
Refraction Refraction is when waves ____ __ or slow down due to travelling in a different _________. A medium is something that waves will travel through. When a pen is placed in water it looks like this: In this case the light rays are slowed down by the water and are _____, causing the pen to look odd. The two mediums in this example are ______ and _______. Words – speed up, water, air, bent

Reflection and Refraction
A lens is an optical device that is used to bend light in a specific way. A converging lens bends light so that the light rays come together to a point. A diverging lens bends light so it spreads light apart instead of coming together.

Reflection and Refraction
Mirrors reflect light and allow us to see ourselves. A prism is another optical device that can cause light to change directions. A prism is a solid piece of glass with flat polished surfaces.

The fine print: There are actually other ways to bend or deflect light, including diffraction gratings and holographic lenses.  These depend on the wave nature of light, and are a little more difficult to explain.   Scientists have also found that gravity can bend light, but it takes a very large object with strong gravity such as a star to bend light very much, so it's not an effect you see every day!

Visible light waves are the only electromagnetic waves we can see
Visible light waves are the only electromagnetic waves we can see. We see these waves as the colors of the rainbow. Each color has a different wavelength. Red has the longest wavelength and violet has the shortest wavelength. When all the waves are seen together, they make white light.

Spectrum Wavelengths

When white light shines through a prism, the white light is broken apart into the colors of the visible light spectrum. Water vapor in the atmosphere can also break apart wavelengths creating a rainbow.

Dispersion and prisms The variation in refractive index with color is called dispersion. A rainbow is an example of dispersion in nature. Tiny rain droplets act as prisms separating the colors in the white light rays from the sun.

Mirrors, Lenses, and Images
We see a world of images created on the retina of the eye by the lens in the front of the eye.

Mirrors, Lenses, and Images
Objects are real physical things that give off or reflect light rays. Images are “pictures” of objects that are formed in space where light rays meet.

Mirrors, Lenses, and Images
The most common image we see every day is our own reflection in a mirror. The image in a mirror is called a virtual image because the light rays do not actually come together. The virtual image in a flat mirror is created by the eye and brain.

Mirrors, Lenses, and Images
Light rays that enter a converging lens parallel to its axis bend to meet at a point called the focal point. The distance from the center of the lens to the focal point is called the focal length. The optical axis usually goes through the center of the lens.

A converging lens bends an incident light ray parallel to the optical axis toward the focal point.
A diverging lens bends an incident light ray parallel to the axis outward, away from the focal point

The image formed by a lens
A lens can form a virtual image just as a mirror does. Rays from the same point on an object are bent by the lens so that they appear to come from a much larger object.

The image formed by a lens
A converging lens can also form a real image. In a real image, light rays from the object actually come back together.

Technological Revolution in Optics
Communication by photons Telephony/data/internet Massive optical data storage CD/DVD Blu-ray disc (25GB) Precision laser machining Laser writing on human hair Photolithography for manufacture of computer chips Laser cutting Medical laser therapy & optical imaging Corrective laser eye surgery 3-D laser imaging of cell

Optics has an important place in history
Telescope observations forged our understanding of the Universe Optics, light & vision has been vital for human survival Today, Optics remains a key scientific diagnostic technique (e.g. imaging). A new revolution in Optics has emerged with the birth of the laser, fibre optics, integration of optics and electronics, etc.. Microscopes revealed a micro-universe Fire – Caveman ---- Fire Beacons (Troy – Spanish Armada (Plymouth to London – York in 12 hrs)) Greek Philosophers Telescope – Galileo – Planets/moons/features/galaxies – Universal laws -Man’s place in universe (not centre) – finding new world’s – origins of Universe Microscope – (The micro-Universe) – fleas/cells (Micro-world) Waves/particles/photons –wave-particle duality – QM – LASERS

Fundamentals of Optics
REFLECTION REFRACTION IMAGING DIFFRACTION INTERFERENCE POLARISATION Continuum of waves Finite no. of waves EM-theory

Our Eyes © 2003 Mike Maloney

Vision Correction with Lenses
c. Hyperopia (farsightedness) a. Normal eye b. Myopia (nearsightedness) Corrected with concave lens Corrected with convex lens No correction needed

A light review: Our primary source of light is the sun.
Light travels in straight lines at a speed of 186,000miles per second. Light waves travel faster than sound waves. Light energy from the sun travels through space , reaches earth, and some of it turns to heat energy and warms the earth’s air. Light from the sun also travels to the cells of green plants (producers) and is stored as energy. When light reaches an object, it is absorbed, reflected, or passes through it.

Electromagnetic radiation can be described in terms of a stream of photons. Each photon is traveling in a wave-like pattern, moving at the speed of light and carrying some amount of energy. The only difference amongst radio waves, visible light, and gamma-rays is the amount of energy of the photons. Radio waves have photons with low energies. Microwaves have a little more energy than radio waves. Infrared has still more energy. As we move down the chart we see visible, ultraviolet, X-rays, and gamma-rays whose photon energies gradually increase. Gamma and Cosmic rays have the highest energy waves. *Slide info from NSTA 2004 conference

Remember radio waves are long…and gamma rays are small
Radio-TV -Microwave- Infrared - VISIBLE -Ultraviolet -X-rays - Gamma- Cosmic

Essential Knowledge Properties of a wavelength

Vocabulary Waves: a disturbance which carries energy and that travels away from the starting point. Wavelength: the distance between one crest of a wave and the next crest (distance between compressions); red is longest and violet is shortest Peak: the highest point of a light wave Trough: the lowest point of a light wave Frequency: the number of waves (light or sound) produced in a given unit of time, such as a second

Vocabulary Reflection: the bouncing of light from a surface
Refraction: the bending of light as it passes through one material to another

Vocabulary Transparent: materials that allow all light to pass through
Translucent: letting light through but scattering it Opaque: materials that do not let light through Transmit: to send (as in sound or light). It also means light passing through an object.

Galileo Galilei Italian Lived from 1564-1642
Developed the refracting telescope Developed the first known example of the microscope

Robert Hooke English Lived from 1635-1703
Devised the compound microscope and illumination system Discovered plant cells Examined fossils with a microscope

Isaac Newton English Lived from 1643-1727
Discovered that white light was not a simple entity, but splits into a range of colors When he passed white light through a glass prism, he noted that a spectrum of light was formed; particle theory of light Reflecting telescope