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Physics in Everyday Life

Physics of Light and Sight

Andrew Robinson

What is Light?

• Light is one of the things we see around us all the time

• The nature of light has been a topic of discussion for centuries

• The Sun is our main source of light.

Thermal Sources of Light

• Objects which are hot or burning may give off light

Observable Optical Effects

• A range of optical effects can be seen in the atmosphere.

Jasper National Park, Alberta

The Nature of Light

• The eminent Dutch scientist, Christiaan Huygens developed a theory that light was a wave

He was a contemporary of Isaac Newton, and they disagreed about many things…

Huygen’s Traité de la Lumiere

Published in 1690 Translation into English only in 1912

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Newton’s Opticks

• Newton postulated that light was a particle: “Corpuscular Theory”

Notice that this 4th edition from 1704 is printed in English, not Latin

Light as a Wave

• Huygen’s Wave Theory was superior to Newton’s Corpuscular Theory and was able to explain many observable phenomena, and so was the favoured theory of most scientists in Europe

– Except England, where Newton’s prestige was so great that nobody dared disagree with his ideas…

Light as a Particle

• The idea of light as a wave was the standard model, until 1905, with the development of “Modern Physics”

• Einstein proposed that some observable phenomena can only be observed if light is a particle, but with wave-like properties

• This is known as “Wave-Particle Duality” and is one of the very challenging concepts in Physics!

Physical Optics

• The optics we are going to discuss here is called Physical Optics.

• It assumes that light travels in straight lines in a single medium

• Light may change direction if it encounters a boundary with a transmission medium with different optical properties

• To explain all of the phenomena discussed, light has to be treated as a wave.

What is a Wave?

• A wave is a disturbance of something, which can carry energy from place to place.

– Light and sound are examples

• Huygens did not know what the disturbance was, but he knew that it had the characteristics of a wave

– We now know that light is a disturbance of electric and magnetic fields. But that’s a story for another day.

The Mexican Wave

• People stand up and sit down in their seats in sequence. The wave moves around the stadium.

http://angel.elte.hu/wave/index.cgi?m=models

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The Wavelength of a Wave

• The distance between two corresponding parts of the wave

• Peak to peak height

wavelength

Change the Wavelength

• This wave has a shorter wavelength than the previous example

The Frequency of the Wave

• The other characteristic of a wave is the Frequency.

• How many wave cycles go past a fixed point per second?

• Frequency is measured in Hertz (which you know from tuning your radio to your favorite station)

– The radio waves have frequencies which are very high. 1 Megahertz is 1 million Hertz

• 1 Hertz (Hz) = 1 wave cycle per second passing a fixed point

The Speed of the Wave

• The speed of the wave is related to both wavelength and frequency

• In Physics the speed of light, which is given the special symbol, c, is extremely important because it is constant everywhere

𝑆𝑝𝑒𝑒𝑑 = 𝐹𝑟𝑒𝑞𝑢𝑒𝑛𝑐𝑦 × 𝑊𝑎𝑣𝑒𝑙𝑒𝑛𝑔𝑡ℎ

What Type of Wave?

• Light is a form of Electro-Magnetic Radiation

• Electric and Magnetic Fields oscillate

• They are produced by oscillating charges

http://www.walter-fendt.de/ph14e/emwave.htm

• Electromagnetic radiation forms a continuous distribution of wavelengths

• Visible Light is a very small part of this

• Visible light is “special” to us because our eyes can detect that range of wavelengths

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Wavelength of Light and Colour

• The property which defines the colour of the light is the wavelength

• Violet light has a wavelength of 400 nanometres - 0.0000004 metres

• Red light has a wavelength of 700 nanometres – 0.0000007 metres

Short Wavelength

Long Wavelength

White Light

• When we see white light, we are adding together all of the contributions from all of the visible wavelengths (Additive Combination).

• Combining red, blue and green light gives us white light

• Our eyes have red, blue and green sensors in the retina, which send a signal to the brain, which interprets them as a colour

Other Parts of the Electromagnetic Spectrum

• Infra-red – “Beyond the red part of the visible spectrum” - heat

Other Parts of the Electromagnetic Spectrum

• Microwaves – used for communications, radar, and microwave ovens

Other Parts of the Electromagnetic Spectrum

• Radio waves – used in radio and TV • High frequency, long wavelength

Other Parts of the Electromagnetic Spectrum

• Ultra violet “beyond violet light” – sun tan, sun burn

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Other Parts of the Electromagnetic Spectrum

• X-rays – medicine, dentistry

Energy Carried by the Radiation

• The HIGH frequency radiation carries more energy and is more dangerous

• The LOW frequency radiation carries less energy and is less dangerous

Light Travelling in a Straight Line

• In many applications, we can assume that light travels in a straight line

• This is known as Geometric Optics.

• Light can change direction when it encounters a mirror or a transparent material

– Most common optical instruments, and the eye, can be described by this assumption.

Speed of Light

• The speed of light is extremely fast, and is extremely difficult to measure.

• Many scientists have made attempts to measure it over the centuries, starting with Galileo.

• The speed of light in a vacuum is:

𝑐 = 299,792,458 𝑚𝑒𝑡𝑟𝑒𝑠 𝑝𝑒𝑟 𝑠𝑒𝑐𝑜𝑛𝑑

• or 1,080,000,000 km/hour!

Light is Fast, But Not That Fast

• Even though light moves at high speed, it still takes over 8 minutes for light to reach us from the sun

We are now seeing the sun as it was 8 minutes ago

Speed of Light in a Solid

• Light slows down when it encounters a transparent medium or object

• Air slows down light very slightly, but only by about 88,000 metres per second which is not much compared with 299,792,458 metres per second.

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Light in Water

• While light is travelling through water, it slows down to about three quarters of c

• When it comes out of the water, it moves at c again

water air air

Speed c Speed c Speed ¾ c

Light in Glass

• The speed of light in glass depends on the chemical composition of the glass

– It can be measured very precisely using a very small piece of glass, and so is very useful in forensic science, where it can identify slivers of glass associated with a crime scene

• Generally, the light slows down to about two thirds of c while in the glass

Reflection

• If light hits a perfectly flat reflecting surface (a mirror), it is reflected at the same angle with which it strikes the surface

• Pied Avocet near Oosterend, Texel island, the Netherlands.

• Still water acts as a mirror surface

Reflective Surfaces

• Polished metals • Mirrors • Gloss Paint

• Paper • Wood • Unpolished metals • Flat or matt Paint

Highly reflective surfaces

Diffuse reflecting surfaces

Reflection and Transmission

• When light reaches a boundary between two transparent materials, some light is reflected, and some is transmitted

• The reflected light still obeys the law of reflection

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Reflection in a Glass Display Case

• Most light is transmitted through the glass

• Some is reflected

Artifacts in the National Museum of Malaysia (Wikimedia)

Refraction

• If light hits a surface of a transparent medium, then some of the light passes into the new medium.

• It slows down in the medium, and this causes a change in wavelength and a change in direction

Air

Water

• Huygens was the one who worked out why the wave changed directions

• The frequency of the wave stays the same, but if the speed changes, then the wavelength must change too

• You can easily see the refraction effect by putting a straw in water in a glass

• You have different speeds of light in the air, the water and the glass, so you see discontinuous images of the straw

Prism

• This change in direction of light can be used to split light into the colours of the spectrum

• We use a specially shaped piece of glass to do this

• The prism splits up white light because each colour has a slightly different wavelength, and travels at a slightly different speed when in the glass

– The speed differences are known as dispersion, and only occur when light passes into a transparent medium

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Rainbows

• This dispersion effect is the cause of many atmospheric optical phenomena, such as rainbows.

• During and after rain, there may be sufficient small droplets of water suspended in the atmosphere to disperse the light coming from the sun.

• Each droplet acts like a tiny prism.

• An individual droplet behaves like this:

Refraction

Reflection from the inside of the drop

Second Refraction

• We see a rainbow because there are many droplets, all refracting the light, but at different heights and positions

An observer a long way from the droplets sees the red light from the upper droplets, and the purple light from the lower droplets

• So we see the rainbow with the red at the top and the violet at the bottom

Secondary Rainbows

• If conditions are favourable, sometimes we can see a second rainbow above the first, with the colours reversed

• In the secondary rainbow, light moves on a different path

• This time there are two reflections inside the droplet.

• The light emerges at a steeper angle than for the primary rainbow, so the observer sees reversed colours.

• The intensity is less because this double reflection is less likely

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Light Bending

• Sometimes, the light does not move in straight lines because it passes through zones where it is refracted and reflected many times, which “bends” it.

• Clouds of ice particles or water droplets are usually responsible

• This gives rise to a range of optical phenomena

Sun-Dogs

• Images of the Sun which have been bent by clouds of suspended ice crystal

Fargo, North Dakota

• The sun dog can occur in cloud formations too

Sun dog over Stonehenge

Solar Halo

• More refraction from ice crystals.

View from the Brocken, Harz, Germany

Mirages

• Sometimes layers of air are arranged in such a way that they refract light to different degrees

• This gives rise to Mirages

Cool air bends light more

Warm air bends light less

The Inferior Mirage

• Images from above are seen on the ground

This type of mirage is not that stable, as the warm air tends to rise.

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Designing Optics

• We use a “Ray diagram” to show the direction that we expect rays to move in, when they pass through a lens, or reflect from a mirror

Principle axis

object

lens

F focal point of lens

Lens Design

• Lenses are designed to either

– make parallel rays diverge (diverging lens) or

– Make parallel rays converge (converging lens)

• The glass or plastic is shaped to make the places where the convergence or divergence a fixed position

– controlled refraction

• Converging Lens

• Diverging Lens

The eye is tricked into believing that the rays from the lens all come from the focal point

F

Image Formation and the Brain

• Your brain takes images from the eye and interpolates back in a straight line

• This means that some images appear to come from a point in space where there is actually no image or object. These images are known as virtual images

The image from a magnifying glass is a good example

Real Image

• For a real image, the rays from the object pass through the image point.

• This means that a real image can always be projected onto a screen (camera, projector, glasses)

Image: Wikimedia

Virtual Image

• Light rays can be traced back to a point from which they appear to diverge.

– Light rays appear to come from that point, even though they don’t really.

– A virtual image cannot be projected onto a screen

– An image from a mirror, or a magnifying glass are examples of a virtual image

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The Camera

• Object more than twice the focal length away from the lens

• Image is real, inverted and smaller than object

2f

• Object is between f and 2f

• Image is real, inverted, larger than object

The Projector

2f

• Image is upright, enlarged and virtual

– Example: Magnifying glass

The Magnifying Glass

2f 2f

f

• Converging lens – object closer than f

Diverging Lens

• Regardless of the position of the object the image formed is always

• Virtual • Upright • Smaller than

the object

2f 2f

f f

Convex Mirror

• A convex mirror curves away from the observer.

• The rays diverge from a focal point that appears to be behind the mirror

• Convex mirrors give a bigger field of view than a plane mirror

• However, they also make objects seem smaller and further away than they really are

Image: Wikimedia

“Objects in the mirror are closer than they appear” Required to be engraved on car side mirrors in US, Canada, India, Australia

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Concave Mirror

• Curves towards the viewer

• Rays are focused at the focal point in front of the mirror

• Concave mirrors produce a magnified image

• Mirrors for cosmetics, shaving and use in dentistry are concave

Satellite dishes do the same thing with radio waves

• Jodrell Bank radio telescope

Mike Peel; Jodrell Bank Centre for Astrophysics, University of Manchester

The sensitive radio receiver goes at the focal point

Multiple Stage Optics

• The performance of one lens or mirror is often limited

• By combining more than one lens and or mirrors, other more powerful optical devices can be made.

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Periscope

Two mirrors

Used in submarines to observe ships while the submarine is under water. Image: US Navy

Telescopes

• Refractor Telescope

• Converging lens near the object (objective lens)

• Diverging lens as the eyepiece

• High magnification

Yerkes 40 inch refractor

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• Newtonian Reflector

Replica of Newton’s original

• Newton used a spherical primary mirror – so the performance wasn’t as good as the refractor telescopes of the day.

• When parabolic mirrors were used (much later), the design proved to be superior to refractors.

Microscopes

• The compound microscope has two lenses to increase magnification to up to x1000

1896 version 21st century version

The Human Eye

• The human eye is an optical device which bends light to form a focussed image on the retina, at the back of the eyeball

Accommodation

• The lens can change shape to allow us to focus on objects far away or nearby.

• Focussing on far objects is done with relaxed ciliary muscles

• Focussing on far objects is done with relaxed ciliary muscles

• As we get older, the ciliary muscles are not able to tense as much.

• We cannot form a good image close up (the near point gets further away)

• We need reading glasses

Rods and Cones

• The rods and cone cells on the retina are there to sense light falling on them

• The rod cells are for imaging in low light conditions, but do not give colour information

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Cones

• The cone cells need more light to function, but they are capable of detecting a range of different colours

• We have three types of cone cells in our eye, which look for red, green and blue light respectively

Sensitivity to Colour

• The cone cells are sensitive to

• Short wavelengths (blue light)

• Medium wavelengths (green light)

• Long wavelengths (red light)

Why are Tennis Balls Green?

Colour Blindness

• Colour blindness occurs if one of more types of cone cells are absent or not working

83 visible if you have normal vision

37 visible unless you are protanopic where you lack the L cones, which see the red-orange-yellow part of the spectrum (1% of human males)

• Someone who is deuteranopic might not see this number (49). Note that the 9 may be difficult to discern even with normal vision.

• Deuteranopia is the loss of the M medium wavelength cones, affecting red – orange – yellow again, slightly differently to the protanopes

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• Someone who is tritanopic might not see this number (56). Image may not be visible on LCD or with excessive screen glare.

• Tritanopes lack the L (Long wavelength) cones, so they have difficulty distinguishing blues and violets

Defects of the Eye

• Myopia

– light focussed in front of the retina

– Eye bends the light too much

• Hyperopia

– Light focussed behind the retina

– Eye doesn’t bend the light enough

Correcting Near Sight

• To correct myopia we put a diverging lens in front of the eye

• The optometrist has to work out the focal length of the lens which gives you corrected vision

F

Correcting Far Sight

• To correct hyperopia, put a converging lens in front of the eye

• This assists the eye by increasing the total angle through which the light is bent

Focal length

Astigmatism • Astigmatic vision is

where you have a different focal length in the horizontal and vertical directions

• In this example vertical lines are focussed correctly, horizontal lines are not

Thermal Source

• Objects that are hot will emit light at all visible frequencies

• They also emit large quantities of infra-red light too

• Incandescent light bulbs get very hot and are using most of the power to produce heat, not visible light, so they are not efficient

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Incandescent Light Bulbs

• Incandescent light bulbs use a hot filament to produce light.

• They produce white light (as a mixture of all colours)

• They produce more infra-red (heat) radiation than visible light, so they are not very efficient

Atomic Emission Lines

• Neon signs use neon (and other gases at low pressure) to produce light. The light is only one wavelength, so we see one colour

Compact Fluorescent Light Bulbs

• Usually have mercury vapour in them, which emits several different wavelengths, but not a continuous spectrum

• They also don’t emit as much infra red radiation, so they are much more efficient

Bioluminescence

• Fireflies

A molecule called Luciferin is responsible for the emission of the green-yellow light