Unit 11 – Light
Unit Big Idea: Visible light is the small part of the electromagnetic spectrum that is essential for human
vision.
Lesson 1 – The Electromagnetic Spectrum
Essential Question: What is the relationship between various EM waves?
By the end of this lesson, you should be able to distinguish between the parts of the electromagnetic
spectrum.
- Light travels as a wave but does not need to travel through a medium.
- Light waves are disturbances in electric and magnetic fields
- When an electrically charged particle vibrates, it disturbs the electric and magnetic fields around
it – called electromagnetic (EM) waves – they carry energy away from the charged particle
- the disturbances are perpendicular to each other and to the direction the wave is traveling in
- radiation – transfer of energy as EM waves
- in a vacuum, all EM waves move at the same speed- the speed of light – 300,000,000 m/s
- EM waves don’t need a medium to travel through but they can travel through gases, liquids, and solids
- EM waves travel slower in gases and fastest in solids
- White light is all the colors of the rainbow combined
- Red, Orange, Yellow, Green, Blue, Indigo, Violet
ROY G BIV
- each color of light has a different wavelength
- red has the longest wavelength and the lowest frequency
- violet has the shortest wavelength and the smallest frequency
- our eyes only register three color ranges of light – the primary colors: red, green, & blue
- all other colors we see are a mixture of these colors
- the color our eyes see if the color that is absorbed by the object, while all the other wavelengths of color
are reflected
- a shirt is red because it absorbs the red wavelength and reflects back all the other colors
- the sky is blue because blue is a shorter wavelength that gets scattered easily – this is why we see it best
- electromagnetic (EM) spectrum – all the frequencies or wavelengths of electromagnetic radiation
- EM Spectrum Waves:
- Radio waves: longest wavelength, used to broadcast signals for radios, TVs, alarm systems, etc
- Microwaves: used to heat food & by cellular phones
- Infrared Light waves: means “below red;” amount of infrared light given off depends on the
objects temperature with red being hottest and violet being coldest
- Visible Light waves: all the colors of the EM spectrum we can see, narrowest part of spectrum
- Ultraviolet Light waves: means “beyond violet;” some animals (like bees) can see with
ultraviolet light
- X-ray waves: can pass through most living tissue but is absorbed by bones
- Gamma ray waves: used to treat illnesses and in making medical images
- the sun gives off radiation in all parts of the EM spectrum
- more of the sun’s energy is given off in the visible light spectrum than all other parts
- not all wavelengths of light get through Earth’s atmosphere the same distance
- Earth’s atmosphere blocks most of the higher frequency radiation (X-rays & gamma rays) from
reaching the ground
- Earth’s atmosphere blocks some of the ultraviolet, infrared, & microwaves
- Earth’s atmosphere doesn’t block visible light and blocks very little radio waves
- ground based telescopes and radio telescopes use radio waves since very little of the
radio waves are blocked by Earth’s atmosphere
- We need to protect ourselves from EM waves
- too much ultraviolet (UV) light can cause sunburn, skin cancer, & damage to the eyes
- sunscreen, sunglasses, hats, long sleeves, & long pants will block UV rays from the sun
- UV light travels through clouds so UV protection is needed even on cloudy days
- Higher frequency EM waves have more energy
- higher frequency EM waves have shorter wavelengths and are more dangerous to humans
- gamma rays have the shortest wavelength (highest frequency) and will damage living tissue
- X-rays & Ultraviolet (UV) waves are also dangerous to humans and precautions are needed
when using them
- Lower frequency EM waves have less energy
- lower frequency EM waves have longer wavelengths and are not that dangerous to humans
- radio waves are even used in baby monitors and walkie-talkies because they are harmless
Lesson 2 – Interactions of Light
Essential Question: How does light interact with matter?
By the end of this lesson, you should be able to explain how light and matter can interact.
- when light comes into contact with a material, it can pass through it, or be transmitted
- the medium can transmit all, some, or none of the light
- transparent – matter that transmits light
- objects that can be seen through clearly
- examples: air, water, clear glass
- translucent – matter that transmits light but not straight through it
- scatters light into many directions but lets some through
- objects seen through it are fuzzy or distorted in appearance
- examples: frosted glass, tissue paper, clear colored glass
- opaque – matter that doesn’t let any light through it
- reflect and/or absorb all the light
- examples: wood, metal, ceramic, brick
- absorption – transfer of light energy to matter
- you only see an object when light from the object enters your eye
- most objects don’t give off, emit, light so you have to see the light that bounces off the surface
of the object
- reflection – bouncing of light off a surface
- when light strikes a rough surface, like on wood or cloth, the light is reflected in many different
directions
- light bounces off an object (reflected angle or ray) at the angle equal to the angle it hit the
surface (incident angle or ray)
- when light strikes a smooth surface, like a mirror, shiny
table, or still water, it reflects in a uniform way – producing an image or reflection of the object
- white light is light that includes all colors
- when white light strikes an object, the object can transmit some or all of the colors of light,
reflect some or all the colors, and absorb some or all the colors
- the perceived color of an opaque object is determined by the colors of light reflected off it
- the color of a transparent or translucent object depends on which color transmitted, or allowed to pass,
through it
- transparent and translucent materials absorb all colors except the one that passes through it – this
is the color it will appear
- perceived color can also depend upon the color of light available
- if you look at different colored objects under a red light, the red light bulb filters out all colors
except red, and some orange and yellow
- a red object would still appear red under a red light bulb
- any object that absorbs red, orange, and yellow light would appear gray or black
- a medium can change the properties of light when it passes through it
- light slows when it passes through matter
- when light changes enters a medium, shorter wavelengths are slowed more than longer
wavelengths
- in a medium, the speed of violet light is less than the speed of red light
- the speed changes each time light changes to a new medium
- refraction – change in direction of a wave as it passes from one medium into another at an angle
- caused by a change in speed as a wave enters a new medium
- a prism separates light in a spectrum of colors because the light waves with the shorter wavelengths
change direction more than the light waves with the longer wavelengths
- scattering – happens when light is sent in many different direction as it passes through a medium
- you can see light shining through fog when you can’t see it shining through the air
- the color of the sky is due to the scattering of light – shorter wavelengths (blues & violets)
scatter more than longer wavelengths (reds & oranges)
- as the sun rises or sets, it’s light is scattered through more of the atmosphere, absorbing all the
shorter wavelengths of light (blues and violets) leaving only the longer wavelengths behind
(reds, oranges, & yellows)
Lesson 3 – Mirrors and Lenses
Essential Question: How do mirrors and lenses work?
By the end of this lesson, you should be able to describe ways that lenses and mirrors form images.
-
light waves move in a straight line until it hits an object, then it can be reflected, or bounce off
most objects have a rough surface that reflects light in many directions
very smooth surfaces, like a mirror, reflect light in a uniform way
normal – the imaginary line that is perpendicular to the mirror’s surface
law of reflection – the angle that the light hits the surface (incident ray) is equal to the angle that
the light is reflected from the surface (reflected ray)
- plane mirrors – mirrors that have a flat surface
- reflection of a plane mirror is right side up but reversed left to right
- your reflection is the same size as you are
- the reflection appears to be the same distance behind the mirror as you are in front of it
- virtual image – image that appears to come from a place that the light does not actually come from
- type of image formed by a plane mirror
- concave mirror - curved inward like the bowl of a spoon
- concave mirrors cause parallel light to converge, or come together
- used for making beams of light and magnifying objects
- when an object is held close to a concave mirror, the reflected waves are extended in a straight
line behind the mirror and form a virtual image where they cross – the image is right side
up and magnified
- when an object is held farther from the concave mirror, the reflected rays cross in front of the
mirror forming a real image
– this real image is upside down and smaller than the object
-
focal point – point where the light rays converge
all images are formed where two or more rays from the same location on an object converge
concave mirrors can form either virtual images or real images
real image – image formed where light from an object converges
- real images can be projected onto a screen whereas a virtual image cannot
- convex mirror - curves outward like the back of a spoon
- cause a beam of light to diverge, or spread apart, as if it came from a focal point behind the
mirror
- creates a virtual image that is right side up and small
- used to make small images of a large area – mirrors used for security in stores and factories;
many cars, buses, and trucks use convex mirrors as side mirrors so they can see more of the
surrounding area
- the reflected rays don’t cross in front of the convex mirror so they can’t form a real image –
instead the reflected rays are extended behind the mirror to form a small, virtual image
-
lens – clear optical tool that refracts light
refraction occurs when a light wave changes speed as it passes from one medium to another
the change in speed makes the light wave bend and either converge or diverge, depending on the lens
light from an object passes through a lens to form a real or virtual image of the object
the type of image depends on the shape of the lens and how close the object is to the lens
- convex lenses form real or virtual images
- convex lens is thicker at the center than at the edges and often convex
on both sides
- parallel rays of light converge at a focal point after they pass through a
convex lens
- focal length – distance between the lens and the focal point
- lenses thicker in the center have a focal length that is longer than lenses that are thinner in the
center
- when an object is held less than one focal length from the convex lens, a virtual image if formed
right side up and larger than the object
- when an object is held more than two focal lengths from a convex lens, the real image forms
upside down
- used to magnify or focus light – used in magnifying glasses, telescopes, microscopes,
binoculars, camera, projectors, and eyeglasses to correct farsightedness
- concave lenses form virtual images
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concave lens is thinner at the center than at the edges
it is concave on both sides
light that passes through a concave lens is refracted outward as if from the focal point
because the light waves are refracted away from each other, parallel light waves passing through
a concave lens do not meet
- a virtual image is formed, that is right side up and smaller than the object
- the distance between the object and the lens doesn’t make a difference in the type of image that
is formed
- used to spread light, many times in combination with other lenses in telescopes and binoculars,
and used in glasses to correct nearsightedness
Lesson 4 – Light Waves and Sight
Essential Question: How do people see?
By the end of this lesson, you should be able to explain how the eye functions and works with the brain to
produce vision.
- you see an object when your eyes detect light and send signals to your brain
- some objects produce their own light, while other object reflect light
- light has to enter your eye before you can see anything
- cornea – this is the part of the eye that light waves enter
- transparent membrane that forms the front part of the eye
- cornea refracts (bends) the light so it passes through the pupil at the center of the iris
- the iris changes the size of the pupil to control the amount of light let in
- the light refracts again as it enters the lens
- muscles around the lens change its thickness so that the object at different distances can be seen in focus
- retina – an image is focused on the retina
- it’s the light sensitive tissue that lines the inside of the eye
- the part of the eye that detects light and sends signals to the brain
- image if focused upside down on the retina
- two types of cells in the retina detect light – the rod cells and the cone cells
- rod cells are very sensitive and can detect even dim light
- cone cells detect brighter light and colors
- cone and rod cells convert the input they receive into electrical signals
- the brain interprets the signal
- optic nerve – bundle of tissue that carries signals to the brain
- different parts of the brain take in the signals and interpret color, shape, movement, and location
of the image
- although the image if upside down, the brain understands the image as being right side up
- the brain also combines slightly different information from the right and left eyes to produce a
sense of distance and depth
- vision problems can happen for a number of different reasons
- common vision problems happen when light is not focused on the retina or when a part of the
eye does not work properly
- nearsightedness (myopia) – happens when a person’s eye is too long or their cornea is curved steeply
- produce an image in front of the retina rather than on the retina
- a nearsighted person can see something only if it is nearby; faraway object look blurry
- can be corrected with a concave lens causing the light to bend away from the thin center of a
concave lens moving the image back to the retina
- farsightedness (hyperopia) – happens when a person’s eye is too short or the cornea is not curved
enough
- produce an image behind the retina rather than on the retina
- a farsighted person can see distant objects most clearly; things nearby look blurry
- people can be born farsighted and grow out of it as they get older
- can be corrected with a convex lens causing the light to bend toward the wide middle of the
convex lens moving the image forward to the retina
- the clouding on an eye lens, called a cataract, interferes with light entering the eye
- color vision deficiency
- about 5 – 8 % of males & 0.5% of females have a color vision deficiency
- this condition is often called color blindness, but very few people can’t see any color
- happens when the cones in the retina do not work properly
- in some people, the cones respond to the wrong colors
- reds & greens are seen as a different color, such as yellow
- color vision deficiency can’t be corrected
- vision problems can be corrected with glasses, contacts, or with laser surgery
- contact lenses correct vision by changing the shape of the cornea
- corrective eye surgery also works by reshaping the patient’s cornea
- reshaping the cornea changes how light is focused on the retina
- during surgery, a thin flap is folded back from the surface of the eye and the cornea is
then reshaped with a laser so the patient gains perfect or nearly perfect vision
Lesson 5 – Light Technology
Essential Question: How can light be used?
By the end of this lesson, you should be able to apply knowledge of light to describe light-related
technologies.
- incandescent light – visible light produced from a very hot material
- electrical current is passed through a thin wire, called a filament, inside the bulb
- the filament gets hot enough to emit (give off) visible light
- inefficient in producing light compared to other types of light bulbs; only 8% of the energy
given off by incandescent light bulbs is in the form of light – the rest is heat
- electric current can energize some gases and produce ultraviolet light, which is invisible to humans
- fluorescent light (compact fluorescent or CFL)– produced when a fluorescent coating inside a bulb
converts the ultraviolet light into visible light
- about 80% of the energy given off by fluorescent bulbs is n the form of visible light
- fluorescent bulbs last about 10 times longer than incandescent bulbs
- light emitting diodes (LED) – contains solid materials that emit light when energized by an electric
current
- emits only one color of light
- about 100% of the energy given off by LEDs is in the form of visible light
- LEDs are very efficient and last a long time
- most traffic lights in the US now use LEDs
- laser – device that produces intense light of a very small range of wavelengths
- produce light in such a way that causes the light to be more concentrated, or intense, than other
types of light
- laser light is coherent (light waves stay together as they travel away from their source)
- the crests and troughs of coherent light waves are aligned so the individual waves behave as one
wave
- some light technologies use light to encode, send, or read signals
- lasers inside a CD or DVD player reads the information stored on the disc
- bar code scanners in retail stores record the price of a purchase and transfer the information to
TV remote controls that use infrared light
- infrared radiation is invisible electromagnetic radiation with a wavelength longer than what our eyes
can detect
- night vision goggles emit infrared radiation to allow people to see objects without visible light
- weather satellites track storms forming at night with infrared pictures
- space-based telescopes determine temperatures of starts and dust clouds by measuring the
infrared radiation coming from them
- optical fiber – thin, transparent glass thread that transmits light over long distances
- bundle of optical fibers transmit information through telephone cables
- also used to network computers and to allow doctors to see inside patients’ bodies without
performing major surgery
- are like pipes that carry light
- light stays inside optical fibers because of total internal reflection – complete reflection of light
back and forth along the inside surface of the material through which it travels
- light is emitted out the end of the fiber
- satellite technologies send TV, radio, and cell phone data to buildings
- weather and government satellites also transmit data using EM waves
- Global Positioning System (GPS) is a network of 24 satellites orbiting Earth
- these satellites continuously send microwave signals
- signals are picked up by a GPS receiver on Earth and used to measure positions on
Earth’s surface
- GPS was originally used by US military but now anyone can use it
- many cars have GPS road maps to help navigate; hikers and campers use GPS receivers
to find their way through the woods
- Doppler Radar uses light in the form of radio waves to measure weather patterns
- radio waves are sent out toward a weather system and bounce off the clouds and back to the
transmitter, which captures the information for analysis
- the frequency of radio waves changes a small amount, depending of how the weather system is
moving
- laser technologies
- light from lasers can be accurately pointed and used to melt and cut different materials
- lasers are used in manufacturing to cut, weld, and engrave certain metals; doctors use lasers for
surgery to make very precise incisions; used to shape the cornea to correct eyesight
- lasers are found in everyday devices; CD drives in computers, in printers, laser pointers, laser
levelers, make holograms, & three-dimensional images seen on credit cards
- microscopes used to see magnified images of tiny, nearby objects
- simple light microscopes have two convex lenses; the objective lens is close to the object being
studied; the eyepiece lens is what you look through
- light from the bottom shines through the object being studied
- telescopes are used to see images of large, distant objects
- used to study unknown objects, like the moon, and to search for undiscovered objects
- telescopes that use visible light are classified as either refracting or reflecting
- refracting telescopes use lenses to collect light
- reflecting telescopes use mirrors to collect light
- the less atmosphere the light travels through, the clearer the image appears so many large
telescopes are located on top mountains
- cameras are used to record images
- digital camera controls the light that enters the camera and uses sensors to detect the light
- the sensors send an electrical signal to a computer in the camera
- the signal contains data about the image that can be stored and transferred
- some cameras also record video using computers