SNC2D Grade 10 Academic Science Physics Type of Lights and Mirrors

Thanks, Tony!

Physics: Chapter 10 Notes

 

Chapter 10.1—Sources and Nature of Light

  • there are many sources of light—some are natural (sun) while others are artificial (candle and light bulbs)
  • some light sources are related to heat—light from hot objects is made up of many different colours mixed together (aka white light)
  • light can also be emitted from objects that aren’t hot (ex.: microorganisms)—these sources often emit one main colour of light
  • for all light sources, atoms within the materials must absorb some form of energy
    • after absorbing the energy, the atoms are considered to be in an excited state
    • then almost immediately, the excited atoms release the energy; it is often released in the form of light

 

THE SUN

  • the most abundant source of light
  • hydrogen atoms in the Sun’s core have so much energy that when they collide, they sometimes combine to form helium (this process is called a fusion reaction)
    • a tremendous amount of energy is released from a fusion reaction
    • the fusion energy is transmitted to the gases on the outer layers of the Sun
    • when these excited atoms release some of their excess energy, they emit light
    • fusion energy is a form of nuclear energy

 

Light from INCANDESCENCE

  • incandescence is light emitted from a material because of the high temperature of the material
  • for many years, the most common source of light at home USED to be the incandescent light bulb
    • an incandescent bulb has a tiny tungsten wire that gets very hot and glows brightly when electric current runs through it (thus electrical energy generates the heat that excites the atoms)
  • an incandescent bulb is inefficient at producing light—only about 5% of the electrical energy used in an incandescent bulb becomes light (the remaining 95% is lost as heat)

 

Light from ELECTRIC DISCHARGE

  • found in some street lights; lightning
  • light is emitted from a heated gas (or vapour) instead of a heated wire
    • this process is called electric discharge
  • a common form of the electric discharge bulb is the sodium vapour bulb
    • there is an electrode at each end of the bulb
    • a drop of sodium and a small amount of mercury is placed in the bulb
    • most of the air is removed from the bulb, and then some of the sodium and mercury form a vapour in the bulb
    • an electric current passes through the vapour and excites the atoms
    • when the excited atoms release their energy, you see as a characteristic yellow

 

FLUORESENCE

  • fluorescence is a light that is emitted during exposure of the source to ultraviolet light
  • a fluorescent bulb is an electric discharge tube with an electrode at each end
    • the bulb contains mercury vapour and an inert gas (ex.: argon, a noble gas)
    • because the bulb contains mercury, the fluorescent bulbs need to be disposed of properly (ex: at hazardous waste centres)
  • the inside of the bulb is coated with a powdery substance, phosphor
  • when electrical energy charges the electrodes (at each end), they emit electrons
    • the electrons travel through the gas, from one electrode to the other
    • as the electrons travel through the gas, they collide with atoms of mercury and excite these atoms
    • the excited mercury atoms release their energy in the form of UV light (which human eyes can’t see)
      • the energy of the UV light is absorbed by the phosphor, which emits visible light
  • fluorescent bulbs last much longer than incandescent bulbs
  • fluorescent lighting is more efficient at producing light than incandescent lighting
    • a compact fluorescent bulb  is 20% efficient (only 80% of energy is lost as heat); an incandescent bulb loses 95% of its energy
  • fluorescent materials are found in many places:
    • many body fluids (blood, urine, semen) contain fluorescent molecules—forensic scientists use UB lights at crime scenes to find these fluids
    • the tongue is fluorescent—if UV light is shined in mouth, the dark (unflurorescent) areas are unhealthy tissue
    • fluorescent materials are used in documents such as cash—detectors use UV light to check legal documents, admission tickets, currency, and clothing for legitimacy
    • paint with fluorescent dyes—used in dark theatres with only UV light shining the audience can only see objects with the fluorescent paint

 

LUMINESCENCE

  • the emission of light by a material or object that has not been heated (ex.: fluorescence—uses UV light to excite atoms of the phosphor)
    • opposite of incandescence
  • types of luminescence:
    • fluorescence
    • phosphorescence—light that is emitted due to exposure of the source to UV light, and that continues to be emitted for some time in the absence of the UV light
      • similar to fluorescence, except the excited atoms in the phosphorescent material retain the energy for several minutes (or even hours)
      • the phosphorescent materials glow long after they have absorbed the UV light (ex.: glow in the dark objects)
    • chemiluminescence—light that is produced by a chemical reaction without a rise in temperature
      • the energy of a chemical reaction causes the light to be generated
      • ex.: glow sticks—two different chemicals are in the stick (one is inside and one is outside a glass capsule); when the glow stick bends, the glass capsule breaks and the 2 chemicals combine; dye in the solutions cause the colour of the light (red dye, yellow dye, blue dye etc.)
    • bioluminescence—light that is produced by a biochemical reaction in a living organism
      • chemical reactions in the living cells produce the light
      • common in marine animals (krill, jellyfish, deep-sea starfish, black dragonfish)

 

THE NATURE OF LIGHT

  • light is the only form of energy that can travel like a wave through empty space and through some materials
    • light behaves like a special kind of wave, called an electromagnetic wave
  • electromagnetic waves involve the movement of energy from one point to another
  • a wave length is the distance from one crest (or trough) of a wave to the next crest (or trough)
  • electromagnetic waves are invisible and can travel through a vacuum—they don’t need particles in order to travel
    • they travel through a vacuum, such as space, at the speed of light (3.00 x 108 m/s)

 

THE ELECTROMAGNETIC SPECTRUM

  • EMR
  • is a diagram that illustrates the range (or spectrum) of electromagnetic waves in order of wavelength or frequency
  • waves in order of *largest to shortest wavelength* and *lowest frequency & energy to highest frequency & energy*:
    • radio waves
    • microwaves
    • infrared
    • visible light—ROY G BIV
    • ultraviolet
    • x-Rays
    • gamma rays
  • the colours of light are just different wavelengths of light
    • the colour red has the longest wavelength, while violet has the shortest wavelength of all the visible light

 

Chapter 10.2—Properties of Light and Reflection

  • all light, regardless of its source, behave the same
  • reflection is the change in direction of a light ray when it bounces off a surface

 

RAYS OF LIGHT

  • light travels in a straight line as long as it is moving through the same medium
    • medium is the substance through with light travels
    • this property of light allows you to make predictions about the appearance of objects, and for example, their shadows
  • a technique known as ray tracing is used to make ray diagrams
    • a ray is a straight line with an arrowhead that shows the direction in which light waves are travelling

 

RAY TRACING

  • rays can be used to predict the location, size, and shape of the shadows of objects

 

FERMAT’S PRINCIPLE

  • this principle predicts the path that light will take after reflecting from a surface or passing through more than one medium
  • according to the principle, light follows the path that will take the least time
  • when light reflects from a surface and remains in one medium, its speed is constant; therefore, the path that takes the least time is the shortest path
  • Fermat’s principle leads to the “laws of reflection”

 

LAWS OF REFLECTION

  • the normal is the line that is perpendicular to a surface where a ray of light meets the surface
    • it’s perpendicular to the point of contact of the incident ray (point of incident)
  • an incident ray is a ray of light that travels from a light source toward a surface
    • the angle of incidence is measure between the incident ray and the normal
  • the reflected ray is a ray which begins at the point where the incident ray and the normal meet
    • the angle of reflection is the angle between the reflected ray and the normal in a ray diagram
  • there are 2 laws of reflection:
    • the incident ray, reflected ray, and normal all lie on the same plane(flat surface)
    • the angle of reflection is equal to the angle of incidence
  • the laws of reflection apply to light and all other forms of waves, such as sound waves
  • useful for billiards—angle at which the ball hits the side of the table, is the angle at which it will bounce off the table

 

DRAWING RAY DIAGARAMS

  • there are 5 steps:
    • draw the incident ray
    • at the contact point where the incident ray hits the surface, draw a normal by measuring a 90 degree angle with a protractor
    • measure the angle of incidence between the incident ray and the normal—make a mark to indicate the same angle on the other side of the normal (this is the angle of reflection)
    • draw the reflected ray from the contact point through the mark (made from step above)
    • label the incident ray, the reflected ray, the angle of incidence, the angle of reflection and the normal

 

IMAGES IN PLANE MIRRORS

  • the object placed in front of a mirror is the object; the likeness that is seen in the mirror is the image
  • using the laws of reflection, rays are drawn going from the object, and you can predict where the image will be and what the image will look like
    • you can predict the characteristics of the image
  • a plane mirror is a mirror with a flat, reflective surface
  • the brain assumes that a light ray travels in a straight line
    • to find out where the eyes “see” the image, extend the rays that reach the eye backward until they meet at a point behind the mirror
    • the extended rays are shown by dashed lines
    • the point at which the dashed lines meet is the location of one point on the object
      • by repeating the process for several points on the blueberry, you can find out exactly where the entire image of the blueberry is located

 

VIRTUAL IMAGES

  • a virtual image is an image formed by rays that appear to be coming from a certain position, but are not actually coming from this position
  • virtual image do not form a visible projection on a screen; if the light ray hits the screen and forms an image on the screen, the image is NOT virtual (it is real)
  • in a virtual image, light rays only APPEAR to be coming from an image
  • if the image is behind a mirror, there is no way light rays can get there—they image MUST be virtual

 

RAY DIAGRAMS AND PLANE MIRRORS

  • in general, an IMAGE has 4 characteristics:
    • location (closer than ,farther than, or the same distance as the object in the mirror)
    • orientation (upright or inverted)
    • size (same size, larger than, smaller than the object)
    • type (real or virtual image)
  • the characteristics of an image can be predicted by drawing a ray diagram to locate the image of an object
  • the characteristics of an image in a plane mirror:
    • same size as the object
    • same distance from the mirror as the object
    • the same orientation as the object (not inverted)
    • a virtual image

 

RAY DIAGRAMS FOR LOCATING IMAGE IN A PLANE MIRROR

  • refer to textbook page 416

 

REFLECTION AND STEALTH TECHNOLOGY

  • Radar was invented in 1935, and was used to detect aircraft from the ground during WWII
  • military aircraft (such as stealth fighter) needed to avoid detection
  • two features of the stealth craft made it invisible to radar
    • paint used on the aircraft absorbs much of the energy from the radar waves
      • the base of the paint allows radar waves to penetrate the surface
      • then the radar waves reflect from one particle to the next, losing energy along the way
      • although paint absorbs much of the energy, some radar waves still reflect off the plane
    • the shape of the airplane—all the surfaces of the plane are flat and all the edges are sharp
      • most of the incoming radar rays will not hit perpendicular to these surfaces
      • when the rays reflect from the surfaces of the stealth, most of the reflected rays will not return to the radar antenna
      • if some of the rays do reflect back to the antenna, it will not be a problem because the signal will be so small that the radar operators will think that the aircraft is a small bird

 

Chapter 10.3—Images in Concave Mirrors

  • in a curved mirror, the size of the image is not identical to the size of the object
  • examples: make-up mirrors, car headlights, flashlight (allows light to be a beam—converged)

 

PROPERTIS OF CONCAVE MIRRORS

  • a concave mirror is the inside of a portion of a sphere
  • a concave mirror is a mirror whose reflecting surface curves inward
  • when you look at objects in a concave mirror, such as people, buildings and cars, the images are distorted—the images are even more distorted toward the edges of the mirror

 

DRAWING RAY DIAGRAMS FOR CONCAVE MIRRORS

  • think of a curved mirror as many small, flat mirrors
  • all the normals of the many small flat mirrors meet at a point called the centre of curvature
    • if an incident ray passes through the centre of curvature, the reflected and incident ray are both 0—the incident ray reflects back upon itself, since anything passing through the centre of curvature will hit the normal of the many small mirrors
  • the principal axis on a concave mirror is the line that passes through the centre of curvature, and is normal to the centre of mirror
    • the principal axis allows you to locate the positions of objects that are placed in front of the mirror
  • the point at which the principal axis cuts the centre of the mirror is called the vertex
  • the focal point is the point on the principal axis through which all reflected rays pass when the incident rays are parallel to and near the principal axis
    • the focal length is the distance between the vertex of a mirror and the focal point’ it is half the distance from the vertex to the centre of curvature
  • if the incident ray is near and parallel to the principal axis, it reflects according to the laws of reflection
    • the reflected ray will always pass through the focal point
    • also, the reverse is true—all rays passing through the focal point will be reflected parallel to the principal axis

 

RAY DIAGRAMS FOR CONCAVE MIRRORS

  • when drawing a ray diagram to predict the position of an image, it is helpful to draw the object so that the bottom is on the principal axis
    • since the principal axis is the normal to the mirror, and ray going toward the mirror along the principal axis will reflect back on itself
      • therefore the bottom of the image will always be on the principal axis
      • it makes life easier—you only have to find the top point of the image
  • to locate the image of an object, you only need to draw 3 incident rays (no need to measure angles); you then trace back the reflected rays to locate the image point
    • first ray travels from the top of the object to the mirror, parallel to the principal axis
      • the reflected ray will pass through the focal point
    • second ray from the top of the object through to the focal point
      • the reflected ray will be parallel to the principal axis
    • third ray travels from the centre of curvature to the top point of object, going towards the mirror
      • the reflected ray will travel the same path as the incident ray
  • an object between the focal point and the mirror (for steps, refer to textbook page 422)
    • the image will be virtual—on the other side of the mirror
    • will be bigger than the object
    • image is upright
  • an object between the focal point and the centre of curvature (for steps, refer to textbook page 423)
    • forms a REAL image—the image is formed when the reflected rays actually meet at the point
    • image is inverted
    • image is bigger than object
    • located behind the centre of curvature
  • an object beyond the centre of curvature (for steps, refer to textbook page 424)
    • image is smaller than object
    • image is inverted
    • located between the centre of curvature and the focal point
    • a real image

 

MIRROR AND MAGNIFICATION EQUATIONS

  • the characteristics of an image (size, inverted/upright, location, real/virtual) can be predicted using two equations:
    • the mirror equation allows you to calculate the location of the image
      • f = focus, di = distance between image and mirror, do = distance between the object and mirror
      • if the image is behind the mirror, di would be a negative number
    • the magnification (the change in size of an optically produced image) equation tells you the height of the image relative to the object, using object and image distances; it also allows you to find the magnification from the object and image distances
      • m = magnification; hi = height of image; ho = height of object; di = distance between image and mirror, do = distance between the object and mirror
      • hi is negative if the image is inverted relative to the object

 

DISTORTION OF IMAGES IN CURVED MIRRORS

  • when light rays that are parallel to the principal axis hit a spherical mirror at points that aren’t close to the centre region of the mirror (principal axis), the reflected rays DON’T meet at the same point
    • this means the focal point is spread out over a large area
  • the image becomes spread out
  • spherical aberration—the irregularities in an image in a curved mirror that result when reflected rays from the outer parts of the mirror do not go through the focal point
  • however, concave mirrors in the shape of a parabola eliminates spherical aberration

 

CASE STUDY: SOLAR OVEN

  • solar ovens use heat from the sun (which is free), to cook food
  • solar ovens are used in countries like Somalia and Tanzania, where most people live in just a few hundred dollars a year—not enough money to pay for propane, electricity or kerosene for stove use
    • kerosene and propane fuel also emit dangerous gases
  • solar ovens don’t use electricity the way conventional ovens do
  • solar ovens are used to replace the use of cooking with wood—smoke from wood fires cause respiratory diseases, and lead to deforestation
  • solar cooking requires no fuel, emits no greenhouse gases, and is smoke-free
  • however, the cost to build a solar oven is quite expensive
  • solar ovens work best in hot and sunny areas—these areas are usually home to the very poor (Somalia, Africa etc.)
  • how a solar oven works: the oven is a parabolic surface (no spherical aberration), and the solar radiation reflects and comes together at the focal point and is converted to heat; this heat is used for cooking
    • surface of oven is a shiny metal (ex: aluminum)

 

RADAR TECHNOLOGY AND CONCAVE SURFACES

  • a radar antenna is basically a concave mirror in the shape of a parabola that can send a receive radio waves
  • a radio wave generator and detector are located at the focal point of the antenna—a pulse of radio waves that lasts a few thousandths of a second hits the antenna and is sent out toward the sky
    • for the next few seconds, the antenna acts as a receiver—any returning radio waves that reach the antenna are directed to the detector at the focal point
  • then, another pulse of radio waves are sent out, the process repeating again

 

Chapter 10.4—Images in Convex Mirrors 

  • a convex mirror has reflecting surfaces bulging/curving outward
  • in a convex mirror, the images are smaller than the object, but are always upright
  • the further you get from the centre of the mirror, the more distorted the images become (spherical aberration)
  • examples: mirrors at corners of parking lots, security mirrors in convenience stores, side view mirrors on cars

 

PROPERTIES OF CONVEX MIRRORS

  • a convex mirror is a spherical mirror (just like a concave mirror)
  • however, the reflecting surface is on the outside of the cut out piece of the sphere (not inside)
  • convex mirrors also have spherical aberration—only the small centre region of a convex mirror give images that aren’t distorted

 

RAY DIAGRAMS FOR CONVEX MIRRORS

  • when you shine rays of light parallel to the principal axis onto a convex mirror, the reflected rays travel out and away from each other
  • in a convex mirror, you have to extend the reflected rays backward behind the mirror, until they meet –this means all images from a convex mirror are virtual

 

DRAWING RAY DIAGRAMS

  • refer to textbook pg. 433 for steps

 

MIRROR AND MAGNIFICATION EQUATIONS

  • the same two equations that are used in concave mirrors
  • however, since the focal point is behind the mirror, the focal length for a convex mirror is negative
  • mirror equation:

 

  • magnification equation

 

 

APPLICATIONS OF CONVEX MIRRORS

  • security mirrors in convenience stores—gives the clerk in the store a very large area of store to view (allows you to see almost everything)
    • convex security mirrors are sometimes used on public transportation buses and also on roads with sharp curves in some countries
  • convex mirrors are attached to the end of a long handle at an angle, and are used at border crossings
    • security guards often need to see the underside of large semitrailers and other vehicles
    • by moving the mirror just under the side of the vehicle, the security guard can see everything on the bottom
  • the convex shape of an aerodynamic plane allows radio waves to hit the plane at nearly a perpendicular angle to many surfaces of the plane
    • the rays will be reflected almost directly backward
    • the radar antenna will detect the reflected rays and locate the aircraft

 

Handouts 

  • a luminous object is something that gives off light; a non luminous object is something that doesn’t give off light
  • light emitted from a material because of high temperatures is called incandescence; light emitted from a material without producing heat is called luminescence

 

  • LED (light emitting diode)—electricity passes through in 1 direction
  • fluorescence light—mercury atoms reacts with gas, and phosphorus absorbs UV light and gives off visible light
  • triboluminescence—breaking of object causing one to be positive and one negative, causing electrons to jump
  • electric discharge—electric current passes through vacuum tube containing a gas; this excites the atoms, and the energy is given off as visible light

 

PROPERTIES OF LIGHT

  • light travels in straight lines in the same medium (air, water etc.)
    • light goes straight if it’s in pure air; if it penetrates air and then water, line isn’t straight
  • light is a form of energy (type of electromagnetic radiation)
  • speed of light in a vacuum (empty space) is:            C = 3.0 x 108 m/s
  • light travels in waves
  • white light is made of a spectrum of colours
  • when light hits an object, it can do the following thing(s): reflect off it, be absorbed by it, or be transmitted through it (passed through it)

 

  • little packets of energy are called photons
  • wavelength (lambda λ)-the distance from one place in a wave to the next similar place on the wave (distance between one crest to another)
    • describes the length of one complete oscillation
  • frequency (f)-measure of the number of times an event happens in a period of time; in wave equations, the number of complete oscillations per second
    • t = the time period—the time it takes the energy to travel one complete wavelength
    • measured in Hz (cycles per second)
  • to calculate speed:    v = λ/t                         OR                                        v = λ f     (lambda in meters; frequency in Hz)
    • speed = v
    • d = one wavelength—λ   (m)
    • f = frequency (Hz)
    • t = time or period (s)
    • therefore speed of a wave = λ / t
  • the higher the frequency of the wave, the more energy it carries

 

CURVED MIRRORS

  • concave mirror is “indented”; convex mirror is “protruding”
    • make up mirrors, headlights, flashlights (concave)
    • security mirrors (convex)
  • concave mirrors are converged; concave mirrors are diverged
  • centre of curvature is the centre of the sphere whose surface has been used to make the mirror
    • a line drawn from C to any point on the mirror is the radius , which is 90 degrees to the mirror’s surface
  • principal axis—line through the centre of curvature to the midpoint of the mirror
  • vertex—the point where the principal axis meets the mirror
  • focus—the point at which light rays parallel to principal axis meet when reflected off a concave mirror
  • real image—light rays meet at one point’ virtual image—light rays don’t meet at a point