Electromagnetism

Electromagnetic Waves

• We are surrounded by electromagnetic waves.

• Electromagnetic wave is produced by the vibration of electric and magnetic fields.

• Electromagnetic waves are time varying electric and magnetic fields that are coupled to each other.

• Electromagnetic waves are waves that travel through empty space or through insulating materials.

• Electromagnetic waves are generated whenever electric charges are accelerated.

• The frequency of the waves created equals the frequency of the alternating current.

• Electromagnetic waves are emitted by accelerating electric charges.

• Electromagnetic waves are traveling as electrical and magnetic transverse waves.

  

Light Waves

• Light is the most evident example of electromagnetic waves.

• Light is a form of electromagnetic radiation.

• Light waves are produced by the vibration of electric and magnetic fields.

James Clerk Maxwell

• 1831-1879

• Scottish Physicist

• Stated that electric current is capable of radiating energy in the form of waves known as electromagnetic waves.

• It is due to his interest on the works of Coulomb, Oersted, Ampere and Faraday on the relationship between electricity and magnetism, he was able to make a theory.

• He formulated a mathematical theory known as Maxwell’s electromagnetic equations.

• Maxwell’s electromagnetic equation states that an oscillating electric current should be capable of radiating energy in the form of waves.

• It is known that electromagnetic waves could travel as the speed of light.

Heinrich Hertz

• 1857-1894

• German Physicist

• Proved the existence of electromagnetic waves through radio waves.

• Heinrich Hertz discovered Hertzian Waves also known as Radio Waves.

• Hertz generated electromagnetic waves by using two circuits generated by A and detected by B. Each circuit has shiny metal balls at each end with very small air gap for a spark to occur each time the electromotive force (emf) reached a peak.

• It shows that electromagnetic waves from A traveled the space between circuit and the receiving loop.

How Electromagnetic Waves are Created?

• Waves are produced by using coil and a capacitor in a parallel circuit.

• The light emitted by a light bulb is caused by thermal motion that accelerates the electrons in the hot filament and eventually produce visible light.

Properties of Electromagnetic Waves

Transverse waves are types of waves that do not need a medium to transfer energy.

The electromagnetic spectrum which is consists of complete range of electromagnetic waves share common properties, such as:

• They exhibit reflection, refraction, diffraction and interference.

• They travel at the speed of light. (3.0 × 10^8 m/s)

• They obey the wave relation. (v=yf)

Each type of electromagnetic wave occupies a particular range of wavelengths we called as a band.

Each type of EM waves come from different sources and has different uses and effects.

Radio Waves

• Radio waves have the longest wavelengths and the lowest frequencies in the electromagnetic spectrum.

• Radio waves are produced by making electricity oscillate in an aerial or antenna.

• Lightning and stars also give off radio waves.

Uses:

• They are used to transmit sound and picture information over long distances.

• Radio waves are used in radio devices that we listen to, such as AM and FM stations, which have very long wavelengths.

• Due to long wavelengths, it is best suited for long distance broadcasts.

Radio waves extend over a very wide range of wavelengths and the frequencies of the different wave bands are the following:

• Very high frequency (VHF) radio waves – this is usually used in communication in civilian aircraft.

• Ultra-high frequency (UHF) radio waves – which has greater frequency compared to VHF. These are commonly used in radio communications and TV transmission.

Danger:

• Large doses are believed to cause cancers, leukemia, and other diseases.

Microwave

• They are described as radio waves of very short wavelength.

• Compared to radio waves, microwaves have shorter wavelengths and higher frequencies.

• Most of the newest technology relies on the utilization of microwaves.

• Due to reflected property of the microwave, they are best used in radar (radio detecting and ranging) .

• This involves the transmission of waves at microwave frequencies and then detecting its echo after reflection from a distant objects.

Uses:

• They are commonly used in satellite communication, early warning radar, missile guidance systems, weather monitoring, mobile phone networks and even for cooking.

• Microwave are used in satellite communication as they can penetrate the ionosphere (a layer in earth’s atmosphere which has a high concentration of charged particles.)

• Microwave are used in mobile phone networks. They operates within the lower end of the microwave range.

• Microwaves can also be used in cooking. In microwave oven, the magnetron of oven generates microwaves that penetrate food being heated and agitate the water molecules within. The movement of the molecules produces heat, hence cooking the food.

• Microwaves are also used by police to track motorists who are driving faster than the speed limit.

Danger:

• Prolonged exposure to microwaves is known to cause "cataracts“ in your eyes and recent research indicates that microwaves from mobile phones can affect parts of your brain

Infrared

• These are waves that lie in the region beyond the red end of the visible spectrum.

• The word “infrared” suggests that this spectrum of wave is “below” to red, which in this case means that the energy and frequency of this wave is lower than red light.

• As to describe the wavelength of these infrared, they are too long to be visible to the naked eye.

• Infrared radiation is given off by hot or warm objects.

Uses:

• Infrared is used to take pictures from satellite with special films to assess the vegetation of the earth’s surface.

• They can also be used to take images used by biologists to track wildlife.

• Scientists also utilized infrared detectors which help in monitoring volcanoes and hotspots.

• They can be used to measure the temperature which give rise to infrared thermometers and scanners which can gets an instant reading of temperature and to show temperature variation of the body for medical diagnosis.

• They are utilized in the aspects of electronic communications. For example, in remote controllers that can send instructions and/or communicate with the television through an infrared beam.

• They are also used in circuit switching. For example, a faucet that turns water on whenever it detects a person’s hand; when the hand is not there anymore, the circuit will turn the water off.

Visible Light

• This is the only part of the electromagnetic spectrum that is visible to human.

• Also known as light wave makes up only a small portion of the entire electromagnetic spectrum.

This light wave appears as white light, and when this white light passes through a prism, it is separated into its constituent colors: ROYGBIV.

• Red, Orange, Yellow, Green, Blue, Indigo, Violet

• 700, 600, 580, 550, 475, 450, 400

• Violet has the shortest wavelength and Red has the longest wavelength.

  

Ultraviolet Waves

• These waves are invisible radiation that lie beyond the violet end of the visible spectrum.

• The name ultraviolet suggests that its frequency and energy is greater compared to violet light.

• Ultraviolet radiation is produced by high-temperature surface such as the sun.

• They can also be produced by electric arcs and specialized lights, such as mercury-vapor lamps, tanning lamps, and black lights.

• Ultraviolet radiation from the sun is classified according to their frequency: UVA, UVB, and UVC.

Uses:

• Production of Vitamin D by the inner layers of the skin.

• Sterilizing water in drinking fountains.

• To prevent counterfeiters. (Through ultraviolet lamps, they can identify fake bank notes and check sensitive documents such as credit cards, passbook, passport, or anything that has UV watermarks.)

• For detectives / crime scenes. (Ultraviolet radiation (UVA) makes certain pigments fluorescence or emit light from within. Detectives and forensics use a UV light source called “black light” on crime scene. Through this black light, it helps reveal blood and other biological fluids and footprints.)

• For entertainment. (Black lights can also be used in entertainment. Some paints that contain fluorescent chemicals or objects sprayed by fluorescent powder may glow in a dark room illuminated only by black light.)

Danger:

• Can damage the retina in your eyes and cause sunburn and even skin cancer.

X-ray

• They are described to have short wavelengths and high frequencies and have a very high penetrating power.

• X-ray are produced and emitted by fast-moving electrons that strike a heavy atom.

Uses:

• Medical Applications. (X-ray with long wavelengths are used extensively in medical applications as these waves can penetrate through the human body. The high energy of X-ray allows them to go through skin and muscle and helps doctors to look inside the body.

• Security. (Another application is in the security aspects. With the use of x-ray machine, it helps scan baggage found at airport terminals.)

• Industry. (Meanwhile, X-ray with short wavelengths are used in industry. These waves can penetrate through metals and can help inspecting welds and manufactured parts.)

Danger:

• Can cause cell damage and cancers

Gamma Waves

• Gamma rays are high-energy waves generated by radioactive atoms and in nuclear explosions.

• They are more penetrating and have shorter wavelengths compared to X-rays.

Uses:

• Sterilizing medical equipment and some applications in astronomy.

• Treat Cancers. (Multiple concentrated beams of gamma rays are directed on the growth in order to kill cancer cells.)

• Radiotherapy

Danger:

• Can cause cancer and mutation

Reflection on Mirrors

Previous Lesson:

• Light is an electromagnetic radiation that has properties of waves and particles.

• Light follows a precise paths of straight lines and angles.

• These light waves can be directed and altered by mirrors and lenses.

• As a wave, there are many wonderful phenomena associated with light such as twinkling of stars, the picturesque colors of a rainbow, bending of light by a medium, and image formation by mirrors.

POWERPOINT

Two of the properties of light are reflection and refraction; these are most easily illustrated using mirrors and lenses.

How does a ray of light behave when it hits a surface?

When a light ray strikes a surface, it is partly absorbed. The portion of light that is reflected may create an image or may just illuminate the surface, depending on the surface.

Reflection

Reflection of light obeys two basic law:

1. The normal line, incident ray, and reflected ray all lie on the same plane.

2. The angle of incidence is equal to the angle of reflection.

Diffuse Reflection:

• Light that is reflected by a rough-textures or uneven surface such as wall, paper and cloth and that scattered in many directions.

Specular Reflection:

• Light that strikes a smooth, flat and shiny surface such as mirror, a piece of metal or undisturbed water, and id reflected in one direction.

Mirrors

• Smooth reflecting surfaces, usually made up of polished metal or glass that has been coated with some metallic substance.

• Can be plane (flat) mirror and curved (spherical) mirror

Plane Mirror

• Image formed by plane mirror can be located and characterized by using ray diagrams.

Plane mirror images all possess the following characteristics:

1. They are located behind the mirror. And such image is described as a virtual image.

2. They are uptight, or erect.

3. They are unmagnified.

4. The image is left-right reversal relative to the object.

Curved Mirror

• A spherical mirror is a curved mirror that has a curvature of a sphere.

• The front and the back of the spoon work like two curved mirror.

• The front is the concave mirror and the back is the convex mirror.

Concave Mirror

• Convergent Mirror

• A concave mirror turns parallel rays of light into convergent rays.

• Concave mirror can form either virtual or real images.

Convergent Mirror

• They are used as magnifying mirrors for shaving and applying makeup, and are also used in reflecting telescopes.

• These are used to make a beam of light in flashlights and car headlights.

Real images are those created when light rays ‘actually’ converge or meet at a point. In front of mirror.

Virtual images are those formed at a location where light rays reflected from a mirror ‘seem’ to diverge. In back of mirror

DLA

Geometric Optics

• When the light from the flashlight or laser hits a surface such as wall, ceiling or

  paper, it bounces off and displays a dot of light on the surface; and what really happens in this case is what we called as diffuse reflection.

• If you direct the light on a smooth, polished surface such as mirror or surface of calm water, you can see an image somewhere else. This is what you called as specular reflection.

Reflection

This orderly reflection of light rays makes the formation of images possible

There are two kinds of reflection, namely diffuse reflection and specular reflection.

Diffuse Reflection

• Happens when light rays strike a dull or rough surface such as paper and clothing which can result to bouncing back of light in different direction.

Specular Reflection

• Occurs when light rays hit a reflective surface and can make light rays reflected in the same direction and/or orderly manner.

Compared to diffuse reflection, specular reflection is more predictable as it obeys the laws of reflection.

Take note that these laws of reflection are applicable to all types of reflecting surfaces such as plane and spherical mirrors.

1. The incident ray, the normal line, and reflected ray all lie on the same plane.

2. The angle of incidence is equal to the angle of reflection.

Mirrors

• Mirrors are smooth reflecting surfaces, usually made up of polished metal or glass that has been coated with some metallic substance.

• They can be flat or curved.

Plane (flat) Mirrors

• The most common type of mirrors.

• When you look directly into a plane mirror, you will see your reflected images and the things around you.

• “Image is always the same distance behind the mirror as the object is in Infront of the mirror.” (Ortiz-Andaya, et.al, 2020)

How do we characterized the image formed on plane mirror?

• The laws of reflection make it possible to draw all types of rays on the same plane and allow us to locate the image.

Virtual Image

• Virtual image is the image you see that is upright and seems to be located behind the mirror.

• Virtual image is also any image formed by extended (or imaginary) rays of light.

• Images are upright, or erect.

• The images are left-right reversed or perverted.

• The images are unmagnified.

Curved Mirrors

• Curved mirrors can produce and show variety of images.

• The front and the back of the spoon would work like two curved mirrors: The front is a concave mirror, and the back is the convex mirror.

• Curved mirror is usually spherical mirror.

Concave Mirror

• Concave mirrors are mirrors with a surface that curves inward.

• These concave mirrors are also known as convergent mirrors as they usually turn parallel rays of light into convergent rays.

Uses: Concave mirrors are commonly used as magnifying mirrors for shaving and applying makeup and in reflecting telescopes. These are also used to make a beam of light in car headlights and flashlights.

How are image formed in concave mirror?

The image forms when the parallel rays of light that hit a concave mirror are reflected inward and meet at one point. Or simply, the image is formed whenever the reflected rays of light intersect.

What are the characteristics of the image formed by concave mirror?

• Concave mirrors can form either virtual or real images.

• If the object is placed beyond the focal point, concave mirror can produce real images. However, if the object is placed between the vertex V and focal point F, concave mirror can produce virtual image.

• Concave mirrors can produce magnified, demagnified or unmagnified image of the object.

• They can also produce either upright or inverted image.

Convex Mirror

• Convex mirrors are mirrors with a surface that curves outward.

• When the parallel rays of light hit a convex mirror, the rays are reflected outwards.

• Those reflected rays appear to spread out from a point (principal focus) behind the mirror.

• You will see that the rays of light usually diverge from this type of curved mirror, that is why convex mirrors are called as diverging mirrors.

Uses: Convex mirrors are commonly used in cars as passenger-side rearview mirrors.

Take note that regardless of the position of the object from the convex mirror and since the rays do not actually meet, the images formed by convex mirrors are:

• always virtual, upright and demagnified.

Ray Diagram

To be able to describe the location, size, orientation, and type of image formed by concave mirror, the technique known as ray diagramming is still used.

• A ray diagram is a graphical representation of the possible paths light can take to get from one place to another.

• This is often from a source or object to an observer or screen. (isaacphysic.com)

• Simply, this diagram is a sketch of the incident rays and reflected rays and can be used to determine the nature of the image and where the image will form.

• In this diagram, light rays are usually represented by an arrow to indicate the direction or the path of light rays.

Labels

Curved mirror has a vertex V, center of curvature C, principal focus or focal point F.

To understand the process of reflecting light and forming images, you have to be more familiar with the given terms.

Principal axis is a horizontal line that serves as our main reference line. Take note that we should measure all the distances we need in this line as we add the following points such as center of curvature C, principal focus or focal point F and vertex V on it.

C is the center of the sphere from which the mirror is formed.

F is the focus or focal point.

V is the vertex, a point at the center of the mirror. This is also where the mirror intersects the principal axis.

The given points above define the following distances:

R is the radius of curvature; it is the distance between the center of curvature and the vertex.

f is the focal length; it is the distance between the focus and the vertex.

Ray Diagram

• Gives us a good graphic of image formation.

• Principal ray diagram provides us a good representation of image formation.

• Through ray diagramming, it helps us identify and describe the characteristics and the nature of the image formed by both plane and curved mirrors.

• A more accurate way to find and describe the image is by calculating the image distance and lateral magnification.

Mirror Equation

Primarily relates the focal length f of the mirror to the object distance and image distance.

• f = focal length

• do= object distance

• di = image distance

Take note that the object and image distance are always measured from the vertex. And in using this equation, you should use a sign convention.

• If the distance measured is in front of the mirror, then it has positive sign while if the distance measured is behind the mirror, then it is negative.

• Distances measured in front of the mirror are positive while those behind the mirror are negative.

Lateral Magnification

Mathematically compare the image size to the object size. (Baguio, 2020)

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