Electromagnetic Radiation
Topic: Electromagnetic Radiation and Review Lecture For Class Test
Subject Code: PHY4201
Lecture No: 3
Teacher Name: Md. Rezaul Karim
Electromagnetic Radiation
Electromagnetic radiation is one of the many ways that energy travels through space. The heat from a burning fire, the light from the sun, the X-rays used by your doctor, as well as the energy used to cook food in a microwave are all forms of electromagnetic radiation. While these forms of energy might seem quite different from one another, they are related in that they all exhibit wavelike properties.
Electromagnetic radiation is one of the many ways that energy travels through space. The heat from a burning fire, the light from the sun, the X-rays used by your doctor, as well as the energy used to cook food in a microwave are all forms of electromagnetic radiation. While these forms of energy might seem quite different from one another, they are related in that they all exhibit wavelike properties.
Visible Light is an electromagnetic radiation that can be detected by the human eyes. When light rays go straight into our eyes, we can see the object
Electromagnetic waves consist of 2waves oscillating perpendicular to one another. One of the waves is an oscillating magnetic field; the other is an oscillating electric field. This can be visualized as follows:
Electromagnetic waves consist of 2waves oscillating perpendicular to one another. One of the waves is an oscillating magnetic field; the other is an oscillating electric field. This can be visualized as follows:
Basic Properties of Light Wave
• Amplitude (a): A wave has a trough (lowest point) and a crest (highest point).The vertical distance between the tip of a crest and the wave’s central axis is known as its amplitude. This is the property associated with the brightness, or intensity, of the wave.
• Wavelength (λ): The horizontal distance between two consecutive troughs or crests is known as the wavelength of the wave.
• Frequency (f) refers to the number of full wavelengths that pass by a given point in space every second; the SI unit for frequency is Hertz (Hz)
• Period (T): It is the length of time takes for one wavelength to pass by a given point in space.Mathematically, the period (T) is simply the reciprocal of the wave’s frequency (f). The unit of Period is Second.
• Amplitude (a): A wave has a trough (lowest point) and a crest (highest point).The vertical distance between the tip of a crest and the wave’s central axis is known as its amplitude. This is the property associated with the brightness, or intensity, of the wave.
• Wavelength (λ): The horizontal distance between two consecutive troughs or crests is known as the wavelength of the wave.
• Frequency (f) refers to the number of full wavelengths that pass by a given point in space every second; the SI unit for frequency is Hertz (Hz)
• Period (T): It is the length of time takes for one wavelength to pass by a given point in space.Mathematically, the period (T) is simply the reciprocal of the wave’s frequency (f). The unit of Period is Second.
Relationship between wavelength and frequency: wavelength and frequency are inversely proportional: that is, the
shorter the wavelength, the higher the frequency, and vice versa. This relationship is given by the following equation:C=f λ
Where λ (the Greek lambda) is the wavelength (in meters, m and ν (the Greek nu) is the frequency (in Hertz, Hz). Their product is the constant c, the speed of light, which is equal to 3.00×108m/s, this relationship reflects an important fact: all electromagnetic radiation, regardless of wavelength or frequency, travels at the speed of light.
⇒ Relation between Period and frequency: T=1/f
shorter the wavelength, the higher the frequency, and vice versa. This relationship is given by the following equation:C=f λ
Where λ (the Greek lambda) is the wavelength (in meters, m and ν (the Greek nu) is the frequency (in Hertz, Hz). Their product is the constant c, the speed of light, which is equal to 3.00×108m/s, this relationship reflects an important fact: all electromagnetic radiation, regardless of wavelength or frequency, travels at the speed of light.
⇒ Relation between Period and frequency: T=1/f
Example 1: A particular wave of electromagnetic radiation has a frequency of 2Х1014 Hz. What is the wavelength of
this wave?
this wave?
Photon Energy
Planck’s discoveries paved the way for the discovery of the photon. A photon is the elementary particle, or quantum, of light. Photons can be absorbed or emitted by atoms and molecules. When a photon is absorbed, its energy is transferred to that atom or molecule. Because energy is quantized, the photon’s entire energy is transferred. The reverse of this process is also true. When an atom or molecule loses energy, it emits a photon that carries an energy exactly equal to the loss in energy of the atom or molecule. This change in energy is directly proportional to the frequency of photon emitted or absorbed. This relationship is given by Planck’s famous equation: E=hν
Where E is the energy of the photon (given in Joules, J), ν is frequency of the photon (given in Hertz, Hz), and h is Planck’s constant, 6.626×10−34 J⋅s
Example 2: A photon has a frequency of 2.0×1024 Hz. What is the energy of this photon?
Electromagnetic Spectrum: Electromagnetic waves can be classified and arranged according to their various
wavelengths/frequencies; this classification is known as the electromagnetic spectrum. The following table shows us this spectrum, which consists of all the types of electromagnetic radiation that exist in our universe.
Planck’s discoveries paved the way for the discovery of the photon. A photon is the elementary particle, or quantum, of light. Photons can be absorbed or emitted by atoms and molecules. When a photon is absorbed, its energy is transferred to that atom or molecule. Because energy is quantized, the photon’s entire energy is transferred. The reverse of this process is also true. When an atom or molecule loses energy, it emits a photon that carries an energy exactly equal to the loss in energy of the atom or molecule. This change in energy is directly proportional to the frequency of photon emitted or absorbed. This relationship is given by Planck’s famous equation: E=hν
Where E is the energy of the photon (given in Joules, J), ν is frequency of the photon (given in Hertz, Hz), and h is Planck’s constant, 6.626×10−34 J⋅s
Example 2: A photon has a frequency of 2.0×1024 Hz. What is the energy of this photon?
Electromagnetic Spectrum: Electromagnetic waves can be classified and arranged according to their various
wavelengths/frequencies; this classification is known as the electromagnetic spectrum. The following table shows us this spectrum, which consists of all the types of electromagnetic radiation that exist in our universe.
As we can see, the visible spectrum—that is, light that we can see with our eyes—makes up only a small fraction of the different types of radiation that exist. To the right of the visible spectrum, we find the types of energy that are lower in frequency (and thus longer in wavelength) than visible light. These types of energy include infrared (IR) rays (heat waves given off by thermal bodies), microwaves, and radio waves. These types of radiation surround us constantly, and are not harmful, because their frequencies are so low. As we have seen in the section, “photon energy,” lower frequency waves are lower in energy, and thus are not dangerous to our health. To the left of the visible spectrum, we have ultraviolet (UV) rays, X-rays, and gamma rays. These types of radiation are harmful to living organisms, due to their extremely high frequencies (and thus, high energies). It is for this reason that we wear suntan lotion at the beach (to block the UV rays from the sun) and why an X-ray technician will place a lead shield over us, in order to prevent the X-rays from penetrating anything other than the area of our body being imaged.
Gamma rays, being the highest in frequency and energy, are the most damaging. Luckily though, our atmosphere absorbs gamma rays from outer space, thereby protecting us from harm.
Gamma rays, being the highest in frequency and energy, are the most damaging. Luckily though, our atmosphere absorbs gamma rays from outer space, thereby protecting us from harm.
Reflection, Refraction and Dispersion
Reflection: When light travelling in a medium encounters a boundary leading to a second medium, part of the light is returned to the first medium from which it came. This phenomenon is called Reflection
Regular Reflection: Reflection of light from smooth surface is called regular or specular reflection
Diffuse Reflection: Reflection of light from rough surface is called diffuse reflection
Laws of Reflection
First Law: The incident ray, the reflected ray and the normal of incidence are in the same plane.
First Law: The incident ray, the reflected ray and the normal of incidence are in the same plane.
Second Law: The angle of reflection is equal to the angle of incidence.
Refraction: When a ray of light travelling through a transparent medium encounters a boundary leading into another transparent medium, part of the ray enters the second medium, the phenomenon of changing the direction of the light ray is known as Refraction of light
Refraction: When a ray of light travelling through a transparent medium encounters a boundary leading into another transparent medium, part of the ray enters the second medium, the phenomenon of changing the direction of the light ray is known as Refraction of light
Angle of Refraction: The angle subtended by the normal and the refracted ray is the angle of refraction
Laws of Refraction
First Law: The incident ray, the refracted ray and the normal at the point of incidence lie in the same plane.
Second Law: The ratio of the sine of the angle of incidence to the sine of the angle of refraction for any two given media is constant. (Snell’s law)
First Law: The incident ray, the refracted ray and the normal at the point of incidence lie in the same plane.
Second Law: The ratio of the sine of the angle of incidence to the sine of the angle of refraction for any two given media is constant. (Snell’s law)
Refractive Index: It is defined as the ratio of velocity of light in a vacuum to the velocity of light in the medium (μ=𝐶/𝜈)
• A medium with a relatively high refractive index is said to have a high optical density
• The refractive index depends not only on the substance but also on the wavelength of the light
• The refractive index depends not only on the substance but also on the wavelength of the light
Dispersion: When a ray of white light falls on a glass prism, it splits up into different colors. This display of colors is known as spectrum of the source of light. This separation of a composite beam into its constituent colors is called dispersion.
Lenses
A lens is an image-forming device. It forms an image by refraction of light at its two bounding surface. In general, lens is made of glass.
Types of Lens:
1. Convex Lens: A convex lens is a converging lens since a parallel beam of light, after refraction, converges to
a point, F
1. Convex Lens: A convex lens is a converging lens since a parallel beam of light, after refraction, converges to
a point, F
2. Concave Lens: A concave lens is a called a diverging lens since ray coming parallel to the principal axis, after refraction, diverge out and seem to come from a point, F.
• A convex lens is thicker at the center than at the edge while a concave lens is thinner at the center than at
the edges
the edges
Terminology of Lens
• A lens has two curved surfaces, each surface having a curvature
• The length of radius of curvature of surface is called the radius of curvature, R
• The line joining the centers of curvature of the two curved surfaces is called the principal axis or simply axis of lens.
• The point F to which set of rays parallel to the principal axis is caused to converge (in case of convex lens) or appear to diverge (in case of concave lens) is the principal focus.
• For every lens, there is a point on the principal axis for which the rays passing through it are not deviated by the lens. Any ray passing through it emerges in a direction parallel to the incident ray. Such a point is called optical centre.
• The distance between the focal point F and the optical center of the lens is called the focal length of the lens.
• The power of a lens is the reciprocal of its focal length (P=1/f)
• A lens has two curved surfaces, each surface having a curvature
• The length of radius of curvature of surface is called the radius of curvature, R
• The line joining the centers of curvature of the two curved surfaces is called the principal axis or simply axis of lens.
• The point F to which set of rays parallel to the principal axis is caused to converge (in case of convex lens) or appear to diverge (in case of concave lens) is the principal focus.
• For every lens, there is a point on the principal axis for which the rays passing through it are not deviated by the lens. Any ray passing through it emerges in a direction parallel to the incident ray. Such a point is called optical centre.
• The distance between the focal point F and the optical center of the lens is called the focal length of the lens.
• The power of a lens is the reciprocal of its focal length (P=1/f)
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