Chapter 13The Characteristics of light

Objectives

• Identify the components of the electromagnetic spectrum.

• Calculate the frequency or wavelength of electromagnetic radiation.

• Recognize that light has a finite speed .

• Describe how the brightness of a light source is affected by distance.

Chapter 13 Vocabulary• Electromagnetic waves – a wave that consists of oscillating electric and

magnetic fields, which radiate outward from the source at the speed of light.

• Reflection – the change in direction of an electromagnetic wave at a surface that causes it to move away from the surface.

• Angle of incidence – the angle between a ray that strikes a surface and the line perpendicular to that surface at the point of contact.

• Angle of reflection – the angle formed by the line perpendicular to a surface and the direction in which a reflected ray moves.

• Virtual image – an image that forms at a point from which light rays appear to come but do not actually come.

• Concave spherical mirror – a mirror whose reflecting surface is a segment of the inside of a sphere.

• Real image – an image formed when rays of light actually pass through a point on the image.

• Convex spherical mirror – a mirror whose reflecting surface is an outward-curved segment of a sphere.

• Linear polarization – the alignment of EM waves in such a way that the vibrations of electric fields in each of the waves are parallel to each other.

Electromagnetic Waves

• Some light cannot be seen by the human eye.

• This is because light is measured in a variety of forms of radiation.

• These forms are examples of electromagnetic waves.

• They vary depending on frequency and wavelength.

• These frequencies are represented on the electromagnetic spectrum.

The electromagnetic spectrum

All electromagnetic waves move at the speed of light

• All forms of electromagnetic light travels at light speed in a vacuum.

• The current accepted value for light speed is 2.99792458 X 108 m/s (or 3.0 x 108 m/s)

he equation to determine wave speed is

• C=f• Which translates:

• speed of light=frequency X wavelength

Illuminance decreases as the square of the distance from the source

• The rate at which light is emitted from a source is called the luminous flux and is measured in lumens (lm)

• The luminous flux decreases as you move away from the light source.

To recap:

Mirrors (Sections 2-3)

• Reflection – the change in direction of an electromagnetic wave at a surface that causes it to move away from the surface

• Angle of incidence – the angle between a ray that strikes a surface and the line perpendicular to that surface at the point of contact

• Angle of reflection – the angle formed by the line perpendicular to a surface and the direction in which a reflected ray moves

• Virtual image – an image that forms at a point from which light rays appear to come but do not actually come

• Concave spherical mirror – a mirror whose reflecting surface is a segment of the inside of a sphere

• Real image – an image formed when rays of light actually pass through a point on the image

• Convex spherical mirror – a mirror whose reflecting surface is an outward-curved segment of a sphere

Mirrors

• Most of us already have a good understanding of plane mirrors “aka flat mirrors” and reflection with plane mirrors. Remember that the angle of incidence and reflection equate to be the same degree.

Convex Mirrors• Although the angle of incidence and reflection

equate to be the same value in a flat mirror, the angles change in a convex mirror due to the outward sphere. This makes the light rays go out into different directions in an organized manner, thus giving a smaller image, but a wide angle view. This is why optical engineers use convex mirrors when a situation calls for a better view of an area, such as in your passenger side mirrors on your car.

Concave Mirrors• A concave mirror is a little harder to understand.

Imagine how when you look at the inside of a spoon, how your image is upside down. That is because when the light ray hits the inward sphere, the rays are reflected to a focus line, a line parallel to the initial light ray. Once you move past the focal point, the area where the light rays come together, the image flips due to the fact that you are picking up a flipped ray, where the top ray is now on bottom and the bottom ray is now on top.

Chapter 14 - Refraction• Refraction – the bending of a wave front as the wave front passes

between two substances in which the speed of the wave differs.• Index of refraction – the ratio of the speed of light in a vacuum to the

speed of light in a given transparent medium.• Lens – a transparent object that refracts light rays such that they

converge or diverge to create an image.• Total internal reflection – the complete reflection that occurs within a

substance when the angle of incidence of light striking the surface boundary is greater than the critical angle.

• Critical angle – the angle of incidence at which the refracted light makes an angle of 90° with the normal (where refraction stops and reflection begins).

• Dispersion – the process of separating polychromatic light into its component wavelengths

• Chromatic aberration – the focusing of different colors of light at different distances behind a lens.

Refraction

• Refraction is the bending of a wave as it enters a new medium at an angle.

• When a wave enters a medium at an angle, refraction occurs because one side of the wave moves more slowly than the other side.

• Refraction only occurs when the two sides of a wave travel at different speeds.

If light travels from one transparent medium to another If light travels from one transparent medium to another at any angle other than straight on, the light ray changes at any angle other than straight on, the light ray changes

direction when it meets the boundary. In case of direction when it meets the boundary. In case of reflection, the angles of incoming and refracted rays are reflection, the angles of incoming and refracted rays are

measured with respect to the normal. When studying measured with respect to the normal. When studying refraction, the normal line is extended into the refracting refraction, the normal line is extended into the refracting

medium. When light moves from one medium to medium. When light moves from one medium to another, part of it is reflected and part is refracted.another, part of it is reflected and part is refracted.

Index of Refraction

An important property of transparent substances is the An important property of transparent substances is the index of refraction. Index of refraction is the ratio of the index of refraction. Index of refraction is the ratio of the

speed of light in a vacuum to the speed of light in a speed of light in a vacuum to the speed of light in a given transparent medium.given transparent medium.

Index of refraction = Index of refraction = Speed of Light (in vacuum)Speed of Light (in vacuum)

Speed of Light (in medium)Speed of Light (in medium)

Snell’s Law

Snell’s law determines the angle of refractionSnell’s law determines the angle of refractionThe index of refraction of a material can be used to The index of refraction of a material can be used to

figure out how much a ray of light will be refracted as figure out how much a ray of light will be refracted as it passes from one medium to another. The greater it passes from one medium to another. The greater the index of refraction, the more refraction occurs.the index of refraction, the more refraction occurs.The angle of refraction was first found in 1621 by The angle of refraction was first found in 1621 by

Willebrord Snell, who experimented with light Willebrord Snell, who experimented with light passing through different media. He then developed passing through different media. He then developed a relationship called Snell’s law, which can be used a relationship called Snell’s law, which can be used

to find the angle of refraction for light travelling to find the angle of refraction for light travelling between any two mediabetween any two media

Snell’s Law

• Snell’s Law = nSnell’s Law = niisinsinθθi i = n= nrrsinsinθθrr

• (Index of refraction of incident material) (Index of refraction of incident material) times the sin(incident angle in degrees) is times the sin(incident angle in degrees) is equal to (index of refraction of refractive equal to (index of refraction of refractive material) times the sin(refracted angle in material) times the sin(refracted angle in degrees)degrees)

Sample Problem

• A light ray of wavelength 589 nm (produced by a sodium lamp) traveling through air strikes a smooth, flat slab of crown glass at an angle of 30.0° to the normal. Find the angle of refraction, Θr.

• Given:Θi = 30.0ni = 1.00nr = 1.52

• Unknown: Θr

• Solve: nniisinsinθθi i = n= nrrsinsinθθrr

n niisinsinθθii / n / nrr = sin = sinθθr r

θθr r = sin= sin-1-1(n(niisinsinθθii / n / nrr))

θ θr r = sin= sin-1-1[(1.00)(sin30.0°)/1.52] [(1.00)(sin30.0°)/1.52]

θθrr = 19.2° = 19.2°

Lenses

Lenses, in a sense, are the complete opposite of mirrors, they use refraction, which is the bending of light as it goes from one medium to the next, to make a focal point, or spread out a view.

Convex Lenses

• Convex Lenses are the complete opposite, in theory, of convex mirrors. They too have an outward sphere appearance, but instead use refraction to create a focal point, compared to a convex mirror which spreads out the light rays to make a larger image. A good example of this would be positive lens glasses, or close up glasses. They center the light into a common focal point and make things easier to distinguish close up.

Concave Lenses

• Concave Lenses are the complete opposite, in theory, of concave mirrors. They too have an inward sphere appearance, but instead use refraction, again the bending of light as it goes from one medium to the next, to spread out the light rays and create a larger image. A good example of this would be a tube television, the screen is concave, which makes the image spread out throughout the whole room, making things easier to see.

Ch. 15 – Interference & Diffraction

• Coherence – the correlation between the phases of two or more waves• Path difference – the difference in the distance traveled by two beams

when they are scattered in the same direction from different points• Order number – the number assigned to interference fringes with respect

to the central bright fringe• Diffraction – a change in the direction of a wave when the wave

encounters an obstacle, an opening, or an edge• Resolving power – the ability of an optical instruments to form separate

images of two objects that are close together• Laser – a device that produces coherent light at a single wavelength

Interference

• There are two types of wave interference: constructive and destructive– In constructive interference, waves combine to form a resultant

wave with the same wavelength but a greater amplitude than either of the original waves.

– In destructive interference, waves combine to form a resultant wave whose resulting amplitude is smaller than the original and whose wavelength is not the same as it was

• There are formulas for calculating constructive and destructive interference: d sin Θ = ±mλ (constructive) and d sin Θ = ±(m+½)λ (destructive).

Diffraction

• Diffraction is a change in the direction of a wave when the wave encounters an obstacle, an opening, or an edge.

• A wave diffracts more if its wavelength is large compared to the size of an opening or obstacle.