Light and Reflection (1)

8/3/2019 Light and Reflection (1)

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Physics B

Light and Reflection


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Characteristics of Light*Electromagnetic waves.

- Light can be separated into six elementary colors ofthe spectrum: red, orange, yellow, green, blue and

violet.

Some light invisible to human eye, has a widerspectrum such as X rays, microwaves and radio waves.They have the same properties as light. Theyre called

electromagnetic waves.


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oLight is a wave composed of oscillating electric and magneticfields. Electromagnetic waves are transverse wavescomposed of electric and magnetic fields are at right angleto each other, and perpendicular to the direction of wave.

oElectromagnetic waves differs in frequencies and

wavelengths which builds up the different colors of the light.It also distinguish visible light from the invisible ones such asX rays.

oAll electromagnetic waves move at the speed of light

oIlluminance as the square of the distance from the source. Asthe distance from the source increase


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*Reflection of light

o Change in direction of light is called reflection.

o All substances absorb some incoming light and reflect the rest.

oThe texture of the surface affect how its reflected. Light reflected fromsmooth shiny surface such as mirrors or water is reflected in one direction

specular reflection). Light that is reflected from a rough texture surface

such as paper or unpolished wood is reflected in many directions (diffuse

reflection).

oIncoming and reflected angles are equal. The angle of reflection and the

angle of incidence are symmetric compared to the normal.


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*Flat Mirrors

o The simplest mirror is the flat mirroroThe image formed by rays that appear to come from

the image point behind the mirror is called virtual

image

oImage location can be predicted with ray diagrams


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*

* Concave Spherical Mirror

o Concave mirrors can be used to form real images

oA spherical mirror with light reflecting from its silvered, concave surface iscalled concave spherical mirror.

oConcave mirrors are used whenever a magnified image of an object is needed asin the case of dressing-table.

oA real image is an image formed when rays of light actually pass through a pointon the image.

o Image created by spherical mirrors suffer from spherical aberration (some raysdo not intersect at the exact point).

o Image location can be predicted with the mirror equation : 1/p + 1/q = 2/R

o Ray diagrams can be used for concave spherical mirrors

oConcave mirrors can produce both real and virtual images


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*Convex Spherical Mirror

o

A convex spherical mirror is a mirror whose reflectingsurface is an outward-curved segment of a sphere

*Parabolic Mirrors

oParabolic mirrors eliminate spherical aberration, whichoccurs when parallel rays converge away from the mirror

focal point.oReflecting telescopes use parabolic mirrors


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*

* reflecting surface that bulges inward (away from the incident light).

* show different image types depending on the distance between theobject and the mirror.


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Object's position (S),focal point (F)

Image Diagram

S < F(Object between focal point and mirror)

Virtual

Up right

Magnified (larger)

S = F

(Object at focal point)Reflected rays are parallel and never meet, so no

image is formed.

In the limit where S approaches F, the image distance

approaches infinity, and the image can be either real

or virtual and either upright or inverted depending on

whether S approaches F from above or below.

F< S < 2F(Object between focus and centre of curvature)

Real

Inverted (vertically)

Magnified (larger

S = 2F

(Object at centre of curvature)

Real


Same size

Image formed at centre of curvature

S > 2F

(Object beyond centre of curvature)

Real


Reduced (diminished/smaller)

As the distance of the object increases, the

imageasymptotically approaches the focal point

In the limit where S approaches infinity, the image size

approaches zero as the image approaches F


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*Mirror Equation : 1/p + 1/q = 1/f

*1/object distance + 1/image distance = 1/focal lenght

*Equation for Magnification = image height/object height = -image distance/object distance

*upright -> + -> virtual

*Inverted -> - -> real


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Reflection


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**The bending of light as it travels from one medium to another is

called refraction.

*The angle between the refracted ray and the normal is called

angle of refraction, there is also the angle of incidence.

*Refraction occurs when lights velocity changes.


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*Angle is bent toward the normal when light

travels from air to glass, air to water and waterto glass.

*Angle is bent away from the normal when light

travels from glass to air, water to air and glass

to water.

*Refraction also occurs with waves.


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* index of refraction = speed of light in vacuum/speed of light in medium

* Index of refraction is always greater than one because light travelsslower in substance than in a vacuum

*The larger the index of refraction is, the slower light travels in thatsubstance and the more a light ray will bend when it passes from avacuum into that material.

*Object appears to be in different positions due to refraction.

*Wavelenght affects the index of refraction.

*Snells law determines the index of refraction.


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Curved surfaces change the direction of light. When rays of lightpass through:

- A converging lens (thicker at the middle), they are bent inward.

- A diverging lens (thicker at the edge), they are bent outward.

Focal length is the image distance for an infinite object distance.Both lens have two focal points but only one focal length.


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*Ray diagrams of thin-lens systems help identify image

height and location.

*Rules for drawing reference rays:

Ray From objectto lens

Fromconverginglens toimage

Fromdiverginglens toimage

Parallel ray Parallel toprincipal

axis

Passingthrough

focal point,

F.

Directedaway from

local point,

F.

Central ray To the

center of

the lens

From the

center of

the lens.

From the

center of

the lens.

Focal ray Passes

through

focal point,

F.

Parallel to

principal

axis

Parallel to

principal

axis.


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Converging lenses can produce real or virtual images of realobjects.

- As the distant object approaches the focal point the imagebecome larger and farther away.

- When the object is at the focal point the light rays fromthe object are refracted so that they exit the lens parallelto each other

- When the object is between a converging lens and its focalpoint, the light rays from the object diverge when theypass through the lens.

- The image created by a diverging lens is always virtual,smaller images.


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The thin-lens equation and magnification.

- Applies when the lense thickness is much smaller than its focal

lenght.- 1/object distance + 1/distance from image to lense = 1/focal lenght

- Applies to converging and diverging lenses

Magnification by lens depends on object and image distances.- Applies to converging and diverging lens

- magnification = image height/object height = - (distance from image

to lens/ distance from object to lens)

- Magnitude of magnification < 1, then image > object- If negative the image is real and inverted.

- If positive image is upright and virtual.


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Eyeglasses and contact lenses

- The cornea acts like a lens, directing light rays toward the

light-sensitive retina in the back of the eye.

- A contact lens is a lens worn over the cornea of the eye.


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Combination of Thin Lenses

The system can be treated in the following manner:

-The first image of the first lens is calculated. Then the lightapproaches the second lens as if it had come from the image

formed by the first lens.

- The image formed by the first lens is considered as the object

for the second lens.

- The image formed by the second lens is the final image of thesystem.

Greater magnification can be achieved by combining two lenses

in a device called compound microscope.

- It consists of two lenses : an objective lens near the objectwith a focal lenght of less than 1cm and an eyepiece with a

focal lenght of a few centimeters.

- The image viewed through a microscope is upside-down with

respect to the actual orientation of the specimen.


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Refracting telescopes also use two converging lenses

- A small inverted image is formed at the focal point of the

objective lens, Fo, because the object is essentially at infinity.

- The eyepiece is positioned so that its focal point lies very close

to the focal of the objective lens, where the images is formed.

- The eyepiece acts like a simple magnifier and allows the viewer

to examine the object in detail.


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*Total Internal Reflection

Total Internal Reflection is the complete reflection of light at the boundary of two

transparent media; this effect occurs when the angle of incidence exceeds thecritical angle.

Critical angle is the angles of incidence at which the refracted light makes an angleof 90degrees with the normal.

Dispersion

Dispersion is the process of separating polychromatic light into its componentwavelength

White light passed through a prism produces a visible spectrum.


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Rainbows are created by dispersion of light in water droplets.

How does it work ?

Sunlight is spread into a spectrum upon entering a sphericalraidrop, then internally reflected on the back side of the

raindrop. The perceived color of each water droplet then

depends on the angle at which that drop is viewed.


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Lens Aberrations

It results from the fact that the focal points of light rays from the

principal axis of a spherical lens are different from the focal points ofrays with the same wavelenght passing near the axis.

Chromatic aberration is another type of aberration that arisses from thewavelenght dependence of refraction.

It can be greatly reduced by the use of a combination of converging anddiverging lenses made from two different type of glass.

Light and Reflection (1)

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