Sound And Light-Chapter 4 Light

The Role of Light to Sight Without light, there would be no sight. The visual ability of humans and other animals is the result of the complex interaction of light, eyes and brain. The appearance of black is a sign of the absence of light. The objects which we see can be placed into one of two categories: Luminous objects are objects which generate their own light. Illuminated objects are objects which are capable of reflecting light to our eyes.

To view an object, you must sight along a line at that object and when you do, light will come from that object to your eye along the line of sight Regardless of the eye location, you will still need to sight along a line in a specific direction in order to view the object.

It is only by reflection of light off objects that we can see colors and so forth. White light composed of all wavelengths of visible light incident on a pure blue object. Only blue light is reflected from the surface.

The incident ray - the light ray approaching the mirror. It intersects the mirror at the same location where your line of sight intersects the mirror. The light ray then reflects off the mirror and travels to your eye; this ray of light is known as the reflected ray. The distance from the mirror to the object (object distance) is equal to the distance from the mirror to the image ( image distance). For all plane mirrors, this equality holds true: Object distance = Image distance

The Law of Reflection The law of reflection states that when a ray of light reflects off a surface, the angle of incidence is equal to the angle of reflection.

The ray of light approaching the mirror is known as the incident ray (labeled I). The ray of light which leaves the mirror is known as the reflected ray (labeled R). When the ray strikes the mirror, point of incidence, a normal line (labeled N) can be drawn perpendicular to the surface of the mirror. It divides the angle between the incident ray and the reflected ray into two = angles. The angle between the incident ray and the normal is known as the angle of incidence. The angle between the reflected ray and the normal is known as the angle of reflection.

Pencil in a Mirror You must sight along a line at the image location. As you sight at the image, light travels to your eye along the path shown in the diagram below. The diagram shows that the light reflects off the mirror in such a manner that the angle of incidence is equal to the angle of reflection.

The Law of Reflection Review Questions

Which one of the angles (A, B, C, or D) is the angle of incidence? Which one of the angles is the angle of reflection? Angle B is the angle of incidence (angle between the incident ray and the normal). Angle C is the angle of reflection (angle between the reflected ray and the normal).

A ray of light is incident towards a plane mirror at an angle of 30-degrees with the mirror surface. What will be the angle of reflection? The angle of reflection is 60 degrees. (The angle of incidence is not 30 degrees; it is 60 degrees since the angle of incidence is measured between the incident ray and the normal.) Hint: angle of the mirror + angle of the light

A ray of light is approaching a set of three mirrors as shown in the diagram. The light ray is approaching the first mirror at an angle of 45-degrees with the mirror surface. Trace the path of the light ray as it bounces off the mirror. Continue tracing the ray until it finally exits from the mirror system. How many times will the ray reflect before it finally exits? The light reflects twice before it finally exits the system. Draw a normal at the point of incidence to the first mirror; measure the angle of incidence (45 degrees); then draw a reflected ray at 45 degrees from the normal. Repeat the process for the second mirror.

Specular vs. Diffuse Reflection Reflection off of smooth surfaces such as mirrors or a calm body of water leads to a type of reflection known as specular reflection. Reflection off of rough surfaces leads to a type of reflection known as diffuse reflection.

Why Does a Rough Surface Diffuses A Beam of Light? The roughness of the material means that each individual ray meets a surface which has a different orientation. The individual rays reflect off the rough surface according to the law of reflection, they scatter in different directions.

Plane Mirror Image Characteristics Are virtual. There is an apparent left-right reversal of the image. For example: if you raise your left hand, you will notice that the image raises what would seem to be it's right hand. If you raise your right hand, the image raises what would seem to be its left hand. This is often termed left-right reversal. There is no top-bottom vertical reversal. If you stand on your feet in front of a plane mirror, the image does not stand on its head. Similarly, the ceiling does not become the floor. The image is said to be upright, as opposed to inverted.

Plane Mirrors

There is a relationship between the object's distance to the mirror and the image's distance to the mirror. For plane mirrors, the object distance (symbol d o ) is equal to the image distance (symbol d i ). That is the image is the same distance behind the mirror as the object is in front of the mirror. If you stand a distance of 2 meters from a plane mirror, you must focus at a location 2 meters behind the mirror in order to view your image.

Plane Mirrors The dimensions of the image are the same as the dimensions of the object. The ratio of the image dimensions to the object dimensions is termed the magnification. Plane mirrors produce images which have a magnification of 1. If a 1.6-meter tall person stands in front of a mirror, he/she will see an image which is 1.6-meters tall. If a penny with a diameter of 18-mm is placed in front of a plane mirror, the image of the penny has a diameter of 18 mm.

Image Formation in Plane Mirrors- What Portion of a Mirror is Required? In order to view his image, the man must look as low as point Y (to see his feet) and as high as point X (to see the tip of his head). The man only needs the portion of mirror extending between points X and Y in order to view his entire image.

Image Formation in Plane Mirrors- What Portion of a Mirror is Required? The distance which a person stands from the mirror will not affect the amount of mirror which the person needs to see their image. A 1:2 ratio between portion of mirror required to view the image and the height of the object is often observed.

Concave Mirrors - The Anatomy of a Curved Mirror The two types of spherical mirrors can be thought of as a portion of a sphere which was sliced away and then silvered on one of the sides to form a reflecting surface. Concave mirrors were silvered on the inside of the sphere and convex mirrors were silvered on the outside of the sphere.

Concave Mirrors - The Anatomy of a Curved Mirror  If a concave mirror is thought of as being a slice of a sphere, then there would be a line passing through the center of the sphere and attaching to the mirror in the exact center of the mirror. This line is known as the principal axis.  The point in the center of the sphere from which the mirror was sliced is known as the center of curvature ( letter C).  The point on the mirror's surface where the principal axis meets the mirror is known as the vertex (letter A) The vertex is the geometric center of the mirror.

Concave Mirrors - The Anatomy of a Curved Mirror  Midway between the vertex and the center of curvature is a point known as the focal point; the focal point- letter F.  The distance from the vertex to the center of curvature is known as the radius of curvature (represented by R). The radius of curvature is the radius of the sphere from which the mirror was cut.  Finally, the distance from the mirror to the focal point is known as the focal length (represented by f). Since the focal point is the midpoint of the line segment adjoining the vertex and the center of curvature, the focal length would be one-half the radius of curvature.

Concave Mirrors - The Anatomy of a Curved Mirror Concave mirrors are capable of producing real images as well as virtual images.

Virtual VS. Real Images Virtual images are images which are formed in locations where light does not actually reach. Light does not actually pass through the location on the other side of the mirror; it only appears to an observer as though the light is coming from this location. When a real image is formed, it still appears to an observer as though light is diverging from the real image location. Only in the case of a real image, light is actually passing through the image location.

Two Rules of Reflection for Concave Mirrors Any incident ray traveling parallel to the principal axis on the way to the mirror will pass through the focal point upon reflection.the principal axisfocal point Any incident ray passing through the focal point on the way to the mirror will travel parallel to the principal axis upon reflection.focal pointthe principal axis

Concave Mirror Image - If the object is outside the focal length, a concave mirror will form a real, inverted image.

Concave Mirror Image - If an object is placed inside the focal length of a concave mirror, and enlarged virtual and erect image will be formed behind the mirror.

Convex Mirrors - Reflection and Image Formation for Convex Mirrors The center of that original sphere is known as the center of curvature ©; principal axis is the line which passes from the mirror's surface through the sphere's center; a focal point (F) which is located along the principal axis, midway between the mirror's surface and the center of curvature. Note that the center of curvature and the focal point are located on the side of the mirror opposite the object - behind the mirror

A convex mirror is sometimes referred to as a diverging mirror due to the fact that incident light originating from the same point and will reflect off the mirror surface and diverge.

Image Characteristics for Convex Mirrors The image is:  located behind the convex mirror  a virtual image  an upright image  reduced in size (i.e., smaller than the object)

Image Characteristics for Convex Mirrors The location of the object does not affect the characteristics of the image. Another characteristic of the images of objects formed by convex mirrors pertains to how a variation in object distance effects the image distance and size.

The diagram below shows seven different object locations (drawn and labeled in red) and their corresponding image locations (drawn and labeled in blue).

Convex Mirror Image - A convex mirror forms a virtual image. Using a ray parallel to the principal axis and one incident upon the center of the mirror, the position of the image can be constructed by back-projecting the rays which reflect from the mirror. The virtual image that is formed will appear smaller and closer to the mirror than the object.

Image Formation by Lenses The Anatomy of a Lens A lens is a carefully ground or molded piece of transparent material which refracts light rays in such as way as to form an image.

A converging lens is a lens which converges rays of light which are traveling parallel to its principal axis A diverging lens is a lens which diverges rays of light which are traveling parallel to its principal axis

Behavior of two incident rays approaching parallel to the principal axis. Two rays converge at a point; this point is known as the focal point of the lens

Behavior of two incident rays traveling through the focal point on the way to the lens. The two rays refract parallel to the principal axis.

Diverging Lenses

Behavior of two incident rays traveling towards the focal point on the way to the lens.

PRIMARY COLORS Primary colors may either be additive or subtractive. The additive primary colors are red, green, and blue. The subtractive primary colors are cyan, yellow, and magenta.

Any two colors of light which produce white light when mixed are considered complementary. Blue + Green = Cyan Red + Blue = Magenta Green + Red = Yellow Blue + Green + Red = White Additive Color Mixing

Subtractive Mixing of Primary Pigment Colors The subtractive mixing of primary pigments produces an array of colors and even black. It's called subtractive because the pigment reflects some colors but absorbs, or cancels out, others. Any two colors of pigment which produce black are considered to be complementary.

Some Common Vision Problems in Human Sight

Nearsightedness (myopia) - Able to see near, but not distance without glasses. This happens because the eyeball is a little too long. The lens focuses the image in front of the retina. Person will need to wear eyeglasses with concave lenses.

Farsightedness (hyperopia) - a person can see distant objects clearly, but nearby objects appear blurry. This happens because the eyeball is too short. The lens focuses the image behind the retina. Person will need to wear eyeglasses with convex lenses.

Astigmatism - rather than a perfectly round ball, the eye is shaped more like a football.

The two different curvatures cause two separate images to be focused on the retina.

This causes double vision to occur in addition to blurring of the image

Presbyopia (loss of near vision) - This is an aging process that affects all people, usually around age 40. The lens of the eye hardens with age and is no longer able to focus at close objects. Most people are myopic or hyperopic because the length of the eye is too long or short. Light does not focus on the retina.

That’s All Folks

No, Really that’s All

This Power Point presentation was constructed and is copyrighted by Roger Price 8/08

The five main diagrams for image formation by a Convex (converging) lens.