Light

Light may be described as natural or artificial.  Natural light is light that comes largely from the sun.  Artificial light is man-made, and is usually associated with burning or with electricity.

Lighted bodies may be described as either luminous or illuminated.  A luminous body, such as the sun or a burning light bulb, manufactures its own light.  An illuminated body, such as the moon or a mirror, is visible because it reflects light.

Light energy is an emission that results from disturbances of the nucleus and/or electrons (electron excitation-relaxation, photon emission).

Emitted energy can take the form of emitted light (electron excitation) or other forms like UV, X-rays, gamma rays (nuclear excitation).

Visible light: visual receptors in the human eye are sensitive to electromagnetic radiation with wavelengths from 7.5 E -7 m (red) to 4 E – 7 m (violet). Another unit system often used is the angstrom (1 angstrom = 1 E – 8 cm).  Visible light then falls from 4000 to 7000 angstroms.  The greatest sensitivity is for green and yellow parts of the spectrum, near the peak of the solar radiation which reaches the earth’s surface (with other forms of solar radiation being filtered out).

Theories of light:

·       Corpuscular theory – Sir Isaac Newton, 17^th century: “Light particles emitted by luminous body are tiny particles of matter or corpuscles – they are projected outward and travel in straight lines.”

·       Wave theory – Christian Huygens – light emitted as series of waves that spread out in all directions (medium for transmission required)

·       Electromagnetic theory – James Clerk Maxwell – light waves possess electrical and magnetic properties and can travel through vacuum. Each type wave in electromagnetic spectrum has a particular wavelength and frequency  (example: radio waves of 1000 m long have frequencies of 3.0 E 8 vibrations/second while X-rays at 1/10 billionth of a meter has a frequency of 3.0 E s18 vibrations/second (shorter λ, greater f)

·       Quantum theory – Max Planck – light waves travel as separate pockets of energy called quanta or photons

 

In some instances each theory seems to work best but since the quantum theory merges the ideas of the three other theories it explains the nature and behavior of light more satisfactorily than does any other theory.

Objects that completely block light and through which we cannot see are called opaque objects.  Objects that readily transmit light and through which we can see clearly are called transparent objects.  Objects that allow light to pass through partially or that distort the light so that we cannot see through them are called translucent objects.

 

Rules for Optical Constructions:

·       Rays of light coming in parallel to the axis come together at the focus and go through

·       Rays of light coming in through the axis will go out parallel to the axis

 

 

Law of Reflection:

·       The light ray that strikes the surface is called the incident ray.  The light ray that bounces off the surface is called the reflected ray.  The line drawn at right angles to the reflecting surface is called the normal.  The angle formed beween the incident ray and the normal is the angle of incidence; the angle between the reflected ray and the normal is angle of reflection. 

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

·       The incident ray, normal, and reflected ray all lie in the same plane.

·       Reflection from a rough surface is called diffuse reflection.

·       Reflection from a smooth flat surface, such as a mirror or a quiet pool of water is called specular reflection.

·       See drawing of reflection here:

 

Reflection from a plane mirror:

·       The size of the image in a plane mirror is the same as the size of the object.

·       The actual distance of the object from a plane mirror is the same as the distance the image appears to be behind the mirror.

·       The image seen in a plane mirror is reversed laterally when compared to the object.

Need notes on curved mirrors here:  concave and convex mirrors

Polarization of Light:

·       A light source, such as the sun or a lamp, emits waves that vibrate vertically, horizontally, and in all directions.  Because the wave is transverse, the vibrations are at right angles to the plane in which the light travels.  Such light waves are said to be unpolarized.

·       Light that passes through a material that only allows passage of waves in a single plane is called polarized light. 

 

Refraction:

Refraction is the bending of light rays as they pass at an angle from one medium, such as water, into another medium, such as air.  The refraction of light rays occurs only when light rays enter a new medium at an oblique angle (any angle that is neither a right angle nor a straight angle). 

In water the speed of light is roughly ¾ of its speed in air. In glass the speed of light decreases to about 2/3 of its speed in air.  This decrease in the speed of light occurs because the molecules in the water and in the glass are more dense (more tightly packed) than the molecules in air.  The closeness of the molecules, called optical density, acts as a barrier to the passage of light.

Law of Refraction:

·       When light rays pass at an oblique angle from a less dense medium into a more dense medium, the ray bends toward the normal. 

·       When a ray passes at an oblique angle from a more dense medium to a less dense the ray is bent away from the normal.

 

see drawing of dispersion

see drawing of light bands entering a more dense media

see drawing of total internal reflection

Lenses:

·       A lens is a thin disk of transparent material, such as glass, whose opposite sides are smooth, curved surfaces.

·       There are two general types of lenses: convex and concave

 

A convex lens is thicker in the middle than at the edges. As parallel rays of light pass through a convex lens they are refracted and are brought together, or converged, at a point called the principal focus (F).  The ray of light passing through the center of the lens is not refracted.  The distance measured from the center of the lens to the principal focus is called the focal length.  The principal axis is a line, normal to the curved surface, that passes through the center of the lens.  Since a convex lens can converge parallel rays of light is often called a converging lens.  In general, the greater the curvature of a convex lens, the shorter the focal length.  Thus, a completely spherical convex lens has the shortest focal length possible for a convex lens.

A concave lens is a lens that is thicker at the edges than in the middle.  When parallel rays of light pass through a concave lens they are refracted and are separated or diverged.  The refracted light rays from a concave lens never meet.  The principal focus of such a lens is found by extending the diverging rays backward through the lens until their extensions meet at a point.  This point is called a virtual focus because the light rays do not actually meet here.  The focal length of this lens is found by measuring the distance from the center of the lens to the point where the extensions of the diverging rays meet.

In general, the images formed by convex lenses are of two types, real and virtual. 

A real image is an image formed by actual rays of light.  All real images have the following characteristics:

1.   a real image is formed by actual rays of light

2.   a real image can be projected onto a screen

3.   a real image is always inverted

 

A virtual image is an imaginary image (it only seems to be formed by rays of light – as when a magnifying glass enlarges the image of a stamp we know the stamp is still the original size).

All virtual images have the following characteristics:

1.   a virtual image is formed by the extensions of light rays

2.   a virtual image cannot be projected onto a screen

3.   virtual images are always upright.

 

The type of image produced by a convex lens depends on the magnitude of the distance of the object from the lens with respect to the principal focus.

 

See drawings of convex lenses:

 

Convex lenses are used in eyeglasses to correct the vision of farsighted people.  In farsightedness, an image falls behind the retina because the lens of the eye is not convex enough.  Thus, the image appears blurred.  A stronger convex lens brings the image forward enough to fall on the retina.

see drawing of eye – farsightedness

 

Combinations of convex lenses are used in refracting telescopes and in microscopes.

 In a refracting telescope the convex objective lens has a long focal length.  This produces a real, smaller, and inverted image of a distant object that is viewed through a convex eyepiece lens.  This image appears at less than a focal length of the eyepiece and produces a virtual, larger, and erect second image.  As the objective lens of a refracting telescope is made larger, its ability to gather light increases.

In a microscope the specimen is examined by a convex objective lens of short focal length.  Since the specimen is beyond one focal length of the lens, the first image is real, larger, and inverted.  This image falls within one focal length of a convex eyepiece of long focal length.  This second image that the eye observes is virtual, larger, and erect, thus producing magnification.  (A concave mirror gathers the necessary light to illuminate the specimen).

Concave lenses are used in eyeglasses to correct the vision of nearsighted people.  In nearsightedness, an image falls in front of the retina.  The light rays that reach the retina are not in focus.  When a concave lens is placed in front of such an eye, this lens diverges the rays and allows a clear image to fall on the retina.

Color

White light entering a prism is refracted as it enters and refracted again as it leaves the prism causing the light to split into a series of colors very much like a natural rainbow (ROY G BIV).  This is called dispersion.

 

See drawing of dispersion and rainbows:

 

To understand why dispersion occurs, we recall that when a light beam enters a more dense medium (like the glass of a prism) at an oblique angle, the speed of the light is decreased and the beam is refracted.  However, the red rays of the white light are slowed down the least, whereas the violet rays of the white light are slowed down the most.  As a result of these differences in speed, the rays are refracted to different degrees.  Red light, which has the longest wavelength, is refracted the least; violet light, which has the shortest wavelength, is refracted the most.  When the rays of colored light leave the prism they are refracted again.  The seven colors spread in a rainbow-like band called a spectrum.

 

A diffraction grating affects a beam of light in the same way.  It is usually a flat piece of transparent material on which thousands of parallel lines have been rules for every square inch of surface.  The space between any two lines is less than one wavelength of light.  When a beam of light enters such a grating, it is spread out or diffracted and a spectrum is formed.

 

The wavelengths of visible light ranges from approximately 40 to 75 millionths of a centimeter.  Electromagnetic waves somewhat longer than those of red light are called infrared rays.  These are invisible to the human eye but can affect types of photographic film or cameras.  Ultraviolet rays have shorter wavelengths than violet light.

 

The color of a transparent object is the color of the light that the object transmits.  Ordinary window glass transmits all colors equally well and, therefore, appears to be colorless in white light. 

·       When white light strikes a piece of red glass, the glass absorbs all the wavelengths except the red.  Since the glass transmits only red light to our eyes, the glass appears red.

·       blue glass transmits only blue light and absorbs all the other colors (all single colors work this way)

·       when a beam of red light shines on a piece of blue glass no light is transmitted because the blue glass absorbs all of the red light.  In such a case, the blue glass may appear black, which is the absence of color. 

 

The color of an opaque object is the color of the light reflected by the object.  White paper appears white in white light because it reflects all colors of light hitting it.

·       When white light falls on a piece of red paper, the paper appears red because the coloring material in the paper reflects red rays and absorbs all the others.

·       An object that reflects no light, but absorbs all light rays that fall on it, appears black

·       When blue cloth is viewed in red light, the cloth appears black because the blue dye in it absorbs the red rays and reflects no light at all.

 

see drawing of primary colors of light and primary colors of pigment

 

 

 

Sun Activity Lab

 

Sun Activity:  How many light bulbs equal the Sun?

 

Purpose:  The purpose of this activity is to measure the sun’s power output and compare that with the power output of a 100 watt light bulb.

 

Procedure:

1.       The apparatus has been built for you.  Be careful with the apparatus.

2.       The apparatus has been taken outside and set so that the absorber face is perpendicular to the rays of the sun.  It was left in this position until the maximum temperature was reached.  This can be found on the chalk board.

3.       The light bulb has been placed at the 50.00 cm mark on the optical bench meter stick.  This allows 4 groups to use to make their measurements.  Position your absorber face so that it is 5.0 cm away from the filament in the bulb.  The absorber must be perpendicular to the filament.  You may need to use textbooks to align the apparatus with the absorber plate.   Turn on the bulb and allow your temperature to stabilize (it is not changing more than 1 full degree over 30 seconds of time.

4.       Slowly move the apparatus, 5.0 cm at a time, away from the light bulb, allowing the temperature to stabilize each time.  As the temperature approaches the chalk board temperature move the apparatus only 1 cm at a time and then only 0.1 cm at a time.  Stop moving the apparatus when the chalk board temperature is reached and maintained for 2 minutes. 

5.       Measure as exactly as possible the distance between the absorber plate of your apparatus and the filament of the bulb in meters.  Record this  value as d(bulb).

6.       For the d(bulb), use the formula below to calculate the wattage of the sun given d(sun) = 1.5 E 11 m:

 

wattage (sun)       =           wattage (bulb)

d² (sun)                             d² (bulb)

 

Summing up:

1.       Use the value of the sun’s wattage to calculate the number of 100-watt light bulbs that would equal the sun’s power.

2.       The accepted value for the sun’s wattage is 3.83 E 26 W.  List 4 factors that might account for the difference between your experimental value and the accepted value.

3.       Calculate the percent error and percent difference between the two answers.  When will these two percents be the same number?

4.       Would it be possible to turn on this many light bulbs at once?  Explain