The Nature of Light

Light Can Act Like Waves or In 1801 Thomas Young an English scientist did an experiment. –Double slit experiment Passed a beam of light through two narrow openings and projected it onto a screen. He found the light produced a striped pattern which meant the light was constructively and destructively interfering. This meant that light is composed of waves.

But Light can Also Behave like a Particle Other observations indicated that light can also act like a particle: –When light hits metal it knocks electrons off the surface. –They found that red light cannot knock electrons off metal no matter how bright it is. –If light were a wave then the brighter light should have more energy. –Photons, light particles contain certain amounts of energy based on the frequency and wavelength of light. –Blue light has a higher frequency and shorter wavelength thus contains more energy than red light.

Light Energy is Proportional to Frequency The higher the frequency of light the more energy. The speed of light depends on the medium it is traveling through. –Light travels the fastest through a vacuum. 3.0 x 10 8 m/s Light slows down in denser mediums.

The Brightness of Light Depends on Intensity Intensity –The quantity of light illuminating a surface. –Depends on the amount of light passing through a certain area or space. –Light spreads out in spherical wave fronts. –Light is more diffuse further from its source.

Reflection and Color Reflection –When light bounces off a surface. –Rough surfaces reflect light rays in many directions. Diffuse reflection Causes a blurry image or no image. Incident ray –Ray hitting the surface Reflected ray –Ray bouncing off the surface Normal –An imaginary line that is perpendicular to the surface the light is reflecting from. Angle of Incidence –Angle between incident ray and the normal. Angle of reflection –Angle between the reflected ray and the normal. The angle of incidence equals the angle of reflection.

Flat Mirrors Plane mirrors (flat mirrors) –Create virtual images Images you see from the apparent light path not the actual path. –Images are the same size as the object and they appear the same distance in the mirror as the object is in front of the mirror.

Curved Mirrors Concave mirrors are curved inwards –Create real images Real images are produced when light is focused in front of the mirror. Can be projected onto a screen. Are usually inverted. –Create virtual images When the object is between the focus and the mirror. The image is larger than the object. These are magnifying mirrors.

Curved Mirrors Convex mirrors are curved outwards –Are diverging mirrors –Create virtual images –Images are always smaller –Panoramic mirrors Security mirrors Side view mirrors on cars

Seeing Colors The frequency and wavelength of light determine what color you will see. –When you optical receptors perceive a wavelength of 550 nm you see green light. –The colors that you see are a result of surfaces reflecting various wavelengths of light back to your eyes. –White light When all of the primary colors are being reflected back to your eyes. –No light (or black) When all of the primary colors are being absorbed and no light is being reflected back to your eyes.

Primary Colors The primary colors are –Red Blue Green Your eyes only have receptors for the frequency of light in these three ranges. Every other color you percieve is a combination of these three wavelengths in various proportions. Adding any two of these colors gives you the secondary colors or primary pigments

Primary Pigments Pigments are substances that absorb light. The primary pigments are: –Cyan Magenta Yellow These are also known as the subtractive colors. –Adding any of these two colors together give you back the primary colors.

Refraction of Light Refraction When light waves bend in a medium due to the change in density. The angle of refraction is NOT equal to the angle of reflection. The angle of incidence will be greater than the angle of refraction if the medium the light is traveling into is denser.

Refraction

Refraction Refraction makes objects appear in different positions from where they actually are. –These are virtual images. –Refraction due to the atmosphere is known as a mirage. –Heating air near the surface of the Earth on a hot day causes the air near the ground to be less dense than the air above it.

Internal Reflection Total internal reflection –Occurs when light is refracted so much that it is reflected back into a substance. Critical angle –When the angle in which light rays meet the boundaries between the two mediums is so large that the boundary acts like a mirror and reflects all of the light back into the medium. Internal reflection is used in fiber optic cables.

Total Internal Reflection

Lenses A curved piece of glass or plastic that changes the direction of light waves. –Light is refracted when it enters glass and then again when it leaves the glass. –Converging lenses Bend light inward and focus light Create virtual and real images Also called convex lenses –Diverging lenses Bend light outward Create virtual images only Also called concave lenses

Convex and Concave Lenses

Concave and Convex Lenses

Convex Lens Application Magnification –When the image is a different size from actual object. MicroscopesTelescopes Lens of the Eye Glasses or contact lenses Cameras

Near Sighted and Far Sighted Near sightedness –Occurs when the eyeball is too long causing the image to be to far in front of the retina. Diverging lenses move the image back so that the image appears on the retina. Far sightedness –Occurs when the eyeball is to short and the image appears to far behind the retina. –Convex lenses move the image up

Anatomy of the Human Eye

The Eye Uses a Convex Lens

Nearsightedness

Diverging Lenses Correct Nearsightedness

Farsightedness

Converging Lenses Correct Farsightedness

Dispersion and Prisms Dispersion –When light is refracted in such a way that it separates light into its individual color components. –Prisms are triangular pieces of glass that bend light. The angled surface causes the difference wavelengths of light to separate because they bend at different angles. Water droplets act like prisms and create rainbows.