Light

What causes light?• Objects that give off their

own light are called luminous.

• They are light sources. • The sun is luminous. The

moon is not.• ALL light energy is produced

when electrons change energy levels within an atom.

• When an electron absorbs extra energy, it jumps to a higher energy level.

• When it drops back to a lower energy level, a packet of light energy is given off.

• A packet of light energy is called a photon.

photon

photon

Models of Light - Wave or Particle? Evidence: Light is a wave.• Light has interference patterns

and particles do not.

• Light behaves like a transverse wave when it is diffracted. Particles cannot be diffracted.

• Light waves can be polarized—(made to vibrate in only one direction). Particles cannot be polarized.

Evidence: Light is a particle.• Blue light produces electricity,

but no amount of red light can.

• If light were waves, then a whole lot of red light should produce electricity. The photoelectric effect proves that light is a particle.

• Changing light intensity (brightness) is best explained if

light is a particle, not a wave.

Einstein’s conclusion: Light is both a wave and a particle!

photon

Speed of Light• In 1926, Albert Michelson first

measured the speed of light accurately.

• The speed of light changes, depending on the medium that the light is traveling through.

• Light travels fastest through empty space--at a speed of about 300,000,000 m/sec !

• Light travels through air almost as fast as through empty space, but it travels more slowly through liquids, and even slower still through solids.

• It takes light 8.3 minutes to get from the Sun to the Earth.

* A light-year is not a measure of time. It measures the distance that light travels through space in one year.

The Electromagnetic Spectrum - 1• Light always has both an

electric field and a magnetic field traveling with it. This is why light is called electromagnetic radiation.

• Light waves are also called electromagnetic waves.

• They are different than other energy waves, because they do not need a medium.

• All electromagnetic waves are not the same.

• They can have different wavelengths, different frequencies, and carry different amounts of energy.

• When all light waves are arranged in order according to their wavelength and frequency, we call this arrangement the electromagnetic spectrum.

Radio Waves Microwaves Visible Spectrum Ultraviolet Gamma Rays

Television Infrared X-Rays

Long wavelength, low frequency Short wavelength, high frequency

low energy photons high energy photons

The Electromagnetic Spectrum

The Electromagnetic Spectrum - 2• The visible spectrum is the part

of the electromagnetic spectrum that we can see.

• The longest wavelength of visible light is 750 nanometers,and is red light. A nanometer is one millionth of a millimeter.

• The shortest wavelength of visible light is 430 nanometers and is violet light.

• The colors of the visible spectrum in order from longest to shortest wavelength are: red, orange, yellow, green, blue, indigo, and violet. (ROY G. BIV)

• The invisible spectrum is the part of the electromagnetic spectrum that we cannot see.

• Radio Waves - have the longest wavelengths.

• Microwaves - High energy radio waves are called microwaves.

• Infrared Waves - have wavelengths slightly longer than visible red light. This type of radiation makes molecules move faster, causing heat!

• Ultraviolet Waves - the main source of UV radiation is the sun. UV light kills bacteria, makes your body produce vitamin D, and causes skin cancer and sunburn.

• X-Rays - high energy light that can travel through body cells.

• Gamma Rays - highest energy light waves are used in radiation treatments for cancer.

Interactions of Light: 1• When light hits an object, the light

can behave in several ways: It can be reflected, or absorbed, or pass through the object.

• Objects that are black absorb all of the light that hits them.

• Objects that are white reflect all of the light that hits them.

• A red object absorbs all other colors and reflects back only red.

• Objects that let almost all of the light pass through are called transparent.

• Objects that let some light pass through but scatter it, are called translucent.

• Objects that don’t let any light pass through are called opaque.

Interactions of Light: 2• Light waves interact with the

surface of an object or with a medium just like other waves do.

• Light waves can be reflected, diffracted, refracted, or experience constructive or destructive interference.

• Light is reflected by a mirror.

• Light is refracted when it changes from one medium to another.

• Constructive interference makes a light brighter, but destructive interference makes darkness.

• Interference produces bands of light and dark.

• All animals do not see in color. Dogs, cats, cattle and horses cannot see colors, but apes, humans and birds can.

• Human eyes have two kinds of receptor cells in the retina: rods which see objects even in dim light but in black and white, and cones which see colors.

• The three primary colors of light are red, green, and blue. When all of these are mixed together, they make white light.

• Paint is different from light. It is made of pigment. The three colors of pigment are magenta, yellow, and cyan. When these are mixed together, they make black pigment.

Light Sources• Objects that produce their

own light are called luminous.

• Light bulbs are luminous. Some common kinds of light bulbs are neon, fluorescent, incandescent, tungsten-halogen, and sodium-vapor lights.

• Even some living things can be luminous--lightning bugs are an example of this process of bioluminescence.

• Objects that reflect light that comes from another source are called illuminated.

Vision - Parts of the Eye• Iris and Pupil: The iris adjusts the size

of the pupil to control how much light enters your eye.

• Cornea and Lens: The transparent cornea lets light into your eye. The lens changes shape to focus the light rays on your retina.

• Retina: The retina is like a movie screen. Light forms an image on the retina. The rod and cone cells are stimulated by the image.

• Optic Nerve: Nerve fibers connected to the rods and cones join together to form the optic nerve, which carries nerve signals from your eyes to your brain.

Optic nerveRetina

Lens

Sclera

Cornea

Pupil

Iris

Lens Muscles

Reflection and Mirrors• A reflection happens any time

light bounces off of an object.

• Anything you can see reflects light to your eye.

• A mirror is a flat or a curved piece of glass that has been painted on the back to give the maximum reflection. Images in a mirror are reversed from left to right.

• Mirrors are either plane, convex, or concave.

• A plane mirror makes an image that seems to be behind the mirror. It is also the same size as the object being reflected. This is a virtual image, which can only be seen in a mirror.

• A concave mirror makes a real image, but the distance between the object and the mirror determines the kind of image reflected.

• A convex mirror forms a virtual image that is larger than the object reflected. The image is always upright.

Convex mirror Concave mirror

Object Plane Mirror Image

Refraction and Lenses• Refraction is the bending of light rays. Light usually travels in straight lines

and at a constant speed unless it changes from one medium to another.

• A lens is a transparent material that refracts light. Lenses usually have one or more curved surfaces. There are two kinds of lenses: convex and concave.

• A concave lens is thin in the middle and thick around the edges. Concave lenses form images that are upright and smaller that the real object.

Convex lens

Object

Image

• A convex lens is thin at the edges and thick in the center. The image it forms depends on how far the object is from the lens. A convex lens is used in a microscope or a camera. The lens in your eye is convex.

Object

Image

Concave lens