Physics (Ch. 16 -18)

Electromagnetic Waves

Light travels almost unimaginably fast and far. Light carries energy and information. Light travels in straight lines. Light bounces and bends when it comes in contact with

objects. Light has color. Light has different intensities, it can be bright or dim.

Properties and Sources of Light

The process of making light with heat is called incandescence.

Incandescent bulbs generate light when electricity passes through a thin piece of metal wire called a filament.

Electric Light

The filament heats up and gives off light.

The other common kind of electric light is the fluorescent bulb.

Electric Light

Fluorescent bulbs convert electricity directly to light without generating a lot of heat.

Fluorescent bulbs use high-voltage electricity to energize atoms of gas that fill the bulb.

Light carries energy and power

Light is a form of energy that travels. The intensity of light is the amount of energy per

second falling on a surface. Most light sources distribute their light equally in all

directions, making a spherical pattern. Because light spreads out in a sphere, the

intensity decreases the farther you get from the source.

Light intensity The intensity of light from a small source follows an

inverse square law because its intensity diminishes as the square of the distance.

The speed of light

The speed at which light travels through air is approximately 300 million m/s. Light travels almost a million times faster than sound. The speed of light is so important in physics that it is given its own symbol, a

lower case c.

c = 3 × 108 m/sec.

1. You are asked for time.2. You are given distance.3. Use v = d ÷ t , rearrange to solve for t = d ÷ v4. Solve for sound: t = (1,609 m) ÷ (340 m/s) = 4.73 s5. Solve for light: t= (1,609 m) ÷ (3 x 108 m/s) = 5.4 x 10-6 s

Calculating time for light and sound

Calculate the time it takes light and sound to travel the distance of 1 mile, which is 1,609 meters.

Reflection

Objects that are in front of a mirror appear as if they are behind the mirror.

This is because light rays are reflected by the mirror. Your brain perceives the light as if it always traveled

in a straight line.

Refraction

An example of refraction of light is the twinkling of a star in the night sky

As starlight travels from space into the Earth’s atmosphere, the rays are refracted.

Since the atmosphere is constantly changing, the amount of refraction also changes.

The light that bends as it crosses a surface into a material refracts and is shown as a refracted ray.

Color and Vision

When all the colors of the rainbow are combined, we do not see any particular color.

We see light without any color. We call this combination of all the colors of light "white

light ".

How the human eye sees color

The retina in the back of the eye contains photoreceptors. (rods and cones)

These receptors release chemical signals. Chemical signals travel to the brain along the optic

nerve.

How we see colors

Which chemical signal gets sent depends on how much energy the light has.

If the brain gets a signal from ONLY green cones, we see green.

How we see other colors The three color receptors in the

eye allow us to see millions of different colors.

The additive primary colors are red, green, and blue.

We don’t see everything white because the strength of the signal matters.

All the different shades of color we can see are made by changing the proportions of red, green, and blue.

How we see the color of things

The light that reaches our eyes can come from two different processes:

1. The light can be emitted directly from the object, like a light bulb or glow stick.

2. The light can come from somewhere else, like when we see objects by reflected light from the sun.

Why are plants green?

Plants absorb light energy and convert it to chemical energy in the form of sugar (food for the plant).

Chlorophyll is an important molecule that absorbs blue and red light.

Plants must reflect some light to avoid absorbing too much energy. The color green is reflected by Chlorophyll.

A plant will die if placed under ONLY green light!

Reflection and Refraction

A lens is an optical device that is used to bend light in a specific way.

A converging lens bends light so that the light rays come together to a point.

A diverging lens bends light so it spreads light apart instead of coming together.

The overall study of how light behaves is called optics.

Mirrors, Lenses, and Images

Light rays that enter a converging lens parallel to its axis bend to meet at a point called the focal point.

The distance from the center of the lens to the focal point is called the focal length.

The optical axis usually goes through the center of the lens.

Mirrors, Lenses, and Images

We see a world of images created on the retina of the eye by the lens in the front of the eye.

Reflection and Refraction

Mirrors reflect light and allow us to see ourselves. A prism is another optical device that can cause light to

change directions. A prism is a solid piece of glass with flat polished surfaces.

Reflection

Images appear in mirrors because of how light is reflected by mirrors.

The incident ray follows the light falling onto the mirror.

The reflected ray follows the light bouncing off the mirror.

Reflection

In specular reflection each incident ray bounces off a smooth/shiny surface in a single direction.

A surface that is not shiny creates diffuse reflection. In diffuse reflection, a single ray of light scatters into

many directions.

Law of Reflection

The angle of incidence equals the angle of reflection.

The incident ray strikes the mirror.The reflected ray bounces off.

2. You are given the angle of incidence.3. Use the law of reflection which states the angle of

reflection equals the angle of incidence.4. Sketch a ray diagram showing the angle of reflection is

30o.

Law of reflection

A light ray is incident on a plane mirror with a 30 degree angle of incidence. Sketch the incident and reflected rays and determine the angle of reflection.

1. You are asked for a ray diagram and the angle of reflection.

Refraction

Light rays may bend as they cross a boundary from one material to another, like from air to water.

This bending of light rays is known as refraction.

The light rays from the straw are refracted (or bent) when they cross from water back into air before reaching your eyes.

Refraction

When a ray of light crosses from one material to another, the amount it bends depends on the difference in index of refraction between the two materials.

Index of refraction

The ability of a material to bend rays of light is described by the index of refraction (n).

Dispersion and prisms

When white light passes through a glass prism, blue is bent more than red.

Colors between blue and red are bent proportional to their position in the spectrum.

Dispersion and prisms

The variation in refractive index with color is called dispersion.

A rainbow is an example of dispersion in nature.

Tiny rain droplets act as prisms separating the colors in the white light rays from the sun.

If you could shake the magnet up and down 450 trillion times per second, you would make waves of red light with a frequency of about 450 THz.

These waves are called electromagnetic waves.

The Electromagnetic Spectrum

Electromagnetic waves have both an electric part and a magnetic part. The two parts exchange energy back and forth.

A 3-D view of an electromagnetic wave shows the electric and magnetic portions.

Light is a transverse wave.

The Electromagnetic Spectrum

The wavelength and amplitude of the waves are labeled λ and A, respectively.

The higher the frequency of the light, the higher the energy of the wave.

Since color is related to energy, there is also a direct relation between color, frequency, and wavelength.

The Electromagnetic Spectrum

Speed of Light

c = f l

Wavelength (m)

Frequency (Hz)

Speed of light3 x 108 m/sec

The speed of light is incredibly fast (3 × 108 m/s) and is represented by its own symbol, c.

1. You are asked for wavelength.2. You are given frequency.3. Use speed of light, c = ƒ l 4. Solve l = c ÷ƒ

◦ l = (3 x 108 m/s) ÷ ( 600 x 1012 Hz)◦ l = 5 x 10 -7 m

Calculating wavelengthCalculate the wavelength in air of blue-green light that has a frequency of 600 × 1012 Hz.

Why is the sky blue? The red color in the sky at sunrise & sunset is due to

an effect called Rayleigh scattering. The sky appears to be blue in the day-time (when the

sun is closest to us) because the Oxygen and Nitrogen in the atmosphere scatter violet and blue light due to their small size.

This blue light is received by the observer.

Why does the image of the sun appear reddish at sunrise & sunset? The light has to travels through more atmosphere during

sunrise and sunset because the distance that the light has to travel from the Sun to an observer is at its greatest.

Blue/violet light is scattered the most & red light is scattered the least.

This means the large amount of blue and violet light has been scattered so the light that is received by an observer is mostly of a longer wavelength and therefore appears to be red.

Visible light is a small part of the energy range of electromagnetic waves.

The whole range is called the electromagnetic spectrum and visible light is in the middle of it.

Light speed and equation Electromagnetic spectrum sections, energy,

wavelength, and frequency Colors of light in order Basic principles of optics (reflection and refraction) Structure of the eye Absorption of light to see colors

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Major points to know