Light

Physics

Ms. Johnson

Early Concepts

• Most philosophers and scientists believed light consisted of particles

• Christian Huygens was the first to argue that light was a wave

• In 1905, Einstein published a paper on the photoelectric effect, where he explained light consisted of particles called photons

• Scientists now agree that light has a dual nature, as a particle and as a wave

Speed of Light

• Several different measurements– Pre - 17th century

– Galileo

– Olaus Roemer 1676

– Michelson (1926)

• Represented by the symbol c =3.0x10^8 m/s• Light-year - distance light travels in one year

Electromagnetic Waves

• Light is energy that is emitted by accelerating electric charges

• This is called an electromagnetic wave

• This is a transverse wave• This wave type includes

radio waves, microwaves, x-rays which are all part of the electromagnetic spectrum

• Visible light has a wavelength from 400nm – 700nm

• Red light has the lowest frequency

• Violet has the highest.• Waves with a frequency

lower than red light are infrared

• Waves with frequencies higher than violet are ultraviolet

Types of Electromagnetic Waves

Transparent & Opaque

• Transparent materials allow light to pass through them

• Light can slow down as it passes through these materials– Ex: glass- .67c water

- .75c

• Opaque materials allow no light to pass through them.

• A material can be transparent to some wavelengths and opaque to others

Shadows

• A thin beam of light is called a ray.

• Any beam of light can be thought of as a bundle of rays

• When a beam of light shines on an object, some of the rays may be stopped while others may continue on their straight-line path

• A shadow is formed where light rays cannot reach.

• A total shadow is called an umbra

• A partial shadow is called a penumbra - it occurs when some light is blocked but other light fills in

Eclipse

• An eclipse is an example of a shadow

• When the moon is between the sun and Earth it casts a shadow– The darkest part of the shadow is the umbra– The lighter, outer part is the penumbra

• There are 2 types of eclipses, solar and lunar

Lunar vs. Solar Eclipse

Polarization

• Light travels in transverse waves that go out in all directions

• When waves are polarized, they are only allowed to travel in one direction

• Filters can be used to create polarized light

• If two polarizing filters are placed at 90° to each other, no light will get through.

Polarized Glasses

3D Viewing

• Seeing in 3 dimensions depends on both eyes giving the same impressions simultaneously.

• A pair of photos placed a short distance apart

• This can be done in slide shows or movies by projecting 2 polarized pictures on the same screen and viewing it through 2 different polarized lenses