Physics Class 6 - Light, Mirrors and Lenses, Quantum Physics

What is electromagnetic radiation?

Transverse wave composed of oscillating electric and magnetic fields

Propagates without medium (unlike all other waves)

What is the speed of light in a vacuum?

c = 3 × 10⁸ m/s

What is the visible light spectrum?

Wavelengths of 700nm(red) - 400 nm (violet)

What is the order of electromagnetic waves, from longest to shortest?

Radio, Micro, Infrared, Visible Light, Ultraviolet, X-Rays

What is energy of a sound wave?

Like sound waves, proportional to square of wave amplitude

Amplitude with electromagnetic waves is electric field strength

What does light do when it hits the interface of two media?

Reflection: bounces back

Transmission: continues through, but refracts

What is the angle of refraction?

Angle of incidence = angle of refraction

Angle measured relative to surface normal (perpendicular)(not the surface)

What is the index of refraction?

How much slower light travels through a medium than a vacuum

n = c/v_medium

n >/= 1

n_vacuum = n_air = 1

What is Snell’s law?

Tells us angle of refraction

n₁ sin(theta₁) = n₂ sin (theta₂)

For angles between 0 and 90, as theta increases, so does sin theta

As n gets larger, theta decreases

What is the equation of lens power?

P = 1/f

f = focal length in centimeters

In diopters

What do glasses with a negative diopter value do?

They form an image of a far away thing f centimeters from the lens or your eye

How do you fix nearsightedness?

A diverging lens

Negative diopter value

How do you fix farsightedness?

Converging lens

Positive diopter

What is total internal reflectance?

Happens when light transmits to a medium of lower refractive index

As you increase the angle of incidence, the angle of refraction increases faster

What is the critical angle?

theta_c = critical angle

Angle of incidence that creates a 90 degree angle of refractance

sin theta_c = n₂/n₁

When theta is greater than the critical angle, you get all energy reflected

What is diffraction?

When waves encounter an obstacle/aperture about the same size of their wavelength, the waves will spread out

What is dispersion?

Effect that allows prisms to split white light into ROYGBV colors → rainbows

First rule of waves (speed depends on medium alone, not frequency) is untrue for light

Higher frequencies have higher indexes of refraction

What is polarization?

One direction of oscillation is privileged over another (usually, light waves have electric fields that oscillate in all directions equally)

Via reflection or transmission through a special fiber/material

What is plane polarization?

Removal of all electric field oscillations except those along one plane parallel to the direction of propagation

What is circular polarization?

Two perpendicular electric field components oscillate 90 degrees out of phase with each other

Vector of the sum of these components creates a field that rotates in a plane perpendicular to the direction of propagation

What waves can be polarized?

Transverse waves

What are the mirror equations?

1/f = 1/o + 1/i

m = -i/o

f = focal length (intrinsic property)

o = object distance; object distance from lens/mirror (almost always positive)

i = image distance; distance from lens/mirror that image is formed

m = magnification (size of image compared to object)

What does a negative magnification mean?

The image is inverted relative to the object

What is the sign of f for a concave mirror, convex mirror, concave lens, and convex lens?

Concave mirror: +

Convex mirror: -

Concave lens: -

Convex lens: +

Where is the image located in a mirror with positive/negative f? Lens?

Mirror with positive f: same side of mirror as the object

Mirror with negative f: opposite side of mirror as object

Lens with positive f: opposite side of lens as object

Lens with negative f: same side of lens as object

When is image distance positive?

Positive where light actually goes (on opposite side of lens) (on same side of mirror)

What is a real image?

Light rays converge

i > o → m = -1/o < o

Inverted image

What is a virtual image?

To the observer, the light rays converge → they do not actually

Eye traces them back to where they intersect and tricks you

i < o → m = -1/o > o

Upright image

How is the photoelectric experiment set up?

Light shines on metal surface → energy to metal → loosest bound electrons ejected → captured by high potential detector → register as current in ammeter

What do you expect from the photoelectric experiment?

If light is a wave, in terms of brighter vs dimmer light of any color

Brighter: higher intensity → liberate more electrons → higher current

Dimmer: lower intensity → fewer liberated electrons → lower current

What do you expect from the photoelectric experiment?

If light is a wave, in terms of how long it takes to see a current

Delayed

What do you expect from the photoelectric experiment?

If light is a wave, if the polarity of battery is flipped and voltage is increased, what do we need to do to the light to see current flow?

As detector becomes more negative (it was positive, normally), more and more electrons are repelled → eventually you get no current

The potential of the detector at which you get no current will depend on intensity of the light

What is the result from the photoelectric experiment?

In terms of brighter vs dimmer light of any color

Brighter light meant more current only for short wavelengths

Some show no current, regardless of brightness

What results did they get from the photoelectric experiment?

In terms of how long it takes to see a current

Current was instantaneous, no delay

What results did they get from the photoelectric experiment?

If the polarity of battery is flipped and voltage is increased, what do we need to do to the light to see current flow?

The potential at which the current stops flowing depends on wavelength, not intensity

Why were the results of the photoelectric experiment weird?

Electromagnetic energy is quantized as particles, photons, containing energy E = hf (aka E = hc/wavelength)

h = planck’s constant

f = frequency

Each photon delivers energy to a single electron → ejected photoelectrons have energy KE_max = hf - phi

phi = work function (binding energy) of the metal target

Because delta KE = - delta PE = -eV for electron striking detector, -eV_stop = KE_max

What is the Heisenberg uncertainty principle?

Light behaves as a particle when it interacts with matter

Particles of matter, like electrons, show wave properties (interference and diffraction) under the right conditions

No particle behaves solely as a particle, and no wave solely as a wave

Waves are spread out. If you create a wave with a specific location (central peak with no spread), it is caused by the interference of an infinite number of different wavelengths

Uncertainty in position and momentum (function of frequency) due to waviness of matter is characterized by delta x * delta p >/= h/2pi

delta x = uncertainty from measuring position

delta p = uncertainty from measuring momentum

h = planck’s constant