Waves and Optics Vocabulary

Flashcards covering key vocabulary and concepts from lecture notes on waves, light, and optics.

Oscillations

Periodic back and forth motion around a central point

Electromagnetic waves

Waves that can travel through empty space; examples include light, microwaves, and X-rays.

note: always transversive

Transverse wave

Waves in which the energy goes left and right, and oscillations are 90 degrees to the direction of wave propagation.

• Direction of the wave is perpendicular to the direction of vibration of the particles

Longitudinal wave

Waves in which the oscillation/vibration of particles is along the wave propagation direction.

• Waves that’s parallel to the direction of the vibration of the particles

Amplitude

Maximum displacement from equilibrium.

Wavelength

Distance from crest to crest.

Period (T)

Time to complete one cycle, measured in seconds.

Wave speed

How fast a wave moves through a medium.

Tension

How much force is pulling on a string.

Photons

Light is made up of…

Reflection

Light bouncing off of something.

Refraction

Light that bends or changes direction.

Incident ray

Ray coming in.

Reflected ray

Ray that’s reflected.

Plane mirror

Flat mirror.

Normal line

Straight line perpendicular from surface.

Specular reflection

Reflection off smooth surface where parallel rays remain parallel.

Diffuse reflection

Reflection off rough surface where rays come out at different angles.

Index of Refraction (n)

Ratio of the speed of light in a vacuum to its speed in a medium.

Wave

a disturbance or vibration that travels through a medium (like air, water, or a string)

How waves carry energy

vibrations in the particles of the medium they travel through (for mechanical waves) or by propagating disturbances in electric and magnetic fields (for electromagnetic waves)

Wave propagation

process of a disturbance traveling through a medium, carrying energy

Frequency (f)

number of wave crests passing a point, per x amount of time

• how many complete waves pass in a given amount of time

Units: cycles / second, or Hertz (Hz)

Period

Time for complete wave to complete its cycle

• Units: seconds

Mechanical wave

Have a material medium (matter)

—> needs a medium to travel through

• Ex: Sound/water

Electromagnetic waves

medium is the electromagnetic field (which can exist in a vacuum, aka no space)

• doesnt need a medium, can travel freely

Displacement - position/distance graphs

• show wavelength

Displacement - time

• shows period of a wave

Wave speed

As wave is the disturbance in a medium, this is the speed of disturbance.

Concept:

Waves can have different frequencies/wavelengths, …

but still move at same speed

Concept:

Higher frequency = …

shorter wavelength

Concept:

Lower frequency = …

longer wavelength

Light and Electromagnetic (transverse) Wave speed in empty space

3.00 x 10⁸ m s^-1

Mechanical Waves - medium: String, wave speed depends on . . . 

Higher tension → faster speed

Mechanical Waves - medium: Fluid, wave speed depends on . . . 

• Density, viscosity 

Mechanical Waves - medium: Sound, wave speed depends on . . . 

• air pressure, temperature, density

  • Usually around 340 - 350 ms^-1 under normal conditions in air.

  • Faster in denser media

Concept:

Sound travels faster in denser mediums because

molecules are packed closer together so when a particle within the medium vibrates, it transfer its energy more efficiently from one particle to the next as they are closely packed

Concept

Regions of higher pressure =

more particles = high density

Compression

when the longitudinal wave is compressed - has more density

Concept

Regions of lower pressure =

fewer particles = lower density

Rarefaction

when longitudinal wave is stretched, not compressed, less density.

Diffraction

the spreading out of a wave

EM Radiation

oscillating electric and magnetic fields,

• doesn’t need medium

High frequency EM waves has

more energy

Low frequency EM waves carry

less energy

Reflection

light bouncing off of something

The ray-wave connection

• Rays show the direction in which waves travel

• When using rays, we typically ignore wave aspects 

Light rays

travel in straight lines from their source, cross one another but doesnt interact, travels forever

Concept:

At an interface (new material), light can be

reflected or refracted (path bends)

A denser medium tends to

slow down the wave (mechanical, except sound) because there are more particles the wave has to move through. Hence, the wave has to do more work to displace the particles, slowing down its speed.

Plane Mirror

Flat mirror

Incident Ray

Ray that’s coming in

Reflected Ray

Ray that has bounced off the surface, going out

Angle of Incidence

Angle between the incident ray and normal line

Angle of Reflection

Angle between the reflected ray and the normal line

Concept - Laws of reflection

The angle of incidence

equals the angle of reflection

Concept - Laws of reflection

The incident ray, the reflected ray, and the normal all

lie in the same plane (same 2-D plane).

Specular Reflection

Reflection from a flat reflective surface.

—> The angles of incidence for these rays are all identical. 

• Angles of reflection are equal

• If incident rays are parallel, then the reflected rays will also be parallel 

Diffuse Reflection

Occurs on surfaces with irregularities/rough/not flat

• Normal lines are in different directions

• Incident rays all have  different angles of incidence.

• Reflected rays  travel in different directions

—> can’t see your reflection because reflected rays are reflecting in many different directions

Refraction

change in direction of a light ray when it travels through a medium

—> the ray that’s leaving is not reflective with the incidence ray

Causes of refraction

• Light rays refract when passing from one medium to another, because of the change in speed.

  • what causes change in speed: density of new medium

• Wavelength changes, but frequency is not changed. (bc frequency doesn’t change)

Frequency (f) generally remains

unchanged. The frequency is determined by the source of the wave.

Concept:

Light rays that slow down will

bend towards the normal. Decrease in wavelength.

Concept:

Light rays that speed up will

bend away from the normal. Increase in wavelength.

Index of Refraction (for a medium)

the ratio of speed of light in empty space, to the speed in that particular medium.

n=c/v

• Index of refraction will always be greater than or equal to 1

• no units

Concept:

The higher the refractive index

the slower the light travels

Substances with a higher index of refraction mean

they are more optically dense. Therefore light will have lower speed in these mediums.

n1 and n2 represents:

index of refraction in medium 1 and 2

Total internal refraction

Light at a boundary is completely reflected, and none is refracted. 

• from denser medium to less dense medium

• When it happens: typically at 50-60^o

• Light speeds up at the boundary

Concept:

As the incident angle increases

The angle of refraction also increases

critical angle

the specific angle of incidence at which the angle of refraction is 90 degrees. Beyond this angle, total internal reflection occurs

Concept

At angles greater than the critical angle, we have

total internal reflection. 

Concept: To calculate critical angle

Use Snell’s law, with refracted angle θ₂ = 90^o

In this case we call the incident angle the critical angle

Apparent depth

Illusion that makes an object submerged in water appear shallower than its actual depth when viewed from above

Concept:

Apparent depth is due to

the increase in light speed when light rays travel from a dense medium to a less dense medium. as this happens, the light will bend (aka, refract), causing it to appear to come from a position closer to the surface