# Physics 4

## 41 terms

### Describe the two types of waves.

1) Longitudinal: (sound, earthquakes) is one in which the medium is displaced parallel to the direction of wave propagation, such as a sound wave in air.

2) Transverse: (light, radio waves, microwaves, a wave on a string, etc.) I s one in which the medium is displaced perpendicularly ot the direction of wave propagation.

### Provide a definition for the following terms. A) Wavelength B) Period C) Velocity D) Amplitude E) Frequency F) Intensity G) Phase

A) Wavelength: (λ) The distance between any point in the wave to the point where the wave begins to repeat itself. (Trough to trough, peak to peak)

B) Period: (T) The reciprocal of frequency. The number of seconds required for one wavelength to pass a fixed point. T=1/ƒ

C) Velocity: (ν) ν=ƒλ

D) Amplitude: (A) The maximum displacement of the wave from zero. Amplitude is always positive.

E) Frequency: (ƒ) The number of wavelengths that pass a fixed point in one second. Measured in hertz (Hz), or cycles per second 1/s.

F) Intensity: (I) Power in waves

G) Phase: A horizontal shift of a wave on a Cartesian graph. Each wavelength represents 360⁰. So half a wavelength represents 180⁰. Two waves that are the same wavelength, and begin at the same point, are said to be in phase with each other. Two waves that are the same wavelngth but travel different distances to arrive at the same point, will be out of phase if that distance is not some multiple of the wavelength.

### What is Intensity proportional to?

Intensity is directly proportional to the amplitude squared and the frequency squared.

### Describe the difference between the richter scale and the decibal system. (What does a 10 fold increase mean in each system?) What is the equation for the decibel system?

The decibel system is similar to the seismic system, both use log scales. 10-fold increases in earthquake intensity are represented by 1.0 unit increases on the Richter scale and 10-fold increases in sound intensity are represented by 10.0 unit increases on the decibel scale.

A sound10 times more intense is rated as 10 decibels higher, a sound 100 times more intense is rated as 20 decibels higher and a sound 1000 times more intense is rated as 30 decibels higher.

Intensity in Decibels = 10*log(I/I₀)
I = intensity of the sound wave
I₀ = threshold of human hearing (value given)

### What is equation for the velocity of a wave? What is the velocity of light wave and a sound wave?

V=λƒ

V=3 x 10⁸ m/s (light)
V=340 m/s (sound)

### What are the 3 cardinal wave rules (dealing with the medium)?

1) Wave speed (velocity) is determined by the medium
2) Frequency NEVER changes medium to medium
3) Wavelength DOES change medium to medium

### What is the proportionality of velocity in a wave with regards to: A) Mass/unit length or density in a string B) Elasticity of the string C) Bulk modulus of the medium in a sound wave D) Density of the medium in a sound wave E) Temperature

A) Mass/unit length or density in a string: Inversely proportional

B) Elasticity of the string: Directly proportional

C) Bulk modulus of the medium in a sound wave: Directly proportional

D) Density of the medium in a sound wave: Inversely proportional

E) Temperature: Directly proportional in GASES ONLY

### What is constructive interference? What is destructive interference?

Constructive interference is when waves add, increasing amplitude. The sum of the displacements results in a greater displacement.

Destructive interference is when waves subtract, decreasing amplitude. When the sum of the displacements results in a smaller displacement.

After passing through each other, waves that interfere will revert to their original shape, unaffected by the interference.

### What is beat frequency and how do you calculate it?

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Beats occur when two waves with slightly different frequencies are superimposed. At some points they will be nearly in phase and experience constructive interference. At other points they will be out of phase and experience destructive interference. These points will alternate with a frequency equal to the difference between the frequencies of the original two waves. This difference is called the beat frequency.

ƒbeat=|ƒ1-ƒ2|

### What are the formulas for the doppler effect? Describe how to use the doppler effect.

∆f/fs=v/c
∆λ/λs=v/c

∆f = |fo-fs|
∆λ = |λo-λs|
When the relative velocity brings the source and observer closer, observed frequency goes up and observed wavelength goes down.

(s→) (←o) = fo↑ and λo↓
(s←) (→o) = fo↓ and λo↑

If the objects are getting farther apart as time passes, subtract ∆f from fs or add ∆λ to λs. If they are approaching each other as time passes, add ∆f to fs or subtract ∆λ from λs.

(s←) (→o):
fo = fs-∆f
λo = λs+∆λ

(s→) (←o):
fo = fs+∆f
λo = λs+∆λ

The relative velocity v is the net speed at which the source and object are approaching each other. For objects moving in the same direction, subtract their individual speeds; for objects moving in opposite directions, add their individual speeds.

←5 and ←6 = 1
→5 and →6 = 1
→5 and ←6 = 11
←5 and →6 = 11

Objects moving in the same direction at the same speed, there is no doppler effect; the relative velocity is zero, so the change in frequency is zero.

C_light=3x10⁸ m/s
C_sound=340 m/s

### What happens to light in the doppler effect?

White light can shift blue if doppler cause an increase in frequency and red if doppler causes a decrease in frequency.

### What are the two equations for harmonics? What harmonics may be used with each equation?

L=nλ/2 (both ends node or antinode)
Gives all harmonics n=1,2,3,...

L=nλ/4 (one end node and one end antinode)
Gives only the odd harmonics n=1,3,5,...
The evens cannot be used because in the equation the L can never be 1/2λ, λ, etc.

### What happens with the frequency at each harmonic?

Each harmonic will have its own new frequency. The frequency is always n*fundamental frequency (i.e., if the firest harmonic is exactly 200 Hz, the 2nd is 400 Hz, the 3rd is 600 Hz, etc.)

### How many antinodes are added with each harmonic?

Each harmonic will have one more antinode than the previous one.

### What is unique about the 2nd harmonic?

The 2nd harmonic is unique, because this is the only time L=λ.

### Draw the first three harmonics for a string attached at only one end. Why can't this have even harmonics?

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The evens cannot be used because in the equation,
L=nλ/4, the L can never be 1/2λ, λ, etc.

### What is the fundamental wavelength?

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The fundamental wavelength is at the first harmonic.

### Describe the particle theory of light. Describe the wave theory of light. Describe einsteins role and what he discovered.

Energy transformation properties of light are usually best described by particle theory. Propagation of light can be described with the wave theory.

Einstein discovered the photoelectric effect thus showing the particle property of light. The photoelectric effect shows that photons exist. He described light as composed of discrete quanta, now called photons, rather than continuous waves.

The photoelectric effect was discovered by shining light on the surface of metal and having electrons emitted. The removal of electrons is solely controlled by the frequency of the light.
KE=hf-φ
φ=hf₀
f₀=minimum (threshold) frequency of the photon required to produce photoelectric emission

### What is the equation to calculate index of refraction and what does it mean.

n=c/v
Light is slower when propagating through a medium. The speed of light propagating through some medium is found using a constant for that medium, called the index of refraction (n). The index of refraction compares the speed c of light in a vacuum to the speed v of light in a particular medium. Since nothing exceeds the speed of light in a vacuum, all media have a refractive index greater than one. The greater the index of refraction for a medium, the slower the light moves through that medium. Water=1.3
Glass=1.5

### What is the order and relative intensity of each class of electromagnetic radiation. How does frequency and wavelength increase or decrease as you move along the spectrum in either direction? Place the following in order of increasing frequency: A. Gamma rays B. Long Waves C. Ultraviolet D. Radio Waves E. X-rays F. Microwaves G. Infrared H. Visible Light

Intensity increases with increasing frequency and decreasing wavelength.

Frequency increases as you move right along the spectrum. Wavelength decreases as you move right along the spectrum.

Wavelength begins with 10⁸ and moves to 10⁻¹⁶
Frequency begins with 10¹ and moves to 10²⁴

Long waves
Microwaves
Infrared
Visible light
Ultraviolet
X-rays
Gamma rays

### What is the visible light spectrum? What is the color order?

390-700nm
ROYGBIV
Red is the lowest energy (frequency) and longest wavelength; violet is the highest energy and shortest wavelength.

### What happens to the frequency and wavelength of light when it crosses into a new medium?

When light crosses into a new medium, the frequency remains the same and the wavelength changes. If the medium's index of refraction is higher, the wavelengths become shorter (velocity decreases); if the index is lower, then the wavelengths become longer (velocity increases).

### What is snell's equation and how is it used? If light hits a denser medium at an angle, will it bend toward the normal line or away from it? When light goes from a denser to a less dense medium which way will it bend?

n₁sinθ₁=n₂sinθ₂

Imagine you are on an ATV. Being in the denser medium (↑n) is like driving in deep mud and being in the less dense medium is like driving on asphault (↓n). If you hit mud at an angle, you'll pivot one direction or the other-depending on which wheel hits the mud, or gets out of the mud, first.

The light will bend towards the normal line when it hits a denser medium.

The light will bend away from the normal line when it hits a less dense medium.

E=hf
E=hv/λ
(v=fλ)

### What are the three types of mirrors?

Concave: The image is Negative (behind the mirror), Virtual, and Upright. It is also diminished in size.

Convex: The image is Positive, Real, Inverted (When the object is placed outside the focal point)

Plane: The image and the object will always be equal distances on either side of the mirror.

### Is a converging lens concave or convex? What type of an image does a converging lens produce? What does a converging lens look like?

CONVerging lens is a CONVex lens, but acts like a concave mirror.

It produces a positive, real, inverted image when the object is outside of the focal point. When the object is inside the focal point it produces a negative, virtual, upright image.

ThiCker Center Converges

### Is a diverging lens concave or convex? What type of image does a diverging lens produce? What does a converging lens look like?

Diverging lens is a concave lens, but acts like a convex mirror.

It ALWAYS produces a negative, virtual, upright image.

Thinner center diverges

### Provide a conceptual definition of converging and diverging lenses.

Converging lenses converge light. Diverging lenses diverge light.

### What type of lens is in the eye? In a magnifying glass?

Eye: Converging
Magnifying Glass: Converging

### What is assumed about the light rays when dealing with lens/mirrors?

We assume parallel light rays, which is approximated by light emanating from a very distant object. The image will occur at the focal point. With non-parallel light the image will not occur at the focal point.

f_mirror=1/2r

### What is the thin lens equation? What are the 4 lens/mirror rules?

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1/f=1/d_i + 1/d_o

1) Object distances (d_o) are ALWAYS (+)
2) Image distances (d_i) or focal point distances (f) are (+) if they are on the same side as the observer and (-) if they are on the opposite side.
3) The observer and the object are on the same side for a mirror and on opposite sides for a lens (you have to be behind your glasses to see thru them to view the object on the other side).
4) PRI/NVU: "Positive, Real, Inverted" and "Negative, Virtual, Upright" always stay together. You only need to know one and you know the other two.

### How do you calculate the magnification of an object? What does a negative value indicate? What are the 4 lens/mirror rules?

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M=-(d_i)/(d_o)=(h_i)/(h_o)

A negative magnification indicates the image is inverted.

1) Object distances (d_o) are ALWAYS (+)
2) Image distances (d_i) or focal point distances (f) are (+) if they are on the same side as the observer and (-) if they are on the opposite side.
3) The observer and the object are on the same side for a mirror and on opposite sides for a lens (you have to be behind your glasses to see thru them to view the object on the other side).
4) PRI/NVU: "Positive, Real, Inverted" and "Negative, Virtual, Upright" always stay together. You only need to know one and you know the other two.

### What does near sighted mean? Where is the image formed in a near-sighted individual?

Near-sighted = Able to focus clearly on close objects, but not on distant objects.

Far images are formed IN FRONT of the retina.

### What does Far sighted mean? Where is the image formed in a Far-sighted individual?

Far-sighted = Able to focus clearly on distant objects, but not on close objects.

Close images are formed BEHIND the retina.

### Why does the image move in front of or behind the retina for near or far sighted people?

Nearsighted: See close don't see far (image converges before retina)
Farsighted: See far don't see close (image converges after retina)

The lens assumes a large curvature (short focal length) to bring nearby objects into focus and a flatter shape (long focal length) to bring a distant object into focus.

In far-sighted a weakening of the ciliary muscles and/or the decreased flexibility of the lens leads to a lens that can no longer assume the high curvature that is required to view nearby objects.

In near-sighted a larger curvature lens causes distant objects to converge before the retina. More likely, it is usually the result of a bulging cornea or an elongated eyeball.

P = 1/f

### How do you calculate magnification and Power in a two lens system?

Magnification: M=m₁m₂
Power: P= p₁ + p₂

### 3. Which of the following statements is/are true of single lens systems? I. All objects are positive II. All images and focal points on the same side of the lens or mirror as the observer are positive. III. All images and focal points on the opposite side of the lens or mirror as the observer are negative. IV. Negative, virtual images are always upright. A. I & II B. I & IV C. I, II & IV D. I, II, III, & IV

3) D ; All of these statements are true. REMEMBER THEM! Objects are always positive as long as it is a SINGLE lens system. As far as everything else is concerned, if its on the same side as the observer, it is positive and if its on the other side its negative. Just make sure that you correctly decide which side is the observer. Statement IV reiterates the fact that NVU and PRI are never violated.

### 5. A child is viewing an ant thru a magnifying glass which approximates a single converging lens. If the child holds the lens 0.5 meters away from his eye, how far should he hold it away from the ant to magnify the ant by a factor of 5? A. 10 cm B. 50 cm C. 2.5 m D. 5.0 m

5) A ; This is an excellent problem to test whether you are actually conceptualizing all of the lens stuff or just memorizing. The distance from the lens to the ant will be do in the equation M = -di/do, and the distance to the child's eye (or retina to be exact) will be the di term. If you can't correctly identify these in the story problem, you'll get it wrong. Plug 5 into the equation for M and solve for the unknown object distance (or "how far should he hold it away from the ant"). WATCH your signs whenever you use lens equations. The distance to the image will be positive because for a converging lens the image and the observer are on the same side. When we solve, this gives us a negative number for distance to the object . . . but I thought all objects were positive! They are, but inverted images are represented by a negative magnification or a negative image height, so your 5 really should be -5, which would give the expected positive object distance.

### 25. Which of the following best describes the propagation of a sound wave at the molecular level? A. molecules of air are compressed into resonating perpendicular bands that push the molecules outward continuously from the source B. molecules vibrate up and down in a cyclical pattern that can be represented by a sine wave C. molecules emanating from the source collide with neighboring molecules, pushing those molecules forward as the original molecules bounce back toward the source. D. molecules compress and relax much like a spring obeying Hooke's Law

25) C; MCAT absolutely LOVES questions like these. This is why we always tell you to know the why and how as much as the what. Answer A is false, but is how most people think of compressional waves based on the drawings they see. We erroneously think that the compressions are actually pushing bands of matter outward from the source; which is not the case. Answer B is false because the molecules do not vibrate up and down perpendicular to the direction of propagation, that is what happens in a transverse wave. Answer D would be true of individual bonds within a molecule, but not of air as a whole. Answer C is the correct answer and describes what actually happens. The molecules only get displaced for a short distance near their original origin; they then quickly run into other molecules, causing them to bounce back to their original positions and the other molecules to continue forward.