MCAT Study Guide Physics Ch. 11 – Electromagnetic Waves 2017-08-15T06:45:05+00:00

## I.          11.1:  ELECTROMAGNETIC WAVES

### A.     EM WAVES

1.     EM wave – created by an oscillating electric charge, composed of oscillating electric and magnetic fields that oscillate with the same frequency at which the electric charge oscillated at

2.     Polarization – the direction in which the electric field oscillates

a)     Most EM waves have electric fields oscillating all all 丄 directions and are ∴ unpolarized

3.     Speed of light in a vacuum:

a)     c = 3 x 108 m/s

4.     Electromagnetic spectrum – the range of EM frequencies

a)     Visible light λ ranges from 400 nm (violet) and 700 nm (red) ### B.     PHOTONS

1.     Photons – unit of energy (E) from EMR

a)     E = hf = h(c/λ)  → h = Planck’s constant

2.     Wave-particle duality – EMR travels like a wave, but interacts with matter like a particle

3.     Interesting note:

a)     In waves, energy is proportional to amplitude2, while in EMR, energy is ∝ to frequency

## II.          11.2:  REFLECTION AND REFRACTION

### A.     REFLECTION

1.     When a beam of light hits water, some light will be reflected, some will pass through the water

2.     Law of reflection – the angle of reflection is equal to the angle of incidence

a)     Note that the angle of incidence is measured from a line 丄 to the medium (called the normal), not the medium itself

3.     Angle of refraction – the angle between the refracted ray and the normal

### B.     INDEX OF REFRACTION 1.     Index of refraction tells us how much slower light travels through that medium than through empty space

a)     Index of refraction = (clight in vacuum)/(clight in medium)

b)     n = c/v

2.     Law of refraction:

a)     n1sinθ1 = n2sinθ2

### C.    TOTAL INTERNAL REFLECTION

1.     When light wave traveling through a medium with high refractive index approaches a medium of lower refractive index, it may or may not escape into the second medium and total internal reflection occurs; depends upon the refractive indices and the angle of approach

2.     Critical angle – the angle that light will undergo total reflection (no refraction)

a)     sinθcrit = n2/n1

(1)   n1 → first medium’s refractive index, n2 → second medium’s refractive index

(2)   The larger the angle (from normal), the closer toward critical angle it becomes

## III.          11.3:  WAVE EFFECTS

### A.     DIFFRACTION

The “bending” of waves around an object or a slit

### B.     POLARIZATION

1.     Polarized light – light whose direction of polarization has been restricted somehow (usually the electric fleld components of the wave vibrate in all planes

### C.    DISPERSION

1.     Waves of light at different frequencies travels through medium (like water, glass) at slightly different speeds, cause the light to be dispersed (think Pink Floyd)

a)     Index of refraction of red light is different than blue light

2.     This is the exception to the Big Rule 1 for waves

## IV.          11.4:  MIRRORS

### A.     CURVED MIRRORS

1.     Focus – halfway point to the center of curvature, or radius

2.     Focal length:

a)     f = ½r

(1)   (+) f → f is on the same side as the observer

(2)   (-) f → f is on the other side of the mirror as the observer

3.     Image formation – we see an image at the point where the reflected rays intersect (concave mirror) OR where the reflected rays seem to intersect (from behind the mirror in convex mirror)

### B.     THE MIRROR EQUATION

Answers the first two of the following questions

1.     4 questions:

a)     Where is the image?

b)     Is the image real or virtual (Do light rays intersect at the image, or just seem to? Compare concave image

[real] vs convex image [virtual])

c)     Is the image upright or inverted?

d)     How tall is the image?

2.     Mirror and lens equation:

a)     1/o + 1/i = 1/f

(1)   o → object’s distance from the mirror (always positive)

(2)   i → image’s distance from mirror (+ if on same side as observer)

(3)   f → focal length of the mirror (+ if on same side as observer)

(a)   If the mirror were spherical, where would the center be? This is the side that f will be on

(b)   f is (+) for concave mirrors and (-) for convex mirrors

### C.    THE MAGNIFICATION EQUATION

1.     m = -i/o

a)     (+) m → image is upright

b)     (-) m → image is inverted

2.     Note – real images are inverted, virtual images are upright

## V.          11.5:  LENSES

### A.     BASICS 1.     For lenses, f is on the opposite side as the analogous mirrors!

2.     Also, real images are on the opposite side of the lens, while virtual images are on the same sided

a)     f is (-) for diverging (concave) lenses

b)     f is (+) for converging (convex) lenses

### B.     LENS POWER

1.     P = 1/f

a)     P → power, in diopters (D)

2.     The combination of two lenses = Σ diopters # 10.

#### Ch. 11 Reflection + Refraction

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