Chapter 8: Light and Optics Flashcards
Electromagnetic waves
Electromagnetic waves are transverse waves because the oscillating electric and magnetic field vectors are perpendicular to the direction of propagation. The electric field and the magnetic field are also perpendicular to each other.
The electromagnetic spectrum
The electromagnetic spectrum describes the full range of frequencies and wavelengths of electromagnetic waves. Wavelengths are often given in the following units: mm (10–3 m), μm (10–6 m), nm (10–9 m), and Å (ångström, 10–10 m). The full spectrum is broken up into many regions, which in descending order of wavelength are radio (109–1 m), microwave (1 m–1 mm), infrared (1 mm–700 nm), visible light (700–400 nm), ultraviolet (400–50 nm), x-ray (50–
10–2 nm), and γ-rays (less than 10–2 nm)
Speed of light
3.0 x 10^8 m/s
To a first approximation—and for
the purposes of all MCAT-related equations—electromagnetic waves also travel in air with this speed. In reference to electromagnetic waves, the familiar equation ν = fλ becomes
c = fλ
where c is the speed of light in a vacuum and, to a first approximation, also in air, f is the frequency, and λ is the wavelength.
To recall the order of the colors in the visible spectrum, remember the grade-school
“rainbow” of ROY G. BV (red, orange, yellow, green, blue, violet).
Visible spectrum
Wavelengths in the visible range are common on the MCAT. Remembering the
boundaries of the visible spectrum (about 400–700 nm) will save you time and energy
on Test Day.
The term blackbody refers to an ideal absorber of all wavelengths of
light, which would appear completely black if it were at a lower temperature than its
surroundings.
Reflection
The law of reflection is
θ1 = θ2
where θ1 is the incident angle and θ2 is the reflected angle, both measured from the normal.
The normal is a line drawn perpendicular to the boundary of a medium; all angles in optics are measured from the normal, not the surface of the medium
Plane mirrors; real or virtual
An image is said to be real if the light actually converges at the position of the image. An image is virtual if the light only
appears to be coming from the position of the image but does not actually converge there.
One of the distinguishing features of real images is the ability of the image to be projected onto a screen
Concave and convex
The passenger-side mirrors in cars are an example of convex mirrors (everything
appears smaller and farther away); the small circular mirrors used for applying
makeup are an example of concave mirrors (everything appears bigger and closer.
Concave mirrors are converging mirrors. Convex mirrors are diverging mirrors. The
reverse is true for lenses.
Any time an object is at the focal point of a converging mirror, the reflected rays will be
parallel, and thus, the image will be at infinity.
Image distance
1/f= 1/o + 1/i =2/r
The focal length (f) is the distance between the focal point (F) and the mirror. Note that for all spherical mirrors, f=r/2 where the radius of curvature (r) is the distance between C and the mirror.
The distance between the object and the mirror is o; the distance between the image and the mirror is i.
On the MCAT, you will most often use this equation to calculate the image distance for all types of mirrors and lenses. If the image has a positive distance (i > 0), it is a real image, which implies that the image is in front of the mirror. If the image has a negative distance (i < 0), it is
virtual and thus located behind the mirror.
Magnification (m)
The magnification (m) is a dimensionless value that is the ratio of the image distance to the object distance:
B= u o I/ 2 pi r
By extension, the magnification also gives the ratio of the size of the image to the size of the object.
The orientation of the image (upright or inverted) can be determined: a negative magnification signifies an inverted image,
while a positive value signifies an upright image. If |m| < 1, the image is smaller than the
object (reduced); if |m| > 1, the image is larger than the object (enlarged); and if |m| = 1, the image is the same size as the object.
The focal length of converging mirrors (and converging lenses) will always be ____
The focal length of diverging mirrors (and diverging lenses) will always be ___
positive, negative
Image types with a single lens or mirror (assuming o is positive): UV NO IR
Image types with a single lens or mirror (assuming o is positive): UV NO IR
Upright images are always virtual
No image is formed when the object is a focal length away
Inverted images are always real
Refraction
Refraction is the bending of light as it passes from one medium to another and changes
speed. The speed of light through any medium is always less than its speed through a vacuum.
speed of light in a vacuum, c, is equal to 3.0 x 10 ^8 m/s
Snells law: Index of refraction
When light is in any medium besides a vacuum, its speed is less than c. For a given medium
(Vb/2)
Refracted rays of light obey Snell’s law as they pass from one medium to another:
n1 sin θ1 = n2 sin θ2
Total internal reflection
Total internal reflection occurs when light cannot be refracted out of a medium and is
instead reflected back inside the medium.
This happens when light moves from a medium with a higher index of refraction to a medium with a lower index of refraction with a high incident angle.
The minimum incident angle at which total internal reflection occurs is called the
critical angle.
