Physics

J. Gabrielse

Unit: Optics

Glencoe Physics: Chapters 17 & 18

Notes:

1. Light Ray: the path of an extremely narrow beam of light

a. all visible objects emit or reflect light rays in all possible directions

b. people only see objects when light rays from those object enter our eyes

2. Reflection: light changes directions when it bounces off surfaces

a. diffuse reflection: light bounces off molecularly rough surfaces randomly

b. regular reflection: light bounces off mirrored (molecularly smooth) surfaces in parallel beams

i. incident angle (θ_i): the angle that the light hits the surface

ii. reflection angle (θ_r): the angle that the light bounces off the surface

iii. Law of Reflection: the incident angle equals the reflected angle (θ_i = θ_r)

3. Refraction: light changes direction when it passes through different mediums

a. incident ray: the beam in the first medium

b. refracted ray: the beam in the second medium

c. normal: a line perpendicular to the surface

d. angle of refraction: the angle that the refracted ray makes with the normal

4. Snell’s Law:  n_isin(θ_i) = n_r sin(θ_r)

a. n_i = index of refraction of the first (incident) medium

b. n_r = index of refraction of the second (refracting) medium

c. θ_i = angle of incidence

d. θ_r = angle of refraction

5. Total Internal Reflection

a. Light is trapped in a medium when the incidence angle is too big for the light to be refracted into the next medium

b. Fiber optics work by transmitting trapped light through a fiber

6. Ray Diagrams

a. Draw rays coming from the top of the object

i. parallel to the optical axis then reflect/refract through the focal point

ii. to the center of the lens then reflect back or refract straight through

iii. through the focal point and then reflect/refract parallel to the optical axis

b. Sight lines: imaginary rays formed by continuing rays back through a mirror/lens

c. If the light rays don’t converge, draw sight lines

7. Mirrors

a. real image: an image formed by converging light rays

b. virtual image: an image formed by converging sight lines

c. plane mirrors

i. no focal point

ii. always form virtual images

d. concave (curved in) mirrors

i. focal point is half the radius of curvature

ii. inverted real images form if the object is before the focal point and the mirror

iii. virtual images form if the object is between the focal point and the mirror

e. convex (curved out) mirrors

i. focal point is half the radius of curvature

ii. always form shrunken right-side-up virtual images

8. lens/mirror equation:

a. ƒ = focal length (negative if behind the mirror)

b. d_o = object distance (negative if behind the mirror)

c. d_i = image distance (negative if behind the mirror)

9. magnification equation:

a. m = magnification

b. h_i = image height (negative if upside down)

c. h_o = object height (negative if upside down)

d. if the magnification is negative the image is inverted (upside down)

10. Lenses

a. Concave Lens

i. Light rays that come in parallel to the optical axis diverge from the focal point

b. Convex Lens

i. Light rays that come in parallel to the optical axis converge at the focal point