Purpose of this Minilab

Purpose of this Minilab • Apply the basics of ray tracing to learn about reflection and refraction of light.

Activity 1: Light Reflection at Plane Surfaces Angle of incidence Angle of reflection Index of refraction of the two materials ni nt Angle of transmission (refraction)

…..the laws…. Law of Reflection: Snell’s Law of Refraction: Incident, reflected, and transmitted ray lie in one plane. Verify the law of reflection using a plane mirror. Verify your homework result on a 90 plane mirror.

Checking the law of reflection with a plane mirror Polar graph paper 45 Qr 0 90 Qi Light Source 45 135 90 180 135 Mirror

Measuring refraction Polar graph paper 45 0 90 Use Snell’s law to determine nplastic. Qi Light Source 45 135 Light must hit the center of the flat side Qt 90 180 135 Semicircular lens nplastic

Measuring angle of total internal reflection Polar graph paper 45 0 90 45 135 Light must hit the center of the flat side Light Source Qcrit 90 180 135 Semicircular lens

Snell’s Law for Critical Angle =1

Light beam displacement by plane parallel plate Light Source Q Q’ t d

Light beam displacement by plane parallel plate Polar graph paper 90 Light Source 135 45 180 Q 0 Q’ Let the beam hit the rectangle in center of the polar paper t 135 45 d • Trace light ray on polar graph paper. • Outline location of rectangular plastic on paper. • Measure angles Q and Q’. • Measure widths d and t. 90

Light beam displacement by plane parallel plate • Use one incident angle Q (and corresponding Q‘ and d and t) •  calculate n. • Use this calculated n to predict the displacement d for a different incident angle. • (Hint: You will also need to use Snell’s Law for this calculation.) • Verify experimentally d for the new angle.

Activity 2: Reflection and Refraction at Spherical Surfaces Getting the radius R of a concave mirror Concave mirror, reflecting side here. R x D

Alternative method to get R ….. Polar graph paper 90 135 45 Move mirror until curvature matches the curvature on polar graph paper. then measure R as shown. 180 0 R 135 45 90

Finding the focal point of the concave mirror Regular graph paper: Trace the rays and determine f. Light Source parallel rays f

Finding the focal point of the convex mirror Regular graph paper: Trace the rays and determine f. Extend the light rays backward to where they seem to come from. Light Source Virtual image (isn’t really there). parallel rays f

Imaging with the convex mirror Regular graph paper: Trace the rays and determine f. Here is our object point Light Source S P Semicircular lens

Thin Lens Equation (how to calculate focal length from the radii of a lens and it’s index of refraction) Each lens has two interface with the air (#1 and #2). Interface #1 is the one that is encountered by the light when entering the lens. Interface #2 is the one that is encountered by the light when exiting the lens. Interface #1 has radius R1. Interface #2 has radius R2.

Thin Lens Equation (how to calculate focal length from the radii of a lens and it’s index of refraction) Sign rules for R1: R1 negative R1 positive R2 positive R2 negative

Example of using the lens equation A double concave lens (concave on interface #1 and also on #2) with both radii being 5cm and the index of refraction n=1.65 : • R1 = - 5 cm and R2 = + 5 cm

The Imaging Equation for Lenses and Mirrors S: Object Distance P: Image Distance f: Focal Length For Mirrors: where R = Radius of Mirror

Sign Rules For Lenses and Mirrors f Means: a positive number Convex Lens: + Concave Lens: - Convex Mirror: - Concave Mirror: + Most objects are real. Real objects: S is positive Virtual objects: S is negative Real images: P is positive Virtual images: P is negative

Example of signs for f, S, and P Convex mirror: f is negative Real object Light Source Virtual image S P positive negative

Using the Desk Lamp Lamp Plug (black) must be plugged into dimmer plug. Dimmer plug (white) must be plugged into power outlet. Dimmer On/Off switch of lamp

Purpose of this Minilab