purpose of this minilab

Optics 1 - Activities with Light Rays

Purpose of this Minilab

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

Optics 1 - Activities with Light Rays

Activity 1: Light Reflection at Plane Surfaces

i r

t

ni

nt

Index of refractionof the two materials

Angle of incidence Angle of reflection

Angle of transmission (refraction)

Optics 1 - Activities with Light Rays

ir Law of Reflection:

Snell’s Law of Refraction: ttii nn sinsin

Incident, reflected, and transmitted ray lie in one plane.

…..the laws….

.

Optics 1 - Activities with Light Rays

Checking the law of reflection with a plane mirror

0

45

45

90

90

135

180

135

Light Source

Polar graphpaper

i

r

Mirror

Optics 1 - Activities with Light Rays

Measuring refraction

0

45

45

90

90

135

180

135

Light Source

Polar graphpaper

i

t

Semicircularlens

Light musthit the centerof the flat side

Use Snell’slaw to determinenplastic.

nplastic

Optics 1 - Activities with Light Rays

Measuring angle of total internal reflection

0

45

45

90

90

135

180

135

Light Source

Polar graphpaper

crit

Semicircularlens

Light musthit the centerof the flat side

Optics 1 - Activities with Light Rays

Snell’s Law for Critical Angle

90sinsin aircriticalplastic nn

criticalplasticn

sin1

=1

Optics 1 - Activities with Light Rays

Light beam displacement by plane parallel plate

Light Source

i

t

dt

Optics 1 - Activities with Light Rays

0

45

45

90

90

135

180

135

Polar graphpaper

Light beam displacement by plane parallel plate

Light Source

i

t

d• Trace light ray on polar graph paper.• Outline location of rectangular plastic on paper.• Measure angles i and t.• Measure widths d and t.

t

Let the beam hit therectangle in centerof the polar paper

Optics 1 - Activities with Light Rays

Light beam displacement by plane parallel plate

t

ii n

tdcos

cos1sin

• Use one incident angle i (and corresponding t 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.

Optics 1 - Activities with Light Rays

180

Polar graphpaper

R0

45

45

90

90

135

135

Move mirror untilcurvature matchesthe curvature onpolar graph paper.then measure Ras shown.

Activity 2: Reflection and Refraction at Spherical Surfaces – Getting the Radius of Curvature

Optics 1 - Activities with Light Rays

Finding the focal point of the concave mirror

Regular graph paper: Trace the rays and determine f.

Light Source

parallel rays

f

Optics 1 - Activities with Light Rays

Finding the focal point of the convex mirror

Regular graph paper: Trace the rays and determine f.

Light Source

parallel rays

f

Extend the light rays backward to where they seem to come from.

Virtual image(isn’t reallythere).

Optics 1 - Activities with Light Rays

Imaging with the convex mirror

Regular graph paper: Trace the rays and determine f.

P

Light Source

Semicircular or Circular lens

Here is ourobject point

S

Optics 1 - Activities with Light Rays

Thin Lens Equation (how to calculate focal length from the radii of a lens and it’s index of refraction)

21

1111RR

nf

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 hasradius R1.

Interface #2 hasradius R2.

Optics 1 - Activities with Light Rays

Thin Lens Equation (how to calculate focal length from the radii of a lens and it’s index of refraction)

21

1111RR

nf

Sign rules for R1:

R1 positive R1 negative

R2 negativeR2 positive

Optics 1 - Activities with Light Rays

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

cmcmcmcmRR

nf 5

25.15

)2(65.05

151165.11111

21

cmf 4

Optics 1 - Activities with Light Rays

The Imaging Equation for Lenses and Mirrors

fPS111

S: Object DistanceP: Image Distancef: Focal Length

2Rf For Mirrors: where R = Radius of Mirror

RPS211

Optics 1 - Activities with Light Rays

Sign Rules For Lenses and Mirrors

Convex Lens: +Concave Lens: -Convex Mirror: -Concave Mirror: +

f

Real objects: S is positiveVirtual objects: S is negative

Real images: P is positiveVirtual images: P is negative

Means: a positive number

Most objects are real.

Optics 1 - Activities with Light Rays

Example of signs for f, S, and P

P

Light Source

S

Real object

Virtual image

positive negative

Convex mirror: f is negative

Optics 1 - Activities with Light Rays

Using the Desk Lamp

Dimmer

Lamp Plug (black) must be pluggedinto dimmer plug.Dimmer plug (white) must be pluggedinto power outlet.

On/Offswitchof lamp