Optics 1 - Activities with Light Rays Purpose of this Minilab Apply the basics of ray tracing to learn about reflection and refraction of light

$Page 1: Optics 1 - Activities with Light Rays Purpose of this Minilab Apply the basics of ray tracing to learn about reflection and refraction of light$
Optics 1 - Activities with Light Rays

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

i r

t

ni

nt

Index of refractionof the two materials

Angle of incidence Angle of reflection

Angle of transmission (refraction)


ir Law of Reflection:

Snell’s Law of Refraction: ttii nn sinsin

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

…..the laws….

.


Checking the law of reflection with a plane mirror

0

45

45

90

90

135

180

135

Light Source

Polar graphpaper

Qi

Qr

Mirror


Measuring refraction

0

45

45

90

90

135

180

135

Light Source

Polar graphpaper

Qi

Qt

Semicircularlens

Light musthit the centerof the flat side

Use Snell’slaw to determinenplastic.

nplastic


Measuring angle of total internal reflection

0

45

45

90

90

135

180

135

Light Source

Polar graphpaper

Qcrit

Semicircularlens

Light musthit the centerof the flat side


Snell’s Law for Critical Angle

90sinsin aircriticalplastic nn

criticalplasticn

sin

1

=1


Light beam displacement by plane parallel plate

Light Source

Qi

Qt

dt


0

45

45

90

90

135

180

135

Polar graphpaper


Light Source

Qi

Qt

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

t

Let the beam hit therectangle in centerof the polar paper



t

ii n

tdcos

cos1sin

• Use one incident angle Qi (and corresponding Qt 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.


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


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


Light Source

parallel rays

f

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

Virtual image(isn’t reallythere).


Imaging with the convex mirror


P

Light Source

Semicircular or Circular lens

Here is ourobject point

S


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

21

111

1

RRn

f

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.


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

21

111

1

RRn

f

Sign rules for R1:

R1 positive R1 negative

R2 negativeR2 positive


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

25.1

5

)2(65.0

5

1

5

1165.1

111

1

21

cmf 4


The Imaging Equation for Lenses and Mirrors

fPS

111

S: Object DistanceP: Image Distancef: Focal Length

2

Rf For Mirrors: where R = Radius of Mirror

RPS

211


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.


Example of signs for f, S, and P

P

Light Source

S

Real object

Virtual image

positive negative

Convex mirror: f is negative


Using the Desk Lamp

Dimmer

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

On/Offswitchof lamp

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Optics 1 - Activities with Light Rays Purpose of this Minilab Apply the basics of ray tracing to learn about reflection and refraction of light