Physics 504

Chapter 1Geometric Optics

Alexander Pope

•Nature and Nature’s laws lay hid in night;

•God said, Let Newton be! •And all was light.

•Epitaph intended for Sir Isaac Newton

Light

• Light is an electromagnetic disturbance whose propagation allows the transportation of light energy.

• Light is considered to travel as a wave AND as particle or photon.

• As a wave, light has different frequencies or cycles per second (cps)

• The unit of frequency is a hertz (Hz).

My Brain Hertz• Frequency = # Cycles / Time (s)

• Period = Time (s) / # Cycles

• So, Period = 1 / Frequency

• Wave Velocity = Wavelength x Frequency

• v (m/s) = λ (lambda) (m) x f (Hz)

• For electromagnetic waves:

• c = λ f

• c (speed of light) = 3x10 8 m/s

Example• If a water tap drips 150 times a minute,

and the distance between the waves is 6 cm, what is the velocity of the waves?

• Wave frequency = # cycles / time• = 150/60s • = 2.5 Hz• Wave velocity, v = λf • = 0.06 m x 2.5 Hz• = 0.15 m/s

Speed of light

• Light travels at a definite speed (c);• c = 3x108 m/s = 300000 km/s• Distance = speed x time• Calculate the distance (km) travelled in:

• 1 minute• 1 hour • 1 day • 1 year - 365.25 days – 1 lightyear

Types of Light Sources• Incandescent: materials emit light when heated to a

high temperature. E.g. Stove element, light bulb• Luminescent: light is emitted through a reaction with

little or no heat released. E.g . Glow sticks, fire flies are bioluminescent

• Fluorescent: materials release light because of the absorption of high energy radiation. E.g. fluorescent light bulbs

• Phosphorescent: similar to fluorescent except that the light continues to be emitted after the source of radiation has been removed. E.g. glow-in-the-dark toys

Light objects

• Luminous: materials that can emit light. E.g. the sun, stars

• Non-luminous: objects that can only be seen by reflected light. E.g. the moon

Light Phenomena• Reflection: light rays bouncing off a face

• Refraction: light rays bending

• Diffusion: light scattering off dust

• LASER = Light Amplification by the Stimulated Emission of Radiation

• Diffraction: light spreading out

• Absorption: light absorbed into objects

• Dispersion: light separating into colours

Transmission of light• Recti-Linear propagation: light travels in straight

lines;• Ray: the path taken by light energy;• Ray diagrams: pictures that show the paths of

light rays;• Diverge: light rays become farther apart;• Converge: light rays come closer together;• Parallel rays: rays that travel the same distance

apart;• Speed of light: c = 3x108 m/s

Pinhole Camera Image

Pinhole camera

• Pinhole cameras demonstrate that light travels in straight lines;

• An image is formed inverted vertically and horizontally on the back of the

camera• The image will be inverted,smaller and real.

Pinhole camera• Magnification = height of image

height of object

• Or, M = h i/ h o

• As well, M = distance to image

distance to object

• Or, M = d I/d O

• So h i/ h o = d I/d O

QuestionA pinhole camera is used to show the image of a turtle on its screen as shown below.

Screen

Pinhole camera

Which of the following is the image of the turtle that will appear on the screen of the pinhole camera?

A)

C)

B)

D)

Question

Alex places a 5.00 cm tall candle 30.0 cm in front of a pinhole camera that has a length of 50.0 cm.What is the magnification of the candle?

A) 1.66

B) 0.166

C) 0.63

D) 0.1

Shadow• Umbra: the area of total

shadow – this area receives no light;

• Penumbra: the area of

partial shadow – this area

receives some light;

• E.g. lunar/solar eclipses

Question

When only the Moon's penumbral shadow strikes Earth, we see a partial eclipse of the Sun from that region.

Sun Moon Earth

A B C

In which region(s) on Earth do we see a partial eclipse of the Sun?

A)

Region A only

B)

Region B only

C)

Regions A and C

D)

Regions A, B and C

Activity

• Page 35, Q. 2,6,7,8,9

Summary• A luminous light source emits its own light.

• A luminous light source can produce its light by incandescence, luminescence, fluorescence, or phosphorescence.

• Light striking an object can be reflected, refracted, absorbed or any combo of the 3.

• Other light phenomena: diffraction, diffusion, scattering, interference, dispersion.

• Light travels in straight line ray paths – rectilinear propagation.

Summary

• When an object creates a shadow, the area of total shadow is the umbra, the area of partial shadow is the penumbra.

• For a pinhole camera, M, the magnification is given by

• M = hi/ho = di/do.

LifeSaversWintOGreen Flash

• Urban Myth? – LifeSavers sparkle

• When sugar crystals are crushed or separated, tiny electrical fields are created, separating pos. and neg. charges which spark when they recombine

• This is Tribo-luminescence.