Lecture 6: What is Light?, Lecture notes of Astronomy

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LIGHT

LECTURE 6

Isaac Newton = 17th century; defined light as a stream of tiny particles called “ corpuscles ” that travels in straight lines. =>Once these particles interact with surfaces, it produces different colors and effects. Christiaan Huygens = 17th century; defined light as a wave not particles. He argued that light spreads out similar to waves. Thomas Young = 1800's; demonstrated that light is a wave via the double-slit experiment. => In this experiment, light first passes through a single slit to create a well-defined laser beam. => Then it travels through the double slit where it produces an interference pattern from overlapping light waves. => The screen at the end shows the resulting pattern of different bright and dark bands due to the interference of light waves.

I. INTRODUCTION

B y a n a l y z i n g s t a r l i g h t a n d o t h e r t y p e s o f r a d i a t i o n , w e g e t a l o t o f i n f o r m a t i o n a b o u t t h e s t a r s a n d g a l a x i e s. W h a t i s l i g h t?

=> Violent has the shortest waves because its peaks are closer together. It also bend the most when passing through a prism due to this reason. => Red light has the longest waves since its peaks are farther apart.

I. INTRODUCTION

N e w t o n ’ s L i g h t D i s p e r s i o n Visible light has very tiny wavelength. Ranges from 400 - 700 nm (400-700) x 10 to the power of -9 meters or 0.0004-0.0007 mm. Violet light has a wavelength of 400 nm; smallest wavelength. Red light has the largest/biggest wavelength. Green has the longest wavelength. Order (shortest to longest): Violet, Blue, Green, Yellow, Orange, Red. L i g h t h a v e a w a v e l e n g t h :

High frequency is associated with shorter wavelengths. Low frequency is associated with longer wavelengths. High frequencies commonly identify hot, forming young stars in the solar system, while lower frequencies tend to reveal cooler objects like dust clouds.

I. INTRODUCTION

S p e e d o f l i g h t i s 3 0 0 , 0 0 0 k i l o m e t e r s p e r s e c o n d ( k m / s ). Roughly equivalent to going around Earth 7.5 times. S p e e d o f l i g h t i s 3 0 0 , 0 0 0 k i l o m e t e r s p e r s e c o n d ( k m / s ). F r e q u e n c y ( f ) = n u m b e r o f c y c l e p e r s e c o n d = > 1 / s H e r t s ( H z ). E l e c t r o m a g n e t i c w a v e s c a r r y e n e r g y. R a d i a t i o n c a r r i e s m o r e e n e r g y. G a m m a r a y s a n d x - r a y s a r e l e s s e n e r g e t i c.

II. TYPES OF

RADIATION

Has a wavelengths from (10-400 nanometers). Used to observe the atmospheres of planets, as well as, study young hot stars.

5. ULTRAVIOLET (UV)

RADIATION

The shortest (less than 0. nanometers). Emitted by the most extremely energetic and violent events in the universe (gamma ray bursts, supernova explosions and other extreme cosmic phenomena).

7. GAMMA RAYS

Much shorter than UV Radiation (0.01- nanometers). A type of high-energy electromagnetic radiation with short wavelengths but high frequencies. Essential in studying the behavior of matter in strong gravitational fields, as well as useful in detecting black holes and neutron stars that might be invisible in other wavelengths.

6. X-RAYS

Earth’s atmosphere blocks gamma rays and x-rays.

Ultraviolet is blocked by the Earth’s ozone player.

Only radio waves and visible light can get through.

Human body will emit infrared.

D E M O : W I R E , L I G H T B U L B

+ I N C R E A S E T E M P E R A T U R E

III. TEMPERATURE

Called Absolute Temperature. Measured on the Kelvin (K) scale (starts at absolute zero - being the coldest). Provide a consistent and absolute measure of temperature of stars, planets and other celestial objects. Higher temperature = higher vibration. 273.15 K correspond with 0 degrees Celsius. 373.15 K correspond with 100 degrees Celsius. DEMO: Wire, light bulb + increase temperature. => connect the light bulb to a power source using a wire. => Human body temperature is 300 K. Connect the light bulb to a power source using a wire. Turn on the light bulb and measure the initial temperature of the bulb. Gradually increase the power supplied to the light bulb and observe the changes. => Notice how the electrical power affects the temperature and brightness of the light bulb. => Bulb radiated more energy when power increased. => Bulb emits infrared. => As temperature increases, first color to appear is red; red changed to orange, then yellow, white and blue-ish.

Model is to explain how electrons move around the nucleus of an atom. electrons orbits around the nucleus (much like planets orbiting the Sun) in a specific, fixed paths. The orbits of electrons are based on their energy levels. The closer to the nucleus, the lower the energy level of the electron in that orbit. An electron can jump from one orbit to another by absorbing or releasing energy. If an electron absorbs energy, it jumps to a higher orbit, which is farther from the nucleus. If it gives away energy, it falls back to a lower orbit. E1 and E2 are lower levels (it has to get rid of energy and it will be carried off by protons of life). If it jumps from E3 to E2, it will emit the same wavelengths. If any level jumps to first 1st level, it will emit visible light.

V. BOHR’S

M O D E L O F H Y D R O G E N

A T O M