Light and Matter

Light

• electromagnetic radiation

• Light is the messenger

• Both a wave and a particle

• Can travel through empty space

Photon

Particle of light

Wavelength

the distance between successive peaks or troughs of a wave, related to colour

Long vs short wavelengths

• Long wavelengths/lower frequencies have redder light

• Short wavelengths/higher frequencies have bluer light

Frequency

The number of times per second that a wave vibrates up and down, commonly measures in nanometers or angstroms, related to colour

Electromagnetic spectrum

The entire range of wavelengths (frequencies) of light is known as the electromagnetic spectrum

The higher the photon energy

The shorter its wavelength

The list of different kinds of light from shortest to longest wavelength

gamma rays, x-rays, ultraviolet, visible, infrared, radio

Black body radiation

• The radiation emitted by an opaque object

• Depends only on temperature

• a perfect emitter and a perfect absorber of light

• an opaque object having a temperature above absolute zero

Wien’s law

The hotter an object is, the shorter is the wavelength of its maximum intensity

Stefan–Boltzmann law

Hotter objects emit more energy than cooler objects of the same size

Wavelength of maximum intensity

• The wavelength at which a perfect radiator emits the maximum amount of energy

• Depends only on the object’s temperature

Emission

Energy in matter can be converted into light that is emitted

Absorption

Matter can absorb energy in the form of light and convert it to another form or re emit it

Transmission

• Transparent objects transmit light

• Opaque objects block (absorb) light

Reflection/scattering

• Light can bounce off objects in one direction (reflection) or random directions(scattering)

• We see objects that emit light directly

• We see others by light reflecting off these objects

Colour of objects

• Objects appear different colours because they absorb some colours (wavelengths) and reflected or transmit other colours.

• The colours we see are the wavelengths that are reflected or transmitted

Interactions of light and matter

• Most colors we see are in reflected light

• The different colored objects in the room are reflecting components of the white light and absorbing the rest.

• Black shirt absorbs all wavelengths

• Blue reflects blue wavelengths, absorbs the rest

• A blue shirt demonstrates that white light contains blue light

Why is the sky blue?

• molecules and small particles in the upper atmosphere scatter blue photons more efficiently than red ones

• When you look away from the Sun, you see blue light that has bounced off the upper atmosphere into your line of sight

Spectroscopy

the process of dispersing light into its spectrum (different wavelengths)

What can we learn by analyzing starlight?

• A star’s temperature

  • peak wavelength of the spectral curve

• A star’s chemical composition

  • dips in the spectral curve or the lines in the absorption spectrum

• A star’s motion

The Doppler effect

The observed change in the wavelength of radiation due to a source moving toward or away from the observer

Redshift

When the source of light is moving away from the observer the wavelength of the emitted light will appear to increase (long wavelengths)

Blueshift

When the source of light is moving towards the observer the wavelength of the emitted light will appear to decrease (short wavelengths)

3 powers of a telescope

1. Light gathering

2. Resolving power

3. Magnifying power

Light gathering power

Depends on the surface area A of the primary lens / mirror, proportional to diameter squared

Angular resolution

a measure of the ability to separate two closely-spaced lights. The human eye has an angular resolution of about 1 arc minute

Resolving power

• the inverse of angular separation or distance between the objects, which can be resolved when viewed via an optical instrument

• The greatest resolution would be provided by blue light (short wavelengths = greater resolving power)

Factors affecting ground based telescopes

• atmospheric interference (including turbulence, weather, and absorption of wavelengths)

• light pollution

• instrumental/engineering challenges

Twinkle of stars

Shifting pockets of air with different temperatures and densities cause light from celestial objects to be refracted (bent) and blurred

Factors that contribute to resolving power of a telescope

Diffraction, optical quality, and atmospheric conditions

Wavelength absorption

• The atmosphere acts as a filter, absorbing significant portions of the electromagnetic spectrum, such as most ultraviolet, X-ray, and far- infrared radiation

• These wavelengths can only be observed effectively by space-based telescopes

• Water vapor and carbon dioxide absorb infrared light. Ultraviolet radiation is absorbed by Ozone in the atmosphere

Weather conditions

Cloud cover, rain, snow, and dust can obstruct observations or make the sky appear dimmer, significantly limiting observing time

Daytime limitations

The brightness of the sun prevents optical telescopes from being used effectively during the day.

Light pollution

Artificial light from cities and other human developments increases the brightness of the night sky, blocking out the faint light of many stars and distant objects

Magnifying power

• Ability of the telescope to make the image appear bigger. The magnification depends on the ratio of focal lengths of the primary mirror/lens and the eyepiece

• A larger magnification does not improve the resolving power of

  the telescope

Refracting telescope

A telescope that forms images by bending light with a lens, lens focuses light onto the focal plane

Reflecting telescope

A telescope that forms images by reflecting light with a mirror (best type), mirro focuses light onto the focal plane

Light pollution

The illumination of the night sky by waste light from cities and outdoor lighting, which prevents the observation of faint objects

Charge coupled device (CCD)

• An electronic device consisting of a large array of light-sensitive elements

• Images are digitalized (converted to numerical data) and can be stored for later analysis

E-M radiation and the atmosphere

• The atmosphere only passes certain `spectral windows’ (either way)

• The atmosphere is transparent to visible light, some parts of the radio and some parts of the Infrared and UV. Most of the UV will not penetrate Earth's atmosphere and reach the ground

Radio waves

Can penetrate the Earth’s atmosphere and can be observed from the ground

Radio telescope

• A reflecting type of telescope.

• uses a large, curved dish (which functions as a mirror for radio waves) to gather and reflect radio waves to a central focal point where a receiver/antenna is located

Interferometer

A way to improve resolving power is to connect two or more telescopes

What is the primary reason to make telescopes larger?

to collect more light to so faint objects can be seen