Diffraction of Light By definition, diffraction refers to the apparent bending of waves around small obstacles and the spreading out of waves past small openings.

Diffraction of Light Newton pointed out in his 1704 book Opticks, that "Light is never known to follow crooked passages nor to bend into the shadow". This concept is consistent with the particle theory, which proposes that light particles must always travel in straight lines.

Diffraction of Light If the particles encounter the edge of a barrier, then they will cast a shadow because the particles not blocked by the barrier continue on in a straight line and cannot spread out behind the edge. On a macroscopic scale, this observation is almost correct, but it does not agree with the results obtained from light diffraction experiments on a much smaller scale.

Diffraction of Light When light is passed through a narrow slit, the beam spreads and becomes wider than expected. This fundamentally important observation lends a significant amount of credibility to the wave theory of light. Light waves encountering the edge of an object appear to bend around the edge and into its geometric shadow, which is a region that is not directly illuminated by the light beam.

Diffraction of Light With the development of quantum physics, scientists came to realize that photons, a tiny elementary particle responsible for all forms of electromagnetic radiation, was in fact the source for visible light. Since all physical objects have wave-like properties at the atomic level, diffraction can be studied in accordance with the principles of quantum mechanics. WAVE PARTICLE DUALITY: All carriers of energy and momentum, such as light and electrons, propagate like a wave and exchange energy like a particle.

Diffraction of Light and Telescopes

The Powers of a Telescope Light Gathering Power Light Gathering Power : Astronomers prefer *large* telescopes. A large telescope can intercept and focus more starlight than does a small telescope. A larger telescope will produce brighter images and will be able to detect fainter objects. Resolving Power Resolving Power : A large telescope also increases the sharpness of the image and the extent to which fine details can be distinguished. Magnification Magnification : The magnifying power is the ability of the telescope to make the image appear large in the field of view.

Size Does Matter D Light-Gathering Power: Depends on the surface area (A) of the primary lens and is proportional to the telescopes diameter.

The Powers of a Telescope Resolving Power: Wave nature of light: The telescope aperture produces fringe rings that set a limit to the resolution of the telescope. Diffraction Fringe – we cannot see any detail smaller than the fringe.

The Powers of a Telescope A larger magnification does not improve the resolving power of the telescope! Magnifying Power The ability of the telescope to make the object’s optical image appear bigger while being observed

Types of Telescopes Refracting Telescopes Refracting Telescopes : Use lenses as the optics to focus and bend light. Galileo used a refracting telescope. The human-eye is partly a refracting telescope.

Refracting Telescope Objective Lens Eyepiece Lens Focal Length Objective Focal Length of Eyepiece Refracting Telescope : Lens focuses light onto the focal plane Focal length

Disadvantages Blue Focus Red Focus Chromatic Aberration Refracting telescopes suffer from Chromatic Aberration. As light passes through a lens, just as a prism will disperse light, the lens will focus bluer wavelengths differently than the redder wavelengths.

Disadvantages Cannot be made very large for a multitude of reasons. Get to be very expensive to maintain. Lenses can grow “cloudy” over time. Lenses can distort over time.

140-ft Hevelius telescope 1673

Types of Telescopes Reflecting Telescopes Telescopes : Use mirrors as the optics to focus and bounce light. The rear view mirror on your car is a simple reflecting telescope. Reflecting Telescope: Concave Mirror focuses light onto the focal plane Most modern telescopes are reflecting telescopes. Focal length

Reflecting Telescope

Advantages Reflecting telescopes do not suffer from Chromatic Aberration. All wavelengths will reflect off the mirror in the same way. Reflecting telescopes can be made very large because the mirrored surfaces have plenty of support. Thus, reflecting telescopes can greatly increase in light gathering and resolving power. Reflecting telescopes are often cheaper ($$$) to make than similarly sized refracting telescopes.

Newton’s Telescope: The first reflecting telescope

Hey! Where are you going? The Doppler Effect The wavelength of light is effected by the relative motion between the source and the observer. Christian Doppler ( ), a professor of mathematics in Prague, pointed out in 1842 that the observed wavelength of light is affected by motion. The frequency increases when the source and observer are approaching each other, and decreases when they are moving away from each other.

The Doppler Effect Stationary Observer Blueshift Redshift Towards Away

Observer

Wavelength Stationary Star Star moving away from us Star moving towards from us How does this help? Strength of the Light with this Wavelength

TelescopesTelescopes

Where to put a Telescope? Far away from civilization – to avoid light pollution

On high mountain-tops – to avoid atmospheric turbulence and other weather effects Where to put a Telescope?

On high mountain-tops – to avoid atmospheric turbulence and other weather effects

“Seeing” Weather conditions and turbulence in the atmosphere set further limits to the quality of astronomical images. Bad seeing Good seeing

Hubble Space Telescope

X-Ray Astronomy X-rays are completely absorbed in the atmosphere. X-ray astronomy has to be done from satellites. NASA’s Chandra X-ray Observatory

Gamma-Ray Astronomy Gamma-rays: most energetic electromagnetic radiation; traces the most violent processes in the Universe The Compton Gamma-Ray Observatory Mission terminated June 2000

Gamma-Ray Astronomy Fermi Gamma-ray Space Telescope Launched 11 June 2008

Gamma-Ray Astronomy Swift Gamma-Ray Burst Mission Swift is a multi-wavelength space- based observatory dedicated to the study of gamma-ray burst (GRB) science. Its three instruments work together to observe GRBs and their afterglows in the gamma-ray, X-ray, ultraviolet, and optical wavebandswavelength

Radio Astronomy

Radio Interferometry

The Very Large Array (VLA): 27 dishes are combined to simulate a large dish of 36 km in diameter. Even larger arrays consist of dishes spread out over the entire U.S. (VLBA = Very Long Baseline Array) or even the whole Earth (VLBI = Very Long Baseline Interferometry)!

The 300-m radio telescope in Arecibo, Puerto Rico