Diffraction of Light

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 LightIf 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 LightWhen 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 LightWith 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 TelescopeLight 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: A large telescope also increases the sharpness of the image and the extent to which fine details can be distinguished.

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 TelescopeResolving 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: Use lenses as the optics to focus and bend light.

Galileo used a refracting telescope.

The human-eye is partly a refracting telescope.

Refracting TelescopeObjective

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

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 TelescopesReflecting 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

AdvantagesReflecting 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

Telescopes

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 AstronomyX-rays are completely absorbed in the atmosphere.

X-ray astronomy has to be done from satellites.

NASA’s Chandra X-ray Observatory

Gamma-Ray AstronomyGamma-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 wavebands

Radio Astronomy

Radio Interferometry

Radio InterferometryThe 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