ASTR 1514 1st Edition Lecture 7Lights and Telescopes Chapter 4.1-4.3Objectives - Understand what light is and its properties. - Understand how a telescope works.Why is light important to astronomers? Light provides the predominate information that astronomers use to understand astronomical objects such as; distance, temperature, and composition. - Most knowledge of the universe beyond Earth comes from observing light. - We use telescopes to collect light and determine astronomical objects temperature, composition, speeds and more. Light- Light travels at a constant speed in a vacuum, 300,000 kilometers per second.- The speed of light was first measured by Roemer when he was observing Jupiter’s moons. - Light will travel faster in less dense materials, it travels fastest in air. - Light is a wave of combined electricity and magnetism, called an electromagnetic wave. - Changing electric and magnetic fields create a self-sustaining electromagnetic wave. These are transverse waves, similar to a drop of water creating ripples. Properties of Waves- Wavelength: The distance between crests of a wave. - Amplitude: The height of the wave. - Frequency (f): The number of waves that pass by each second. - Period (P): The time it takes a wave to complete a full cycle. - Wavelength has an inverse relationship with frequency. So a long wavelength means a low frequency. A short wavelength means a high frequency. - Wavelength = Speed of light (300,000km/s) divided by the frequency of the wave. - In visual band, violet light has a shorter wavelength than red light. - Here is the order of wavelength colors in the visual band from smallest to largest. Violetlight<Indigo light<Bluelight<Green light<Yellow light<Orange<Red light.- Wavelengths of light are very small, for example Red light has a wavelength of approximately 6.5 x 10 -8- Light behaves as both a wave and particle. - Photon: A particle of light.1. Photons carry energy: E = hfE represent energy. h is a constant, and f is the frequency. 2. High energy photons: Light with a High frequency. (Ex: Ultraviolet light)3. Low energy photons: Light with low frequency (Ex: Infrared light)4. Higher frequency = higher energy.Light is a wave because- Waves exhibit a property called interference, waves interact with one another to becomestronger. - Light from a laser pointer exhibits interference, therefore light must be a wave. Telescopes- In a perfectly dark site (away from city lights) you can see about 4,500 stars in one hemisphere of the sky with the naked eye. - In a larger city, more light pollution, only about 35 stars can be seen per hemisphere. - Telescopes collect light to see distant objects in the sky. - A telescopes aperture, space through which light passes in a whole or opening, is much larger than the human eye, therefore they collect more light. - The bigger the aperture, the more light they collect. How telescopes work - Because light is a wave, it is refracted, when light changes direction when entering at anangle, when entering another medium (glass, air, water)- Refraction bends light. - The lens focuses the light onto the focal plane, the image contains all of the light going through the lens. - Image is then small enough to fit on a detector. - Focal length: Distance between the lens and the image, a longer focal length yields a larger image. Reflecting Telescopes- Reflecting telescopes use mirrors.- The shape of a mirror bends the light, forming an image on the focal plane, which is now between the object and the mirror. - Reflecting lens has a primary and secondary mirror. - The secondary mirror focuses the light from the primary mirror to a convenient location. - Folding the light path allows for the telescope to be shorter for the same focal length, you only have to configure one side of the mirror. Problems with Telescopes - Glass bends different colors by different amounts- Chromatic Aberration: When different colors have different focal lengths, the images will appear blurry. Detectors- By-eye observing there is no permanent record. Short exposure, only 100ms – impossibleto see faint objects. Low quantum efficiency – only a small fraction (10%) of incident, falling on or striking a surface, photons are detected. - The eye detects wavelengths between 400nm (Deep violet) and 700nm (Far red). The photons are collected from the retina, then our brain interprets the image. Photography- Photography opened the door to modern astronomy.- Refined in the late 1800s. - Faint images require very long exposure times, this is expensive. - Astronomers use CCDS, Charge-coupled devices, such as digital cameras. - Electronic detectors record the photons as pixels. - Photons create a signal in the array. Spectrum: Light sorted by frequency. - Visible spectrum: The rainbow: Red, Orange, Yellow, Green, Blue, Indigo, Violet. - Red has the longest wavelength (FM waves, Infrared waves)- Violet has the shortest wavelength. (Gamma rays, Ultraviolet rays)- Visible spectrum is only a small part of the full electromagnetic spectrum. - Spectrographs or spectrometers break up incoming light into different