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Basic Optical Devices
Part 1
Mirrors, Prisms,
Diffraction Gratings
and Spectroscopes
Basic Optical Devices are used to…
1. Change the Direction of a Ray of Light
2. Gather parallel rays of light and focus them at the
Focus or Focal Point This focus can be…
Real, that is the light rays really meet there
Virtual that is the rays only appear to meet there
3. Disperse white light into a spectrum
4. Create an Image which is…
Real and can be Projected onto a screen (its really there)
Virtual which cannot be projected
(it just appears to be there!)
Mirrors are smooth reflecting surfaces
There are 3 basic types of mirrors by shape;
Flat: Used to change the direction of a ray
or to create a virtual image of equal size
Convex: (Curves out)
• Has a virtual focus
• always creates a smaller virtual image
• it is rarely used in telescopes
Concave: (Curves in)
•Used to gather light & focus it into a real focus,
•Creates an erect virtual and larger image
of an object placed closer than its focus
•Create inverted real image
Of an object placed farther away than the focus
These images get smaller the farther away the
object is
•Concave mirrors are often used in telescopes
Prisms are flat sided pieces of glass, plastic or
crystal, They are used to:
• disperse white light into a spectrum.
• change the direction of a ray of light
Diffraction Gratings
Diffraction Gratings are a series of parallel slits
usually in a plastic film.
They use diffraction to disperse white light into
a spectrum
A Spectroscope is a devise that uses a prism or
diffraction grating to study spectra