OPTICAL+INSTRUMENTS

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 * WAVES AND SOURCES || OPTICAL INSTRUMENTS || TWO-SOURCE INTERFERENCE || DIFFRACTION GRATING || X-RAYS || THIN-FILM INTERFERENCE ||

RAY DIAGRAM - a diagram in which the position and nature of the image can be identified.

Define the terms principal axis, focal point, focal length and linear magnification as applied to a converging (convex) lens. PRINCIPAL AXIS - The line perpendicular to the plane of a lens. FOCAL POINT (PRINCIPAL FOCUS) - the point to which all rays parallel to the principal axis are refracted. FOCAL LENGTH - The distance from the centre of the lens to the principal focus.

Define the power of a convex lens and the dioptre.

POWER OF A LENS - the degree to which the lens diverts light, according to the formula P=1/f. A fat lens is more powerful. DIOPTRE - Unit of lens power. Symbol is m -1



Define linear magnification.

LINEAR MAGNIFICATION - how much larger the image is than the object

Construct ray diagrams to locate the image formed by a convex lens. Students should appreciate that all rays incident on the lens from the object will be focused, and that the image will be formed even if part of the lens is covered. Distinguish between a real image and a virtual image.

REAL IMAGE - an image from which the rays actually originate. Can be formed on a screen.

VIRTUAL IMAGE - an image from which the rays appear to originate. Cannot be formed on a screen.



Apply the convention “real is positive, virtual is negative” to the thin lens formula. THIN LENS FORMULA - relates the focal length (f) to the positions of the object (u) REAL IS POSITIVE CONVENTION - A sign convention used with the thin lens formula.

Solve problems for a single convex lens using the thin lens formula.

The simple magnifying glass
Define the terms far point and near point for the unaided eye. For the normal eye, the far point may be assumed to be at infinity and the near point is conventionally taken as being a point 25 cm from the eye.

FAR POINT - The point where the object must be placed to be focused by the unaccommodating eye. This is taken to be infinity (from where the rays will be parallel).

NEAR POINT (D) - The nearest point where an object can be placed and focused by the eye. This is taken to be D = 25 cm for the normal eye.

Define angular magnification.

ANGULAR MAGNIFICATION - The angular magnification measures how much bigger an optical instrument makes an object look.



Derive an expression for the angular magnification of a simple magnifying glass for an image formed at the near point and at infinity.

MAGNIFYING GLASS: Forms a virtual image of an object placed closer than the focal point. Its magnification varies depending on the position of the image.



Construct a ray diagram for a compound microscope with final image formed close to the near point of the eye (normal adjustment). Students should be familiar with the terms objective lens and eyepiece lens.
 * The compound microscope and astronomical telescope. **

COMPOUND MICROSCOPE: A two lens device in which the eyepiece lens forms a virtual image of a real image. The real image is of an object viewed by the object lens.

Construct a ray diagram for an astronomical telescope with the final image at infinity (normal adjustment).

ASTRONOMICAL TELESCOPE: A two lens device in which the eyepiece lens forms a virtual image at infinity of a real image. The real image is of a distant object whose rays arrive parallel.

State the equation relating angular magnification to the focal lengths of the lenses in an astronomical telescope in normal adjustment.

Solve problems involving the compound microscope and the astronomical telescope. Problems can be solved either by scale ray diagrams or by calculation. media type="custom" key="25416798"

Explain the meaning of spherical aberration and of chromatic aberration as produced by a single lens. SPHERICAL ABERRATION: Distortion of the image by a real lens due to its curvature which causes the rays further from the axis to have a shorter focal length. The image is no longer sharp and its straight lines are curved. It is corrected with a ‘stopper’ which only allows rays near to the axis of the lens. CHROMATIC ABERRATION: Distortion of the image by a real lens due to dispersion of different wavelengths of light. Correction is by ‘achromatic doublet’ lens made of two different glass lenses, one convex, the other concave.
 * Aberrations **

Describe how spherical aberration in a lens may be reduced.

SPHERICAL ABERRATION CORRECTION - A circular diaphragm, known as a stopper, allows only the rays near the axis to form the image. The disadvantage is a loss of brightness. Describe how chromatic aberration in a lens may be reduced. CHROMATIC ABERRATION CORRECTION - A composite lens, known as an achromatic doublet, made from two different glasses, one convex lens, the other concave, brings the divergent rays together again.

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