A2/A-level Physics - Lenses 

Lenses

• Principal focus
  • Rays parallel to the principle axis of the lens converge onto a point called the principle focus parallel rays that aren’t parallel to the principle axis converge somewhere else on the focal plane.
• Focal length of converging lens
  • Focal length, f,
    • Is the distance between the lens axis and the principle focus.
  • Converging lens
    • Brings light rays together
    • Changes direction of light rays by refraction
    • A more powerful (thicker) lens converges the rays more strongly and will have a shorter focal length.

Formation of images by a converging lens

• A real image is formed when light rays from an object are made to pass through another point in space. The light rays are actually there, and the image can be captured on screen.

• A virtual image is formed when light rays from an object appear to have come from another point in space. The light rays aren’t really where the images appear to be, so the images can’t be captured on a screen.
• Converging lenses can form real and virtual images

Ray diagrams

You must know how to draw ray diagrams in A-level Physics! Practice makes perfect! 👩‍🏫

Lens equation

Astronomical telescope consisting of two converging lenses

• A telescope, in normal adjustment, is set up so that the principal focus of the objective lens is in the same position as the principal focus on the eye lens, so the final magnified image appears to be infinity.

Angular magnification in normal adjustment

• The objective lens converges the rays from the object to form the real image
• The eye lens acts as a magnifying glass on this image to form a magnified virtual image.
• If you assume that the object is infinity, then the rays from it are parallel, and the real image is formed on the focal plane.
• Telescopes magnify the angular diameter of objects
• Image appears closer than object
• The magnification, m, of the telescope can be calculated in terms of angles, or the focal length the angle magnification is the angle subtended by the image, Ѳ_i, over the angle subtended (be opposite to) by the object, Ѳ_o , at the eye.

Focal length of the lenses

If you assume the object is an infinity, then the rays from it are parallel, and the real image is formed on the focal plane magnification in terms of focal length.

• Telescopes magnifying power is a ratio of apparent size of an object seen through a telescope to its size when seen through the eye alone.
• Value of the magnifying power depends on eyepiece.

Reference:

https://getrevising.co.uk/resources/lenses_and_optical_telescopes

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