OPTICS - types of lenses, ray diagrams

Doc Brown's Physics Revision Notes

Suitable for GCSE/IGCSE Physics/Science courses or their equivalent

This page will answer many questions e.g.

What is a convex lens?

What is a concave lens?

What is the focal length of a lens?

How can you measure the focal length of a convex lens?



  • REMINDERS:

      • 1. No refraction when a light ray strikes a different medium at 90o to the surface ie 'down' the normal.

        • The same applies to 3 and 4 for the central ray in the diagram.

      • 2. Double refraction through a rectangular glass block at the air/glass interfaces, note that when the ray emerges back into air its path is parallel to the original incident ray.

      • 3. Refraction of two rays at the two surfaces of a diverging concave lens.

      • 4. Refraction of two rays at two of the surfaces of a triangular glass or plastic prism.

      • 5. Refraction of two rays at the two surfaces of converging concave lens.

  • Be able to explain how to measure the focal length of a converging lens using a distant object (see Ray diagram 2 below).

  • You should revise any investigations on the behaviour of converging lenses, including real and virtual images.

  • Ray diagram 1 (below): Conventions in light ray diagrams

    • F is the abbreviation for focal length. 2F means twice the focal length.

    • Focal length is defined and explained in Ray diagram 2 below.

  • Ray diagram 2 (below): Ray diagram to show how to measure the focal length of a convex lens.

    • The image is real and inverted (upside down!). The image is a distance F from the lens.

    • -

  • Ray diagram 3 (below): Ray diagram showing the formation of an image from an object at a distance of 2F (twice the focal length) beyond the convex lens.

    • The image is real, inverted and the same size as the object. The image is a distance 2F beyond the lens.

    • -

  • Ray diagram 4 (below): The formation of a virtual image by a convex lens when the object is between F and 2F.

    • The image is real, inverted and larger than the object. The image is further than 2F from the lens.

    • -

  • Ray diagram 5 (below): The formation of a virtual image by a convex lens when the object is between F and the lens.

    • The image is virtual, right way up (NOT inverted), bigger than the object (a 'magnifying glass' effect). The image is on the same side of the lens as the object.

    • -

  • Revise any investigations on the use of converging lenses to:

  • a) measure the focal length using a distant object

    • b) investigate factors which affect the magnification of a converging lens (formulae are not needed)

    • Be able to explain how the eyepiece of a simple telescope magnifies the image of a distant object produced by the objective lens (ray diagrams are not necessary). diagram to do

  • Eye defects and correction still to write up

 


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