Lesson 9 of 11 · 7 min
Simple and compound microscope
NCERT § "The Microscope"
Held up to one eye, Kabir's +10 D lens makes a printed letter look three and a half times bigger. With a second, stronger lens it can reach a hundred and fifty.
The lesson in notes
In short
A simple microscope (magnifier) is a converging lens of short focal length held close to the eye with the object within its focal length, giving a virtual, erect, enlarged image.
Its angular magnification is m = 1 + D/f if the image forms at the near point (D = 25 cm), and m = D/f for an image at infinity (relaxed eye).
A compound microscope has an objective of short focal length that forms a real, inverted, enlarged image, which the eyepiece then magnifies further as a simple microscope.
Total magnification m = m_o × m_e. For the objective, the linear magnification is m_o = L/f_o, where L, the tube length, is measured from the objective's second focal point to the eyepiece's first focal point.
With the final image at infinity, m = (L/f_o)(D/f_e); with the final image at the near point, the eyepiece contributes (1 + D/f_e).
The final image in a compound microscope is inverted with respect to the object.
High magnification needs short focal lengths for both objective and eyepiece.