Ray Optics — Part 3: Microscopes and telescopes: visual lesson, CBSE Class 12 Physics

Lesson

Optical instruments

Part 3 — using lenses and mirrors to see the very small and the very far.

In this lesson you will

Derive the magnifying power of a simple microscope
Derive the magnifying power of a compound microscope and an astronomical telescope
Describe a reflecting telescope and its advantages

Concept

Magnifying power and the simple microscope

Magnifying power $m = \dfrac{\beta}{\alpha}$ : angle subtended at the eye by the image ÷ angle subtended by the object at the near point ( $D = 25$ cm). For a convex lens of focal length $f$ held close to the eye: $m = 1 + \dfrac{D}{f}\ \ (\text{image at } D),\qquad m = \dfrac{D}{f}\ \ (\text{image at infinity, relaxed eye})$

Table of the magnifying power of a simple microscope for an image at the near point and at infinity
Image position	Magnifying power
Near point (25 cm)	1 + D/f
Infinity (relaxed eye)	D/f

Worked example

Worked example 9: a magnifying glass

A jeweller uses a convex lens of focal length 5.0 cm as a simple microscope. Find its magnifying power when the image is (a) at the near point, (b) at infinity.

(a) $m = 1 + \dfrac{D}{f} = 1 + \dfrac{25}{5.0} = 6.0$ .
(b) $m = \dfrac{D}{f} = \dfrac{25}{5.0} = 5.0$ .
Note — a shorter focal length gives a larger magnification.

Worked table: magnifying power 6 with the image at the near point and 5 with the image at infinity
Image at	m
25 cm	6
Infinity	5

Cookies