1.08. Telescopes and magnifiers

Magnification formula

Magnification = new image size / old actual image size

Angular magnification Definition

The relationship between the angular size of the actual object relative to the optical axis AND the angular size of the image relative to the optical axis

Hand magnifier

Positive spherical lens

Distance between object and lens is set by observer

It is usually set to the focal length of the lens which forms a virtual image of the object (virtual - viewed by the eye)

How to calculate magnification of hand magnifier

Equivalent power of magnifier / 4

How to calculate equivalent power of 2 lenses separated by a distance

Fe = F1 + F2 - (d x F1 x F2)

Fe = equivalent power

F1 = power of magnifier lens

F2 = accommodation exerted by patient (or the powerr of the reading addition worn by patient)

d = distance between magnifier and eye (metres)

Stand magnifier

Positive spherical lens

Distance between object and lens is set by the frame of the stand magnifier (kept at constant distance from a near task)

Usually shorter than the focal length of the lens

Forms a virtual image of the object

Telescopes

Constist of:

Objective lens or mirror

Eyepeice

What is a telescope with 2 lenses

Refracting telescope

What is a telescope with a mirror and a lens

Reflecting telescope

What are the 2 types of refracting telescope

Astronomical and galilean

Astronomical telescope

Objective - positive lens

Eyepeice - positive lens

Creates an inverted image

How does an astronimcal telescope create a magnified image

When the focal length of the eyepeice is shorter than the focal length of the objective

Shorter focal length. = higher power so we want this as the eyepeice to get a magnified image

So the doptric power of the eyepeice is greater than objective

How to calculate total length of telescope

Add focal lengths

How to calculate magnification for an astronomical telescope

Prisms in astronomical telescopes

Prisms re-invert the image to put it thhe correct way up

Porro prism assembly

Roof prism assembly

Porro prisms

Arranged as a pair to invert the image

Found in binoculars and slit lamps

Ingoing and outgoing rays are not co axial

Invert and translate the image in terms of their beam path to adjust system to match your pd

What dos it mean when Ingoing and outgoing rays are not co axial

Image goes through prism and comes out at a slightly different place

Roof prisms

Found in compact binoculars and many low vision aids

Ingoing and outgoing rays are co axial

Enables the objective and eyepeice lenses to alse be co axial

What does it mean when Ingoing and outgoing rays are co axial

Point straight at eachother

the beam exits along the same line it entered

Galilean telescope

Objective - positive lens

Eyepeice - negative lens

Creates an upright image - no prisms needed

Creates a larger (magnified) image if the focal length of the eyepiece is shorter than the focal length of the objective

Eyepeice has more dioptres

Reflecting telescope

Objective - curved mirror

Why do reflecting telescopes have curved mirrors

Curved mirrors dont suffer with chromatic aberration (when a lens makes colors separate, causing blurry edges or color outlines in an image)

Refelcting telescopes - aspects of performance