Examples for optical instruments
a.) Grandpa's far point is 75 cm. What is the prescription (refractive power in diopters) for Grandpa's contact lenses?
A diverging lens will make an image of a far away object at its focal length. Therefore, the focal length is negative 75 cm, and the prescription is
b.) What is his prescription (in diopters) for eye glasses?
The image must be brought to 74 cm. Therefore the prescription is
c.) Grandma's near point is 75 cm. What is the prescription (in diopters) for Grandpa's contact lenses?
Choose the focal length such that an object at the normal near point of 25 cm produces an image where Grandma can see it, at 75 cm. -> f = 37.5 cm, and the prescription is
d.) What is her prescription for eye glasses?
The same as above, only the normal near point is 24 cm from the glasses and her near point is 74 cm from the lenses, -> f = 35.5 m.
a.) A simple two-lens telescope's eyepiece has a focal length of 10 cm. If one wishes the magnification of the telescope to by a factor of 40, what is the length of the telescope?
The second lens must have a focal length 40 times longer, 400 cm. The overall length is thus
Optical instrument's index