Lenses and Optics Flashcards
What is the strength of a lens, conceptually?
The strength of a lens (measured in diopters) is a measure of the len’s refractive power. The greater the strength, the more the light will bend.
What is the formula for lens strength?
Strength = (1/Focal Length)
This yields the strength of a lens in diopters.
How does the strength of a lens relate to focal length?
More powerful lenses have a shorter focal length. This is because the light needs to bend more at the shorter focal point.
What do negative and positive values mean for lens strength?
Negative value: lens is diverging (i.e., concave)
Positive value: lens is converging (i.e., convex)
How does a converging lens refract parallel rays of light?
A converging (convex) lens refracts parallel rays of light so they converge at a single point on the other side of the lens called the focal point.
Is a converging lens convex or concave?
A converging lens is convex.
CONVerging = CONVex
Is a diverging lens convex or concave?
Concave
What is the sign of the focal length (F) for diverging and converging lenses?
Diverging (concave): negative
Converging (convex): positive
What kind of image does a diverging lens produce?
Virtual, upright, and reduced. Note that virtual images are always upright!
So, this means that the image distance (Di) for diverging lenses is always negative.
How does a diverging lens refract parallel rays of light?
A diverging (concave lens) spreads parallel rays of light away from its focal point.
What is myopia and how do we treat it?
Myopia is nearsightedness and results when light from distance objects is focused in front of the retina. NEARsightedness = light focused in front of retina.
To treat myopia, we need to use a diverging (concave) lens. This will “push” the image back to the retina.
What is hyperopia and how do we treat it?
Hyperopia is farsightedness and results when light focuses behind the retina. FARsightedness = light focused too FAR back.
To treat hyperopia, we need to use a converging lens that will focus the light sooner.
Image distance (Di)
The image distance is the distance from the lens or mirror to the image.
What does the sign of the image distance tell us?
Positive image distance: real image
Negative image distance: virtual image
Object distance (Do)
The object distance is the distance from the object to the lens.
How to solve for the focal length
Focal length = 0.5(R), where r is the radius of curvature.
Note that if we have a diverging (concave) lens, we need to make the focal length negative!
What is chromatic aberration and how does light frequency relate?
Chromatic aberration refers to the formation of blurry images due to the effects of dispersion (light separation) through a lens.
Note that the refractive index of light increases with frequency, so high frequency light (e.g., violet) will refract more.
What is spherical aberration?
Spherical aberration occurs in converging lenses and is due to the geometry to a spherical lens. Light rays near the edge of a spherical lens refract more than predicted due to its thickness, which can lead to multiple focal points. As a result, blurriness can occur.
How to calculate magnification for lenses placed in series?
M total = M1 * M1 * … * Mn
How can we fix spherical aberration?
Reduce the thickness of the lens.
What kind of image does a convex (diverging) mirror produce?
Diverging mirrors always produce a virtual, upright image.
What kind of lens is the eye?
The eye is a converging lens.
What kind of image is produced in a converging lens or mirror if the object is placed within the focal length (i.e., O less than F)?
A upright and virtual image is formed.
What kind of image is produced in a converging lens or mirror if the object is placed outside the focal length (i.e., O>F)?
An inverted and real image is formed.
What kind of image is produced in a converging lens or mirror if the object distance equals the focal length (i.e. O=F)?
No image is produced.
