Geometric optics (11 questions)

Question 1

(T/F) There will be approximately 11 questions of geometric optics on boards and there are 11 main equations

Answer 1

TRUE

Question 2

An image or an object converges or diverges light, is it real or virtual?

Answer 2

#1 General Vergence equation: L'=F+L, F=L'-L
-An optical system alters the wavefront of the light that strikes it.
*(-L)=RO=light diverges as it leaves object
*(-L')=VI=light diverges as it leaves the lens
*(+L)=VO=light converges as it leaves object
*(+L')=RI=light converges as it leaves lens
L=object space and L'=image space

Question 3

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Answer 3

2 Object and image locations: l=n1/L, l’=n2/L’,

• vergence is related to object and image locations
• l=distance to object from optical system (lens)
• l’=distance to image from optical system (lens)
• n1=index light travels thru b4 striking optical system
• n2=index light travels thru after leaving optical system
• concave=wraps around lower n =diverges light
• convex=wraps around higher n =converges light

Question 4

A lens (biconvex, biconcave, plano convex, etc) is suspended in air, you have the n value of air and the lens material, they give you a radius. Find the power.

Answer 4

3 Single surface refractive index (SSRI): F=(n2-n1)/r

• n1=index of original media
• n2=index of final media
• if it looks like a “C” its (+) radius, a backwards “C” is a (-) radius (if you get n1 or n2 mixed up or forget the neg on the radius just remember how to make pos/neg= concave will be neg/convex will be positive)

*We do 337.5/r to get K values because aqueous in cornea is an index of 1.3375

Question 5

You are given image height and object height (or incoming vergence vs outgoing vergence or image distance vs object distance), find magnification

Answer 5

4 Lateral magnification: m=hi/ho=L/L’=l’/l

• hi/ho=height of image/height of object
• L/L’=incoming vergence/outgoing vergence
• l’/l=image distance from lens/object distance from lens

Question 6

You are looking at a fish under water. You know the index for air and water, the actual depth of the fish but you need to know the apparent depth

Answer 6

#1: F=L'-L and #2: L=n1/l, L'=n2/l'
#1+2: F=zero bc surface is flat so L'=L, so we can set up as a ratio (n1/l=n2/l' apparent depth)

A fish is 30cm below water…n1=1.0, l=(-)0.3M, n2=1.5, l’=(-)0.2M (20cm). n1=index of first media (water), n2=2nd media (air) bc image of fish is traveling from inside the water to our eye.

Question 7

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Answer 7

5: effective vergence Leff=L/(1-xL)

• L=vergence of light at r=0
• Leff=vergence of light at r=x

Question 8

SRx glasses=(-)8.00D, the vertex distance is 12mm, what would the contact lens Rx be?

SRx glasses=(+)8.00D, the vertex distance is 12mm, what would the contact lens Rx be?

Answer 8

6: vertex power: Fc=Fg/(1-dFg)

• Fg=power of correcting lens at vertex dist of d
• Fc=desired power at cornea
• Plus lenses get more minus when they move closer to the cornea, hyperopes req more plus in CL
• Minus lenses get more plus when they move closer to the cornea, myopes req less minus in CL
• For both plus and minus lenses, more plus is necessary when the lens is moved from the spectacle plane to the cornea
• vertex distance is always neg when going from CL to specs and always pos when going from specs to CL.

Fg=(-)8.00/(1-(+0.012x-8)=(-)7.29
Fg=(+)8.00/(1-(+0.012x+8)=(+)8.84

Question 9

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Answer 9

#7: Equivalent power: Fe=F1+F2-t/n2(F1F2)
-takes into account the thickness of a lens to turn it into a thin lens. In a thin lens, you are able to directly add front and back powers to get total power.  
F2=back surface power
F1=front surface power
Fe=equivalent power
t=thickness of lens in meters
n2=index of lens material

Question 10

you are given index, front surface power, thickness, and back surface power, what is the back vertex power?

Answer 10

#8: Back vertex power: Fv=F2+[(F1)/(1-{(t/n2)F1})]
-Most prescriptions are specified using back vertex power
F2=back surface power
F1=front surface power
Fv=back vertex power
t=thickness of lens in meters
n2=index of lens material

Question 11

What is the spherical equivalent of -2.00-1.00 x180?

Answer 11

#9: spherical equivalent aka circle of least confusion
Fse=(Fc/2) + Fs

ans: -2.50 (half of the cyl gets added to the sphere and axis disappears)

Question 12

you are given radius of curvature of a mirror in air, what is the power, is it concave or convex?

Answer 12

10: Power of a mirror: F=(-)2n/r

n=usually air
r=radius of curvature (in meters)
-radius is pos if mirror looks like a “(“ (convex), therefore the power will be neg (a convex mirror diverges light)
-radius is neg if mirror looks like a “)” (concave), therefore the power will be pos (a concave mirror converges light)

Question 13

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Answer 13

#11: Lens/mirror combination: F=2F1+F2
F1=power of front surface of the lens
F2=power of the mirror