When o of the mirror equation is positive and negative:

Positive: object is in front of mirror (real image)

Negative: object is behind mirror (virtual image)

When i of the mirror equation is positive and negative:

Positive: image is in front of mirror (real image)

Negative: image is behind mirror (virtual image)

When r of the mirror equation is positive and negative:

Positive: concave mirrors

Negative: convex mirrors

When f of the mirror equation is positive and negative:

Positive: concave mirrors

Negative: convex mirrors

When m of the mirror and lens equations is positive and negative:

Positive: image is upright (erect)

Negative: image is inverted

When absolute value of m of the mirror and lens equations is greater than 1 and les than 1:

Greater than 1: enlarged

Less than 1: reduced

Equal to 1: same size

When o of the lens equation is positive and negative:

Positive: object on side of lens light is coming from (virtual side)

Negative: object on side of lens light is going to (real side)

When i of the lens equation is positive and negative:

Positive: image on side of lens light is going to (real side)

Negative: image on side of lens light is coming from (virtual side)

When r of the lens equation is positive and negative:

Positive: when on real side (convex surface as seen from side light is coming from)

Negative: when on virtual side (cocave surface as seen from side light is coming from)

When f of the lens equation is positive and negative:

Positive: converging lens

Negative: diverging lens

Law of Reflection equation:

θ_{1} = θ_{2}

where θ_{1} is the incident angle and θ_{2} is the reflected angle

the angles are always measured from normal (a line perpendicular to the surface of the medium)

Equation to determine the object or image distance from a mirror, focal length, or radius of curvature:

^{1}/_{o }+ ^{1}/_{i} =^{ 1}/_{f} = ^{2}/_{r}

where o is the distance of the object from the mirror, i is the distance of the image from the mirror, f is the distance from the focal point to the mirror (focal length), and r is the distance between the center of curvature and the mirror (for spherical mirrors)

units = m^{-1}

Equation to find the magnification of an image for both mirrors and lenses:

m = - ^{i}/_{o}

where o is the distance of the object from the mirror and i is the distance of the image from the mirror or lens

if the absolute value of m is less than 1, the image is reduced

if the absolute value of m is greater than 1, the object is enlarged

Equation to determine the index of refraction:

n = ^{c}/_{v}

where c is the speed of light in a vacuum (3.8 X 10^{8}), v is the speed of light in the given medium, and n is the index of refraction

Equation to find the degree of refraction of a light ray upon entering a new medium:

n_{1}sinθ_{1} = n_{2}sinθ_{2}

theta is always measured with respect to the perpendicular to the boundary

Equation to determine the critical angle:

derived from Snell's Law

sinθ_{c} = n_{2} / n_{1}

Equation to determine object distance (o), image distance (i), focal length (f), and magnification (m) for lenses:

^{1}/_{o }+ ^{1}/_{i} = ^{1}/_{f}

m = ^{-i}/_{o}

Equation to determine the focal length in lenses where thickness cannot be ignored:

^{1}/_{f }= (n-1)(^{1}/r_{1} - ^{1}/r_{2})

where n is the index of refraction of the lens material, r_{1 }is the radius of the curvature of the first lens surface, and r_{2} is the radius of curvature of the second lens surface